NAME | SYNOPSIS | DESCRIPTION | OPTIONS | DIAGNOSTICS | SETUID INSTALLATION | NOTES | BUGS | HISTORY | REPORTING BUGS | SEE ALSO | COLOPHON

STRACE(1)                  General Commands Manual                 STRACE(1)

NAME         top

       strace - trace system calls and signals

SYNOPSIS         top

       strace [-CdffhikqrtttTvVxxy] [-In] [-bexecve] [-eexpr]...  [-acolumn]
       [-ofile] [-sstrsize] [-Ppath]... -ppid... / [-D] [-Evar[=val]]...
       [-uusername] command [args]

       strace -c[df] [-In] [-bexecve] [-eexpr]...  [-Ooverhead] [-Ssortby]
       -ppid... / [-D] [-Evar[=val]]... [-uusername] 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
       option.

       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
       completes.

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

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

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

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

       Structure pointers are dereferenced and the members are displayed as
       appropriate.  In all 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(1, 3), ...}) = 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 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.

       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 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" 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

   Output format
       -a column   Align return values in a specific column (default column
                   40).

       -i          Print the instruction pointer at the time of the system
                   call.

       -k          Print the execution stack trace of the traced processes
                   after each system call (experimental).  This option is
                   available only if strace is built with libunwind.

       -o filename Write the trace output to the file filename rather than
                   to stderr.  Use filename.pid if -ff is used.  If the
                   argument begins with '|' or with '!' then 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.

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

       -qq         If given twice, suppress messages about process exit
                   status.

       -r          Print a relative timestamp upon entry to each system
                   call.  This records the time difference between the
                   beginning of successive system calls.

       -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.

       -t          Prefix each line of the trace with the time of day.

       -tt         If given twice, the time printed will include the
                   microseconds.

       -ttt        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.

       -T          Show the time spent in system calls.  This records the
                   time difference between the beginning and the end of each
                   system call.

       -x          Print all non-ASCII strings in hexadecimal string format.

       -xx         Print all strings in hexadecimal string format.

       -y          Print paths associated with file descriptor arguments.

       -yy         Print protocol specific information associated with
                   socket file descriptors.

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

       -C          Like -c but also print regular output while processes are
                   running.

       -O overhead Set the overhead for tracing system calls to overhead
                   microseconds.  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 -c.

       -S sortby   Sort the output of the histogram printed by the -c option
                   by the specified criterion.  Legal values are time,
                   calls, name, and nothing (default is time).

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

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

                             [qualifier=][!][?]value1[,[?]value2]...

                   where qualifier is one of trace, abbrev, verbose, raw,
                   signal, read, write, fault, or inject 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.  Question mark before the
                   syscall qualification allows suppression of error in case
                   no syscalls matched the qualification provided.  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 trace=set
                   Trace only the specified set of system calls.  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 trace=/regex
                   Trace only those system calls that match the regex.  You
                   can use POSIX Extended Regular Expression syntax (see
                   regex(7)).

       -e trace=%file

       -e trace=file (deprecated)
              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 in the list.  Betchya woulda forgot
              that one.

       -e trace=%process

       -e trace=process (deprecated)
              Trace all system calls which involve process management.  This
              is useful for watching the fork, wait, and exec steps of a
              process.

       -e trace=%network

       -e trace=network (deprecated)
              Trace all the network related system calls.

       -e trace=%signal

       -e trace=signal (deprecated)
              Trace all signal related system calls.

       -e trace=%ipc

       -e trace=ipc (deprecated)
              Trace all IPC related system calls.

       -e trace=%desc

       -e trace=desc (deprecated)
              Trace all file descriptor related system calls.

       -e trace=%memory

       -e trace=memory (deprecated)
              Trace all memory mapping related system calls.

       -e trace=%stat
              Trace stat syscall variants.

       -e trace=%lstat
              Trace lstat syscall variants.

       -e trace=%fstat
              Trace fstat and fstatat syscall variants.

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

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

       -e trace=%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.

       -e trace=%%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 expression.

       -e abbrev=set
              Abbreviate the output from printing each member of large
              structures.  The default is abbrev=all.  The -v option has the
              effect of abbrev=none.

       -e verbose=set
              Dereference structures for the specified set of system calls.
              The default is verbose=all.

       -e raw=set
              Print raw, undecoded arguments for the specified set of system
              calls.  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.

       -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 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
              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 inject=set[:error=errno|:retval=value][:signal=sig][:when=expr]
              Perform syscall tampering for the specified set of syscalls.

              At least one of error, retval, or signal 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, 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 :signal=sig option is specified without :error=errno or
              :retval=value options, then only a signal sig is delivered
              without a syscall fault injection.  Conversely, :error=errno
              or :retval=value option without :signal=sig option injects a
              fault without delivering a signal.

              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.

              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 one of the following:

                first
                  For every syscall from the set, perform an injection for
                  the syscall invocation number first only.

                first+
                  For every syscall from the set, perform injections for the
                  syscall invocation number first and all subsequent
                  invocations.

                first+step
                  For every syscall from the set, perform injections for
                  syscall invocations number first, first+step,
                  first+step+step, and so on.

              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.

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

              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=set[:error=errno][:when=expr]
              Perform syscall fault injection for the specified set of
              syscalls.

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

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

       -v     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.

   Tracing
       -b syscall  If specified syscall is reached, detach from traced
                   process.  Currently, only execve 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.

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

       -f          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.

       -ff         If the -o filename option is in effect, each processes
                   trace is written to filename.pid where pid is the numeric
                   process id of each process.  This is incompatible with
                   -c, since no per-process counts are kept.

       -I interruptible
                   When strace can be interrupted by signals (such as
                   pressing ^C).  1: no signals are blocked; 2: fatal
                   signals are blocked while decoding syscall (default); 3:
                   fatal signals are always blocked (default if '-o FILE
                   PROG'); 4: fatal signals and SIGTSTP (^Z) are always
                   blocked (useful to make strace -o FILE PROG not stop on
                   ^Z).

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

       -E 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.

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

       -F          This option is now obsolete and it has the same
                   functionality as -f.

       -h          Print the help summary.

       -V          Print the version number of strace.

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() value, etc)
       between traced process and its parent are not preserved unless -D is
       used.

       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.

SETUID INSTALLATION         top

       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 non-setuid version of strace for ordinary
       lusers to use.

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, on System V-derived systems
       the true time(2) system call does not take an argument and the stat
       function is called xstat and takes an extra leading argument.  These
       discrepancies are normal but idiosyncratic characteristics of the
       system call interface and are accounted for by C library wrapper
       functions.

       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
       fadvise64_64(2).

       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
       call.

BUGS         top

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

       A traced process runs slowly.

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

       The -i option is weakly supported.

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.

REPORTING BUGS         top

       Problems with strace should be reported to the strace mailing list at
       <strace-devel@lists.sourceforge.net>.

SEE ALSO         top

       ltrace(1), time(1), ptrace(2), proc(5)

COLOPHON         top

       This page is part of the strace (system call tracer) project.
       Information about the project can be found at 
       ⟨http://sourceforge.net/projects/strace/⟩.  If you have a bug report
       for this manual page, send it to strace-devel@lists.sourceforge.net.
       This page was obtained from the project's upstream Git repository 
       ⟨git://git.code.sf.net/p/strace/code⟩ on 2017-05-03.  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 man-pages@man7.org

                                 2010-03-30                        STRACE(1)

Pages that refer to this page: ltrace(1)ptrace(2)proc(5)vdso(7)