strace(1) — Linux manual page


STRACE(1)                  General Commands Manual                 STRACE(1)

NAME         top

       strace - trace system calls and signals

SYNOPSIS         top

       strace [-ACdffhikqqrtttTvVwxxyyzZ] [-I n] [-b execve] [-e expr]...
              [-O overhead] [-S sortby] [-U columns] [-a column] [-o file]
              [-s strsize] [-X format] [-P path]... [-p pid]...
              [--seccomp-bpf] { -p pid | [-DDD] [-E var[=val]]...
              [-u username] command [args] }

       strace -c [-dfwzZ] [-I n] [-b execve] [-e expr]... [-O overhead]
              [-S sortby] [-U columns] [-P path]... [-p pid]...
              [--seccomp-bpf] { -p pid | [-DDD] [-E var[=val]]...
              [-u username] command [args] }

DESCRIPTION         top

       In the simplest case strace runs the specified command until it
       exits.  It intercepts and records the system calls which are called
       by a process and the signals which are received by a process.  The
       name of each system call, its arguments and its return value are
       printed on standard error or to the file specified with the -o

       strace is a useful diagnostic, instructional, and debugging tool.
       System administrators, diagnosticians and trouble-shooters will find
       it invaluable for solving problems with programs for which the source
       is not readily available since they do not need to be recompiled in
       order to trace them.  Students, hackers and the overly-curious will
       find that a great deal can be learned about a system and its system
       calls by tracing even ordinary programs.  And programmers will find
       that since system calls and signals are events that happen at the
       user/kernel interface, a close examination of this boundary is very
       useful for bug isolation, sanity checking and attempting to capture
       race conditions.

       Each line in the trace contains the system call name, followed by its
       arguments in parentheses and its return value.  An example from
       stracing the command "cat /dev/null" is:

           open("/dev/null", O_RDONLY) = 3

       Errors (typically a return value of -1) have the errno symbol and
       error string appended.

           open("/foo/bar", O_RDONLY) = -1 ENOENT (No such file or directory)

       Signals are printed as signal symbol and decoded siginfo structure.
       An excerpt from stracing and interrupting the command "sleep 666" is:

           sigsuspend([] <unfinished ...>
           --- SIGINT {si_signo=SIGINT, si_code=SI_USER, si_pid=...} ---
           +++ killed by SIGINT +++

       If a system call is being executed and meanwhile another one is being
       called from a different thread/process then strace will try to
       preserve the order of those events and mark the ongoing call as being
       unfinished.  When the call returns it will be marked as resumed.

           [pid 28772] select(4, [3], NULL, NULL, NULL <unfinished ...>
           [pid 28779] clock_gettime(CLOCK_REALTIME, {1130322148, 939977000}) = 0
           [pid 28772] <... select resumed> )      = 1 (in [3])

       Interruption of a (restartable) system call by a signal delivery is
       processed differently as kernel terminates the system call and also
       arranges its immediate reexecution after the signal handler

           read(0, 0x7ffff72cf5cf, 1)              = ? ERESTARTSYS (To be restarted)
           --- SIGALRM ... ---
           rt_sigreturn(0xe)                       = 0
           read(0, "", 1)                          = 0

       Arguments are printed in symbolic form with passion.  This example
       shows the shell performing ">>xyzzy" output redirection:

           open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3

       Here, the second and the third argument of open(2) are decoded by
       breaking down the flag argument into its three bitwise-OR
       constituents and printing the mode value in octal by tradition.
       Where the traditional or native usage differs from ANSI or POSIX, the
       latter forms are preferred.  In some cases, strace output is proven
       to be more readable than the source.

       Structure pointers are dereferenced and the members are displayed as
       appropriate.  In most cases, arguments are formatted in the most C-
       like fashion possible.  For example, the essence of the command "ls
       -l /dev/null" is captured as:

           lstat("/dev/null", {st_mode=S_IFCHR|0666, st_rdev=makedev(0x1, 0x3), ...}) = 0

       Notice how the 'struct stat' argument is dereferenced and how each
       member is displayed symbolically.  In particular, observe how the
       st_mode member is carefully decoded into a bitwise-OR of symbolic and
       numeric values.  Also notice in this example that the first argument
       to lstat(2) is an input to the system call and the second argument is
       an output.  Since output arguments are not modified if the system
       call fails, arguments may not always be dereferenced.  For example,
       retrying the "ls -l" example with a non-existent file produces the
       following line:

           lstat("/foo/bar", 0xb004) = -1 ENOENT (No such file or directory)

       In this case the porch light is on but nobody is home.

       Syscalls unknown to strace are printed raw, with the unknown system
       call number printed in hexadecimal form and prefixed with "syscall_":

           syscall_0xbad(0x1, 0x2, 0x3, 0x4, 0x5, 0x6) = -1 ENOSYS (Function not implemented)

       Character pointers are dereferenced and printed as C strings.  Non-
       printing characters in strings are normally represented by ordinary C
       escape codes.  Only the first strsize (32 by default) bytes of
       strings are printed; longer strings have an ellipsis appended
       following the closing quote.  Here is a line from "ls -l" where the
       getpwuid(3) library routine is reading the password file:

           read(3, "root::0:0:System Administrator:/"..., 1024) = 422

       While structures are annotated using curly braces, simple pointers
       and arrays are printed using square brackets with commas separating
       elements.  Here is an example from the command id(1) on a system with
       supplementary group ids:

           getgroups(32, [100, 0]) = 2

       On the other hand, bit-sets are also shown using square brackets, but
       set elements are separated only by a space.  Here is the shell,
       preparing to execute an external command:

           sigprocmask(SIG_BLOCK, [CHLD TTOU], []) = 0

       Here, the second argument is a bit-set of two signals, SIGCHLD and
       SIGTTOU.  In some cases, the bit-set is so full that printing out the
       unset elements is more valuable.  In that case, the bit-set is
       prefixed by a tilde like this:

           sigprocmask(SIG_UNBLOCK, ~[], NULL) = 0

       Here, the second argument represents the full set of all signals.

OPTIONS         top

       -e expr     A qualifying expression which modifies which events to
                   trace or how to trace them.  The format of the expression


                   where qualifier is one of trace (or t), abbrev (or a),
                   verbose (or v), raw (or x), signal (or signals or s),
                   read (or reads or r), write (or writes or w), fault,
                   inject, status, quiet (or silent or silence or q),
                   decode-fds (or decode-fd), or kvm, and value is a
                   qualifier-dependent symbol or number.  The default
                   qualifier is trace.  Using an exclamation mark negates
                   the set of values.  For example, -e open means literally
                   -e trace=open which in turn means trace only the open
                   system call.  By contrast, -e trace=!open means to trace
                   every system call except open.  In addition, the special
                   values all and none have the obvious meanings.

                   Note that some shells use the exclamation point for
                   history expansion even inside quoted arguments.  If so,
                   you must escape the exclamation point with a backslash.

       -E var=val
                   Run command with var=val in its list of environment

       -E var
       --env=var   Remove var from the inherited list of environment
                   variables before passing it on to the command.

       -p pid
                   Attach to the process with the process ID pid and begin
                   tracing.  The trace may be terminated at any time by a
                   keyboard interrupt signal (CTRL-C).  strace will respond
                   by detaching itself from the traced process(es) leaving
                   it (them) to continue running.  Multiple -p options can
                   be used to attach to many processes in addition to
                   command (which is optional if at least one -p option is
                   given).  -p "`pidof PROG`" syntax is supported.

       -u username
                   Run command with the user ID, group ID, and supplementary
                   groups of username.  This option is only useful when
                   running as root and enables the correct execution of
                   setuid and/or setgid binaries.  Unless this option is
                   used setuid and setgid programs are executed without
                   effective privileges.

       -b syscall
                   If specified syscall is reached, detach from traced
                   process.  Currently, only execve(2) syscall is supported.
                   This option is useful if you want to trace multi-threaded
                   process and therefore require -f, but don't want to trace
                   its (potentially very complex) children.

                   Run tracer process as a grandchild, not as the parent of
                   the tracee.  This reduces the visible effect of strace by
                   keeping the tracee a direct child of the calling process.

                   Run tracer process as tracee's grandchild in a separate
                   process group.  In addition to reduction of the visible
                   effect of strace, it also avoids killing of strace with
                   kill(2) issued to the whole process group.

                   Run tracer process as tracee's grandchild in a separate
                   session ("true daemonisation").  In addition to reduction
                   of the visible effect of strace, it also avoids killing
                   of strace upon session termination.

                   Trace child processes as they are created by currently
                   traced processes as a result of the fork(2), vfork(2) and
                   clone(2) system calls.  Note that -p PID -f will attach
                   all threads of process PID if it is multi-threaded, not
                   only thread with thread_id = PID.

                   If the --output=filename option is in effect, each
                   processes trace is written to where pid is
                   the numeric process id of each process.

       --follow-forks --output-separately
                   Combine the effects of --follow-forks and
                   --output-separately options.  This is incompatible with
                   -c, since no per-process counts are kept.

                   One might want to consider using strace-log-merge(1) to
                   obtain a combined strace log view.

       -I interruptible
                   When strace can be interrupted by signals (such as
                   pressing CTRL-C).

                   1, anywhere    no signals are blocked;
                   2, waiting     fatal signals are blocked while decoding
                                  syscall (default);
                   3, never       fatal signals are always blocked (default
                                  if -o FILE PROG);
                   4, never_tstp  fatal signals and SIGTSTP (CTRL-Z) are
                                  always blocked (useful to make strace -o
                                  FILE PROG not stop on CTRL-Z, default if

       -e trace=syscall_set
                   Trace only the specified set of system calls.
                   syscall_set is defined as [!]value[,value], and value can
                   be one of the following:

                   syscall      Trace specific syscall, specified by its
                                name (but see NOTES).

                   ?value       Question mark before the syscall
                                qualification allows suppression of error in
                                case no syscalls matched the qualification

                   /regex       Trace only those system calls that match the
                                regex.  You can use POSIX Extended Regular
                                Expression syntax (see regex(7)).

                   syscall@64   Trace syscall only for the 64-bit

                   syscall@32   Trace syscall only for the 32-bit

                   syscall@x32  Trace syscall only for the 32-on-64-bit

                   file         Trace all system calls which take a file
                                name as an argument.  You can think of this
                                as an abbreviation for
                                -e trace=open,stat,chmod,unlink,...  which
                                is useful to seeing what files the process
                                is referencing.  Furthermore, using the
                                abbreviation will ensure that you don't
                                accidentally forget to include a call like
                                lstat(2) in the list.  Betchya woulda forgot
                                that one.  The syntax without a preceding
                                percent sign ("-e trace=file") is

                   process      Trace system calls associated with process
                                lifecycle (creation, exec, termination).
                                The syntax without a preceding percent sign
                                ("-e trace=process") is deprecated.

                   network      Trace all the network related system calls.
                                The syntax without a preceding percent sign
                                ("-e trace=network") is deprecated.

                   signal       Trace all signal related system calls.  The
                                syntax without a preceding percent sign ("-e
                                trace=signal") is deprecated.

                   ipc          Trace all IPC related system calls.  The
                                syntax without a preceding percent sign ("-e
                                trace=ipc") is deprecated.

                   desc         Trace all file descriptor related system
                                calls.  The syntax without a preceding
                                percent sign ("-e trace=desc") is

                   memory       Trace all memory mapping related system
                                calls.  The syntax without a preceding
                                percent sign ("-e trace=memory") is

                   %creds       Trace system calls that read or modify user
                                and group identifiers or capability sets.

                   %stat        Trace stat syscall variants.

                   %lstat       Trace lstat syscall variants.

                   %fstat       Trace fstat, fstatat, and statx syscall

                   %%stat       Trace syscalls used for requesting file
                                status (stat, lstat, fstat, fstatat, statx,
                                and their variants).

                   %statfs      Trace statfs, statfs64, statvfs, osf_statfs,
                                and osf_statfs64 system calls.  The same
                                effect can be achieved with
                                -e trace=/^(.*_)?statv?fs regular

                   %fstatfs     Trace fstatfs, fstatfs64, fstatvfs,
                                osf_fstatfs, and osf_fstatfs64 system calls.
                                The same effect can be achieved with
                                -e trace=/fstatv?fs regular expression.

                   %%statfs     Trace syscalls related to file system
                                statistics (statfs-like, fstatfs-like, and
                                ustat).  The same effect can be achieved
                                with -e trace=/statv?fs|fsstat|ustat regular

                   %clock       Trace system calls that read or modify
                                system clocks.

                   %pure        Trace syscalls that always succeed and have
                                no arguments.  Currently, this list includes
                                arc_gettls(2), getdtablesize(2), getegid(2),
                                getegid32(2), geteuid(2), geteuid32(2),
                                getgid(2), getgid32(2), getpagesize(2),
                                getpgrp(2), getpid(2), getppid(2),
                                get_thread_area(2) (on architectures other
                                than x86), gettid(2), get_tls(2), getuid(2),
                                getuid32(2), getxgid(2), getxpid(2),
                                getxuid(2), kern_features(2), and
                                metag_get_tls(2) syscalls.

                   The -c option is useful for determining which system
                   calls might be useful to trace.  For example,
                   trace=open,close,read,write means to only trace those
                   four system calls.  Be careful when making inferences
                   about the user/kernel boundary if only a subset of system
                   calls are being monitored.  The default is trace=all.

       -e signal=set
                   Trace only the specified subset of signals.  The default
                   is signal=all.  For example, signal=!SIGIO (or
                   signal=!io) causes SIGIO signals not to be traced.

       -e status=set
                   Print only system calls with the specified return status.
                   The default is status=all.  When using the status
                   qualifier, because strace waits for system calls to
                   return before deciding whether they should be printed or
                   not, the traditional order of events may not be preserved
                   anymore.  If two system calls are executed by concurrent
                   threads, strace will first print both the entry and exit
                   of the first system call to exit, regardless of their
                   respective entry time.  The entry and exit of the second
                   system call to exit will be printed afterwards.  Here is
                   an example when select(2) is called, but a different
                   thread calls clock_gettime(2) before select(2) finishes:

                       [pid 28779] 1130322148.939977 clock_gettime(CLOCK_REALTIME, {1130322148, 939977000}) = 0
                       [pid 28772] 1130322148.438139 select(4, [3], NULL, NULL, NULL) = 1 (in [3])

                   set can include the following elements:

                   successful   Trace system calls that returned without an
                                error code.  The -z option has the effect of
                   failed       Trace system calls that returned with an
                                error code.  The -Z option has the effect of
                   unfinished   Trace system calls that did not return.
                                This might happen, for example, due to an
                                execve call in a neighbour thread.
                   unavailable  Trace system calls that returned but strace
                                failed to fetch the error status.
                   detached     Trace system calls for which strace detached
                                before the return.

       -P path
                   Trace only system calls accessing path.  Multiple -P
                   options can be used to specify several paths.

                   Print only syscalls that returned without an error code.

                   Print only syscalls that returned with an error code.

   Output format
       -a column
                   Align return values in a specific column (default column

       -e abbrev=syscall_set
                   Abbreviate the output from printing each member of large
                   structures.  The syntax of the syscall_set specification
                   is the same as in the -e trace option.  The default is
                   abbrev=all.  The -v option has the effect of abbrev=none.

       -e verbose=syscall_set
                   Dereference structures for the specified set of system
                   calls.  The syntax of the syscall_set specification is
                   the same as in the -e trace option.  The default is

       -e raw=syscall_set
                   Print raw, undecoded arguments for the specified set of
                   system calls.  The syntax of the syscall_set
                   specification is the same as in the -e trace option.
                   This option has the effect of causing all arguments to be
                   printed in hexadecimal.  This is mostly useful if you
                   don't trust the decoding or you need to know the actual
                   numeric value of an argument.  See also -X raw option.

       -e read=set
       --read=set  Perform a full hexadecimal and ASCII dump of all the data
                   read from file descriptors listed in the specified set.
                   For example, to see all input activity on file
                   descriptors 3 and 5 use -e read=3,5.  Note that this is
                   independent from the normal tracing of the read(2) system
                   call which is controlled by the option -e trace=read.

       -e write=set
       --write=set Perform a full hexadecimal and ASCII dump of all the data
                   written to file descriptors listed in the specified set.
                   For example, to see all output activity on file
                   descriptors 3 and 5 use -e write=3,5.  Note that this is
                   independent from the normal tracing of the write(2)
                   system call which is controlled by the option
                   -e trace=write.

       -e quiet=set
                   Suppress various information messages.  The default is
                   quiet=none.  set can include the following elements:

                   attach           Suppress messages about attaching and
                                    detaching ("[ Process NNNN attached ]",
                                    "[ Process NNNN detached ]").
                   exit             Suppress messages about process exits
                                    ("+++ exited with SSS +++").
                   path-resolution  Suppress messages about resolution of
                                    paths provided via the -P option
                                    ("Requested path "..." resolved into
                   personality      Suppress messages about process
                                    personality changes ("[ Process PID=NNNN
                                    runs in PPP mode. ]").
                   superseded       Suppress messages about process being
                                    superseded by execve(2) in another
                                    thread ("+++ superseded by execve in pid
                                    NNNN +++").

       -e decode-fds=set
                   Decode various information associated with file
                   descriptors.  The default is decode-fds=none.  set can
                   include the following elements:

                   path    Print file paths.
                   socket  Print socket protocol-specific information,
                   dev     Print character/block device numbers.
                   pidfd   Print PIDs associated with pidfd file

       -e kvm=vcpu
       --kvm=vcpu  Print the exit reason of kvm vcpu.  Requires Linux kernel
                   version 4.16.0 or higher.

                   Print the instruction pointer at the time of the system

                   Print the syscall number.

                   Print the execution stack trace of the traced processes
                   after each system call.

       -o filename
                   Write the trace output to the file filename rather than
                   to stderr. form is used if -ff option is
                   supplied.  If the argument begins with '|' or '!', the
                   rest of the argument is treated as a command and all
                   output is piped to it.  This is convenient for piping the
                   debugging output to a program without affecting the
                   redirections of executed programs.  The latter is not
                   compatible with -ff option currently.

                   Open the file provided in the -o option in append mode.

                   Suppress messages about attaching, detaching, and
                   personality changes.  This happens automatically when
                   output is redirected to a file and the command is run
                   directly instead of attaching.

                   Suppress messages attaching, detaching, personality
                   changes, and about process exit status.

       --quiet=all Suppress all suppressible messages (please refer to the
                   -e quiet option description for the full list of
                   suppressible messages).

                   Print a relative timestamp upon entry to each system
                   call.  This records the time difference between the
                   beginning of successive system calls.  precision can be
                   one of s (for seconds), ms (milliseconds), us
                   (microseconds), or ns (nanoseconds), and allows setting
                   the precision of time value being printed.  Default is us
                   (microseconds).  Note that since -r option uses the
                   monotonic clock time for measuring time difference and
                   not the wall clock time, its measurements can differ from
                   the difference in time reported by the -t option.

       -s strsize
                   Specify the maximum string size to print (the default is
                   32).  Note that filenames are not considered strings and
                   are always printed in full.

                   Prefix each line of the trace with the wall clock time in
                   the specified format with the specified precision.
                   format can be one of the following:

                   none          No time stamp is printed.  Can be used to
                                 override the previous setting.
                   time          Wall clock time (strftime(3) format string
                                 is %T).
                   unix          Number of seconds since the epoch
                                 (strftime(3) format string is %s).

                   precision can be one of s (for seconds), ms
                   (milliseconds), us (microseconds), or ns (nanoseconds).
                   Default arguments for the option are

                   Prefix each line of the trace with the wall clock time.

                   If given twice, the time printed will include the

                   If given thrice, the time printed will include the
                   microseconds and the leading portion will be printed as
                   the number of seconds since the epoch.

                   Show the time spent in system calls.  This records the
                   time difference between the beginning and the end of each
                   system call.  precision can be one of s (for seconds), ms
                   (milliseconds), us (microseconds), or ns (nanoseconds),
                   and allows setting the precision of time value being
                   printed.  Default is us (microseconds).

       --no-abbrev Print unabbreviated versions of environment, stat,
                   termios, etc.  calls.  These structures are very common
                   in calls and so the default behavior displays a
                   reasonable subset of structure members.  Use this option
                   to get all of the gory details.

                   Print all non-ASCII strings in hexadecimal string format.

                   Print all strings in hexadecimal string format.

       -X format
                   Set the format for printing of named constants and flags.
                   Supported format values are:

                   raw       Raw number output, without decoding.
                   abbrev    Output a named constant or a set of flags
                             instead of the raw number if they are found.
                             This is the default strace behaviour.
                   verbose   Output both the raw value and the decoded
                             string (as a comment).

                   Print paths associated with file descriptor arguments.

                   Print all available information associated with file
                   descritors: protocol-specific information associated with
                   socket file descriptors, block/character device number
                   associated with device file descriptors, and PIDs
                   asociated with pidfd file descriptors.

                   If strace and tracee are in different PID namespaces,
                   print PIDs in strace's namespace, too.

                   Count time, calls, and errors for each system call and
                   report a summary on program exit, suppressing the regular
                   output.  This attempts to show system time (CPU time
                   spent running in the kernel) independent of wall clock
                   time.  If -c is used with -f, only aggregate totals for
                   all traced processes are kept.

       --summary   Like -c but also print regular output while processes are

       -O overhead
       --summary-syscall-overhead =overhead
                   Set the overhead for tracing system calls to overhead.
                   This is useful for overriding the default heuristic for
                   guessing how much time is spent in mere measuring when
                   timing system calls using the -c option.  The accuracy of
                   the heuristic can be gauged by timing a given program run
                   without tracing (using time(1)) and comparing the
                   accumulated system call time to the total produced using

                   The format of overhead specification is described in
                   section Time specification format description.

       -S sortby
                   Sort the output of the histogram printed by the -c option
                   by the specified criterion.  Legal values are time (or
                   time-percent or time-total or total-time), min-time (or
                   shortest or time-min), max-time (or longest or time-max),
                   avg-time (or time-avg), calls (or count), errors (or
                   error), name (or syscall or syscall-name), and nothing
                   (or none); default is time.

       -U columns
                   Configure a set (and order) of columns being shown in the
                   call summary.  The columns argument is a comma-separated
                   list with items being one of the following:

                   time-percent (or time)              Percentage of
                                                       cumulative time
                                                       consumed by a
                                                       specific system call.
                   total-time (or time-total)          Total system (or wall
                                                       clock, if -w option
                                                       is provided) time
                                                       consumed by a
                                                       specific system call.
                   min-time (or shortest or time-min)  Minimum observed call
                   max-time (or longest or time-max)   Maximum observed call
                   avg-time (or time-avg)              Average call
                   calls (or count)                    Call count.
                   errors (or error)                   Error count.
                   name (or syscall or syscall-name)   Syscall name.

                   The default value is
                   time-percent,total-time,avg-time,calls,errors,name.  If
                   the name field is not supplied explicitly, it is added as
                   the last column.

                   Summarise the time difference between the beginning and
                   end of each system call.  The default is to summarise the
                   system time.

       -e inject=syscall_set[:error=errno|:retval=value][:signal=sig][:syscall=syscall][:delay_enter=delay][:delay_exit=delay][:when=expr]
                   Perform syscall tampering for the specified set of
                   syscalls.  The syntax of the syscall_set specification is
                   the same as in the -e trace option.

                   At least one of error, retval, signal, delay_enter, or
                   delay_exit options has to be specified.  error and retval
                   are mutually exclusive.

                   If :error=errno option is specified, a fault is injected
                   into a syscall invocation: the syscall number is replaced
                   by -1 which corresponds to an invalid syscall (unless a
                   syscall is specified with :syscall= option), and the
                   error code is specified using a symbolic errno value like
                   ENOSYS or a numeric value within 1..4095 range.

                   If :retval=value option is specified, success injection
                   is performed: the syscall number is replaced by -1, but a
                   bogus success value is returned to the callee.

                   If :signal=sig option is specified with either a symbolic
                   value like SIGSEGV or a numeric value within 1..SIGRTMAX
                   range, that signal is delivered on entering every syscall
                   specified by the set.

                   If :delay_enter=delay or :delay_exit=delay options are
                   specified, delay injection is performed: the tracee is
                   delayed by time period specified by delay on entering or
                   exiting the syscall, respectively.  The format of delay
                   specification is described in section Time specification
                   format description.

                   If :signal=sig option is specified without :error=errno,
                   :retval=value or :delay_{enter,exit}=usecs options, then
                   only a signal sig is delivered without a syscall fault or
                   delay injection.  Conversely, :error=errno or
                   :retval=value option without :delay_enter=delay,
                   :delay_exit=delay or :signal=sig options injects a fault
                   without delivering a signal or injecting a delay, etc.

                   If both :error=errno or :retval=value and :signal=sig
                   options are specified, then both a fault or success is
                   injected and a signal is delivered.

                   if :syscall=syscall option is specified, the
                   corresponding syscall with no side effects is injected
                   instead of -1.  Currently, only "pure" (see -e
                   trace=%pure description) syscalls can be specified there.

                   Unless a :when=expr subexpression is specified, an
                   injection is being made into every invocation of each
                   syscall from the set.

                   The format of the subexpression is:


                   Number first stands for the first invocation number in
                   the range, number last stands for the last invocation
                   number in the range, and step stands for the step between
                   two consecutive invocations.  The following combinations
                   are useful:

                   first             For every syscall from the set, perform
                                     an injection for the syscall invocation
                                     number first only.
                   first..last       For every syscall from the set, perform
                                     an injection for the syscall invocation
                                     number first and all subsequent
                                     invocations until the invocation number
                                     last (inclusive).
                   first+            For every syscall from the set, perform
                                     injections for the syscall invocation
                                     number first and all subsequent
                   first..last+      For every syscall from the set, perform
                                     injections for the syscall invocation
                                     number first and all subsequent
                                     invocations until the invocation number
                                     last (inclusive).
                   first+step        For every syscall from the set, perform
                                     injections for syscall invocations
                                     number first, first+step,
                                     first+step+step, and so on.
                   first..last+step  Same as the previous, but consider only
                                     syscall invocations with numbers up to
                                     last (inclusive).

                   For example, to fail each third and subsequent chdir
                   syscalls with ENOENT, use
                   -e inject=chdir:error=ENOENT:when=3+.

                   The valid range for numbers first and step is 1..65535,
                   and for number last is 1..65534.

                   An injection expression can contain only one error= or
                   retval= specification, and only one signal=
                   specification.  If an injection expression contains
                   multiple when= specifications, the last one takes

                   Accounting of syscalls that are subject to injection is
                   done per syscall and per tracee.

                   Specification of syscall injection can be combined with
                   other syscall filtering options, for example, -P
                   /dev/urandom -e inject=file:error=ENOENT.

       -e fault=syscall_set[:error=errno][:when=expr]
                   Perform syscall fault injection for the specified set of

                   This is equivalent to more generic -e inject= expression
                   with default value of errno option set to ENOSYS.

       --debug     Show some debugging output of strace itself on the
                   standard error.

       -F          This option is deprecated.  It is retained for backward
                   compatibility only and may be removed in future releases.
                   Usage of multiple instances of -F option is still
                   equivalent to a single -f, and it is ignored at all if
                   used along with one or more instances of -f option.

       --help      Print the help summary.

                   Try to enable use of seccomp-bpf (see seccomp(2)) to have
                   ptrace(2)-stops only when system calls that are being
                   traced occur in the traced processes.  This option has no
                   effect unless -f/--follow-forks is also specified.
                   --seccomp-bpf is also not applicable to processes
                   attached using -p/--attach option.  An attempt to enable
                   system calls filtering using seccomp-bpf may fail for
                   various reasons, e.g. there are too many system calls to
                   filter, the seccomp API is not available, or strace
                   itself is being traced.  In cases when seccomp-bpf filter
                   setup failed, strace proceeds as usual and stops traced
                   processes on every system call.

       --version   Print the version number of strace.

   Time specification format description
       Time values can be specified as a decimal floating point number (in a
       format accepted by strtod(3)), optionally followed by one of the
       following suffices that specify the unit of time: s (seconds), ms
       (milliseconds), us (microseconds), or ns (nanoseconds).  If no suffix
       is specified, the value is interpreted as microseconds.

       The described format is used for -O, -e inject=delay_enter, and -e
       inject=delay_exit options.

DIAGNOSTICS         top

       When command exits, strace exits with the same exit status.  If
       command is terminated by a signal, strace terminates itself with the
       same signal, so that strace can be used as a wrapper process
       transparent to the invoking parent process.  Note that parent-child
       relationship (signal stop notifications, getppid(2) value, etc)
       between traced process and its parent are not preserved unless -D is

       When using -p without a command, the exit status of strace is zero
       unless no processes has been attached or there was an unexpected
       error in doing the tracing.


       If strace is installed setuid to root then the invoking user will be
       able to attach to and trace processes owned by any user.  In addition
       setuid and setgid programs will be executed and traced with the
       correct effective privileges.  Since only users trusted with full
       root privileges should be allowed to do these things, it only makes
       sense to install strace as setuid to root when the users who can
       execute it are restricted to those users who have this trust.  For
       example, it makes sense to install a special version of strace with
       mode 'rwsr-xr--', user root and group trace, where members of the
       trace group are trusted users.  If you do use this feature, please
       remember to install a regular non-setuid version of strace for
       ordinary users to use.


       On some architectures, strace supports decoding of syscalls for
       processes that use different ABI rather than the one strace uses.
       Specifically, in addition to decoding native ABI, strace can decode
       the following ABIs on the following architectures:

       │Architecture       ABIs supported          │
       │x86_64             │ i386, x32 [1]; i386 [2] │
       │AArch64            │ ARM 32-bit EABI         │
       │PowerPC 64-bit [3] │ PowerPC 32-bit          │
       │s390x              │ s390                    │
       │SPARC 64-bit       │ SPARC 32-bit            │
       │TILE 64-bit        │ TILE 32-bit             │
       [1]  When strace is built as an x86_64 application
       [2]  When strace is built as an x32 application
       [3]  Big endian only

       This support is optional and relies on ability to generate and parse
       structure definitions during the build time.  Please refer to the
       output of the strace -V command in order to figure out what support
       is available in your strace build ("non-native" refers to an ABI that
       differs from the ABI strace has):

       m32-mpers      strace can trace and properly decode non-native 32-bit
       no-m32-mpers   strace can trace, but cannot properly decode non-
                      native 32-bit binaries.
       mx32-mpers     strace can trace and properly decode non-native
                      32-on-64-bit binaries.
       no-mx32-mpers  strace can trace, but cannot properly decode non-
                      native 32-on-64-bit binaries.

       If the output contains neither m32-mpers nor no-m32-mpers, then
       decoding of non-native 32-bit binaries is not implemented at all or
       not applicable.

       Likewise, if the output contains neither mx32-mpers nor no-
       mx32-mpers, then decoding of non-native 32-on-64-bit binaries is not
       implemented at all or not applicable.

NOTES         top

       It is a pity that so much tracing clutter is produced by systems
       employing shared libraries.

       It is instructive to think about system call inputs and outputs as
       data-flow across the user/kernel boundary.  Because user-space and
       kernel-space are separate and address-protected, it is sometimes
       possible to make deductive inferences about process behavior using
       inputs and outputs as propositions.

       In some cases, a system call will differ from the documented behavior
       or have a different name.  For example, the faccessat(2) system call
       does not have flags argument, and the setrlimit(2) library function
       uses prlimit64(2) system call on modern (2.6.38+) kernels.  These
       discrepancies are normal but idiosyncratic characteristics of the
       system call interface and are accounted for by C library wrapper

       Some system calls have different names in different architectures and
       personalities.  In these cases, system call filtering and printing
       uses the names that match corresponding __NR_* kernel macros of the
       tracee's architecture and personality.  There are two exceptions from
       this general rule: arm_fadvise64_64(2) ARM syscall and
       xtensa_fadvise64_64(2) Xtensa syscall are filtered and printed as

       On x32, syscalls that are intended to be used by 64-bit processes and
       not x32 ones (for example, readv(2), that has syscall number 19 on
       x86_64, with its x32 counterpart has syscall number 515), but called
       with __X32_SYSCALL_BIT flag being set, are designated with #64

       On some platforms a process that is attached to with the -p option
       may observe a spurious EINTR return from the current system call that
       is not restartable.  (Ideally, all system calls should be restarted
       on strace attach, making the attach invisible to the traced process,
       but a few system calls aren't.  Arguably, every instance of such
       behavior is a kernel bug.)  This may have an unpredictable effect on
       the process if the process takes no action to restart the system

       As strace executes the specified command directly and does not employ
       a shell for that, scripts without shebang that usually run just fine
       when invoked by shell fail to execute with ENOEXEC error.  It is
       advisable to manually supply a shell as a command with the script as
       its argument.

BUGS         top

       Programs that use the setuid bit do not have effective user ID
       privileges while being traced.

       A traced process runs slowly (but check out the --seccomp-bpf

       Traced processes which are descended from command may be left running
       after an interrupt signal (CTRL-C).

HISTORY         top

       The original strace was written by Paul Kranenburg for SunOS and was
       inspired by its trace utility.  The SunOS version of strace was
       ported to Linux and enhanced by Branko Lankester, who also wrote the
       Linux kernel support.  Even though Paul released strace 2.5 in 1992,
       Branko's work was based on Paul's strace 1.5 release from 1991.  In
       1993, Rick Sladkey merged strace 2.5 for SunOS and the second release
       of strace for Linux, added many of the features of truss(1) from
       SVR4, and produced an strace that worked on both platforms.  In 1994
       Rick ported strace to SVR4 and Solaris and wrote the automatic
       configuration support.  In 1995 he ported strace to Irix and tired of
       writing about himself in the third person.

       Beginning with 1996, strace was maintained by Wichert Akkerman.
       During his tenure, strace development migrated to CVS; ports to
       FreeBSD and many architectures on Linux (including ARM, IA-64, MIPS,
       PA-RISC, PowerPC, s390, SPARC) were introduced.  In 2002, the burden
       of strace maintainership was transferred to Roland McGrath.  Since
       then, strace gained support for several new Linux architectures
       (AMD64, s390x, SuperH), bi-architecture support for some of them, and
       received numerous additions and improvements in syscalls decoders on
       Linux; strace development migrated to git during that period.  Since
       2009, strace is actively maintained by Dmitry Levin.  strace gained
       support for AArch64, ARC, AVR32, Blackfin, Meta, Nios II, OpenRISC
       1000, RISC-V, Tile/TileGx, Xtensa architectures since that time.  In
       2012, unmaintained and apparently broken support for non-Linux
       operating systems was removed.  Also, in 2012 strace gained support
       for path tracing and file descriptor path decoding.  In 2014, support
       for stack traces printing was added.  In 2016, syscall fault
       injection was implemented.

       For the additional information, please refer to the NEWS file and
       strace repository commit log.

REPORTING BUGS         top

       Problems with strace should be reported to the strace mailing list

SEE ALSO         top

       strace-log-merge(1), ltrace(1), perf-trace(1), trace-cmd(1), time(1),
       ptrace(2), proc(5)

       strace Home Page ⟨

AUTHORS         top

       The complete list of strace contributors can be found in the CREDITS

COLOPHON         top

       This page is part of the strace (system call tracer) project.
       Information about the project can be found at ⟨⟩.
       If you have a bug report for this manual page, send it to  This page was obtained from the
       project's upstream Git repository
       ⟨⟩ on 2020-11-01.  (At that time,
       the date of the most recent commit that was found in the repository
       was 2020-10-21.)  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-to-
       date source for the page, or you have corrections or improvements to
       the information in this COLOPHON (which is not part of the original
       manual page), send a mail to

strace              2020-09-23                        STRACE(1)

Pages that refer to this page: ltrace(1)strace-log-merge(1)ptrace(2)seccomp(2)proc(5)procfs(5)capabilities(7)vdso(7)