core(5) — Linux manual page


CORE(5)                   Linux Programmer's Manual                  CORE(5)

NAME         top

       core - core dump file

DESCRIPTION         top

       The default action of certain signals is to cause a process to
       terminate and produce a core dump file, a file containing an image of
       the process's memory at the time of termination.  This image can be
       used in a debugger (e.g., gdb(1)) to inspect the state of the program
       at the time that it terminated.  A list of the signals which cause a
       process to dump core can be found in signal(7).

       A process can set its soft RLIMIT_CORE resource limit to place an
       upper limit on the size of the core dump file that will be produced
       if it receives a "core dump" signal; see getrlimit(2) for details.

       There are various circumstances in which a core dump file is not

       *  The process does not have permission to write the core file.  (By
          default, the core file is called core or, where pid is
          the ID of the process that dumped core, and is created in the
          current working directory.  See below for details on naming.)
          Writing the core file fails if the directory in which it is to be
          created is not writable, or if a file with the same name exists
          and is not writable or is not a regular file (e.g., it is a
          directory or a symbolic link).

       *  A (writable, regular) file with the same name as would be used for
          the core dump already exists, but there is more than one hard link
          to that file.

       *  The filesystem where the core dump file would be created is full;
          or has run out of inodes; or is mounted read-only; or the user has
          reached their quota for the filesystem.

       *  The directory in which the core dump file is to be created does
          not exist.

       *  The RLIMIT_CORE (core file size) or RLIMIT_FSIZE (file size)
          resource limits for the process are set to zero; see getrlimit(2)
          and the documentation of the shell's ulimit command (limit in

       *  The binary being executed by the process does not have read
          permission enabled.  (This is a security measure to ensure that an
          executable whose contents are not readable does not produce a—
          possibly readable—core dump containing an image of the

       *  The process is executing a set-user-ID (set-group-ID) program that
          is owned by a user (group) other than the real user (group) ID of
          the process, or the process is executing a program that has file
          capabilities (see capabilities(7)).  (However, see the description
          of the prctl(2) PR_SET_DUMPABLE operation, and the description of
          the /proc/sys/fs/suid_dumpable file in proc(5).)

       *  /proc/sys/kernel/core_pattern is empty and
          /proc/sys/kernel/core_uses_pid contains the value 0.  (These files
          are described below.)  Note that if /proc/sys/kernel/core_pattern
          is empty and /proc/sys/kernel/core_uses_pid contains the value 1,
          core dump files will have names of the form .pid, and such files
          are hidden unless one uses the ls(1) -a option.

       *  (Since Linux 3.7) The kernel was configured without the
          CONFIG_COREDUMP option.

       In addition, a core dump may exclude part of the address space of the
       process if the madvise(2) MADV_DONTDUMP flag was employed.

       On systems that employ systemd(1) as the init framework, core dumps
       may instead be placed in a location determined by systemd(1).  See
       below for further details.

   Naming of core dump files
       By default, a core dump file is named core, but the
       /proc/sys/kernel/core_pattern file (since Linux 2.6 and 2.4.21) can
       be set to define a template that is used to name core dump files.
       The template can contain % specifiers which are substituted by the
       following values when a core file is created:

           %%  A single % character.
           %c  Core file size soft resource limit of crashing process (since
               Linux 2.6.24).
           %d  Dump mode—same as value returned by prctl(2) PR_GET_DUMPABLE
               (since Linux 3.7).
           %e  The process or thread's comm value, which typically is the
               same as the executable filename (without path prefix, and
               truncated to a maximum of 15 characters), but may have been
               modified to be something different; see the discussion of
               /proc/[pid]/comm and /proc/[pid]/task/[tid]/comm in proc(5).
           %E  Pathname of executable, with slashes ('/') replaced by
               exclamation marks ('!') (since Linux 3.0).
           %g  Numeric real GID of dumped process.
           %h  Hostname (same as nodename returned by uname(2)).
           %i  TID of thread that triggered core dump, as seen in the PID
               namespace in which the thread resides (since Linux 3.18).
           %I  TID of thread that triggered core dump, as seen in the
               initial PID namespace (since Linux 3.18).
           %p  PID of dumped process, as seen in the PID namespace in which
               the process resides.
           %P  PID of dumped process, as seen in the initial PID namespace
               (since Linux 3.12).
           %s  Number of signal causing dump.
           %t  Time of dump, expressed as seconds since the Epoch,
               1970-01-01 00:00:00 +0000 (UTC).
           %u  Numeric real UID of dumped process.

       A single % at the end of the template is dropped from the core
       filename, as is the combination of a % followed by any character
       other than those listed above.  All other characters in the template
       become a literal part of the core filename.  The template may include
       '/' characters, which are interpreted as delimiters for directory
       names.  The maximum size of the resulting core filename is 128 bytes
       (64 bytes in kernels before 2.6.19).  The default value in this file
       is "core".  For backward compatibility, if
       /proc/sys/kernel/core_pattern does not include %p and
       /proc/sys/kernel/core_uses_pid (see below) is nonzero, then .PID will
       be appended to the core filename.

       Paths are interpreted according to the settings that are active for
       the crashing process.  That means the crashing process's mount
       namespace (see mount_namespaces(7)), its current working directory
       (found via getcwd(2)), and its root directory (see chroot(2)).

       Since version 2.4, Linux has also provided a more primitive method of
       controlling the name of the core dump file.  If the
       /proc/sys/kernel/core_uses_pid file contains the value 0, then a core
       dump file is simply named core.  If this file contains a nonzero
       value, then the core dump file includes the process ID in a name of
       the form core.PID.

       Since Linux 3.6, if /proc/sys/fs/suid_dumpable is set to 2
       ("suidsafe"), the pattern must be either an absolute pathname
       (starting with a leading '/' character) or a pipe, as defined below.

   Piping core dumps to a program
       Since kernel 2.6.19, Linux supports an alternate syntax for the
       /proc/sys/kernel/core_pattern file.  If the first character of this
       file is a pipe symbol (|), then the remainder of the line is
       interpreted as the command-line for a user-space program (or script)
       that is to be executed.

       Since kernel 5.3.0, the pipe template is split on spaces into an
       argument list before the template parameters are expanded.  In
       earlier kernels, the template parameters are expanded first and the
       resulting string is split on spaces into an argument list.  This
       means that in earlier kernels executable names added by the %e and %E
       template parameters could get split into multiple arguments.  So the
       core dump handler needs to put the executable names as the last
       argument and ensure it joins all parts of the executable name using
       spaces.  Executable names with multiple spaces in them are not
       correctly represented in earlier kernels, meaning that the core dump
       handler needs to use mechanisms to find the executable name.

       Instead of being written to a file, the core dump is given as
       standard input to the program.  Note the following points:

       *  The program must be specified using an absolute pathname (or a
          pathname relative to the root directory, /), and must immediately
          follow the '|' character.

       *  The command-line arguments can include any of the % specifiers
          listed above.  For example, to pass the PID of the process that is
          being dumped, specify %p in an argument.

       *  The process created to run the program runs as user and group

       *  Running as root does not confer any exceptional security bypasses.
          Namely, LSMs (e.g., SELinux) are still active and may prevent the
          handler from accessing details about the crashed process via

       *  The program pathname is interpreted with respect to the initial
          mount namespace as it is always executed there.  It is not
          affected by the settings (e.g., root directory, mount namespace,
          current working directory) of the crashing process.

       *  The process runs in the initial namespaces (PID, mount, user, and
          so on) and not in the namespaces of the crashing process.  One can
          utilize specifiers such as %P to find the right /proc/[pid]
          directory and probe/enter the crashing process's namespaces if

       *  The process starts with its current working directory as the root
          directory.  If desired, it is possible change to the working
          directory of the dumping process by employing the value provided
          by the %P specifier to change to the location of the dumping
          process via /proc/[pid]/cwd.

       *  Command-line arguments can be supplied to the program (since Linux
          2.6.24), delimited by white space (up to a total line length of
          128 bytes).

       *  The RLIMIT_CORE limit is not enforced for core dumps that are
          piped to a program via this mechanism.

       When collecting core dumps via a pipe to a user-space program, it can
       be useful for the collecting program to gather data about the
       crashing process from that process's /proc/[pid] directory.  In order
       to do this safely, the kernel must wait for the program collecting
       the core dump to exit, so as not to remove the crashing process's
       /proc/[pid] files prematurely.  This in turn creates the possibility
       that a misbehaving collecting program can block the reaping of a
       crashed process by simply never exiting.

       Since Linux 2.6.32, the /proc/sys/kernel/core_pipe_limit can be used
       to defend against this possibility.  The value in this file defines
       how many concurrent crashing processes may be piped to user-space
       programs in parallel.  If this value is exceeded, then those crashing
       processes above this value are noted in the kernel log and their core
       dumps are skipped.

       A value of 0 in this file is special.  It indicates that unlimited
       processes may be captured in parallel, but that no waiting will take
       place (i.e., the collecting program is not guaranteed access to
       /proc/<crashing-PID>).  The default value for this file is 0.

   Controlling which mappings are written to the core dump
       Since kernel 2.6.23, the Linux-specific /proc/[pid]/coredump_filter
       file can be used to control which memory segments are written to the
       core dump file in the event that a core dump is performed for the
       process with the corresponding process ID.

       The value in the file is a bit mask of memory mapping types (see
       mmap(2)).  If a bit is set in the mask, then memory mappings of the
       corresponding type are dumped; otherwise they are not dumped.  The
       bits in this file have the following meanings:

           bit 0  Dump anonymous private mappings.
           bit 1  Dump anonymous shared mappings.
           bit 2  Dump file-backed private mappings.
           bit 3  Dump file-backed shared mappings.
           bit 4 (since Linux 2.6.24)
                  Dump ELF headers.
           bit 5 (since Linux 2.6.28)
                  Dump private huge pages.
           bit 6 (since Linux 2.6.28)
                  Dump shared huge pages.
           bit 7 (since Linux 4.4)
                  Dump private DAX pages.
           bit 8 (since Linux 4.4)
                  Dump shared DAX pages.

       By default, the following bits are set: 0, 1, 4 (if the
       CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS kernel configuration option is
       enabled), and 5.  This default can be modified at boot time using the
       coredump_filter boot option.

       The value of this file is displayed in hexadecimal.  (The default
       value is thus displayed as 33.)

       Memory-mapped I/O pages such as frame buffer are never dumped, and
       virtual DSO (vdso(7)) pages are always dumped, regardless of the
       coredump_filter value.

       A child process created via fork(2) inherits its parent's
       coredump_filter value; the coredump_filter value is preserved across
       an execve(2).

       It can be useful to set coredump_filter in the parent shell before
       running a program, for example:

           $ echo 0x7 > /proc/self/coredump_filter
           $ ./some_program

       This file is provided only if the kernel was built with the CON‐
       FIG_ELF_CORE configuration option.

   Core dumps and systemd
       On systems using the systemd(1) init framework, core dumps may be
       placed in a location determined by systemd(1).  To do this,
       systemd(1) employs the core_pattern feature that allows piping core
       dumps to a program.  One can verify this by checking whether core
       dumps are being piped to the systemd-coredump(8) program:

           $ cat /proc/sys/kernel/core_pattern
           |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %e

       In this case, core dumps will be placed in the location configured
       for systemd-coredump(8), typically as lz4(1) compressed files in the
       directory /var/lib/systemd/coredump/.  One can list the core dumps
       that have been recorded by systemd-coredump(8) using coredumpctl(1):

       $ coredumpctl list | tail -5
       Wed 2017-10-11 22:25:30 CEST  2748 1000 1000 3 present  /usr/bin/sleep
       Thu 2017-10-12 06:29:10 CEST  2716 1000 1000 3 present  /usr/bin/sleep
       Thu 2017-10-12 06:30:50 CEST  2767 1000 1000 3 present  /usr/bin/sleep
       Thu 2017-10-12 06:37:40 CEST  2918 1000 1000 3 present  /usr/bin/cat
       Thu 2017-10-12 08:13:07 CEST  2955 1000 1000 3 present  /usr/bin/cat

       The information shown for each core dump includes the date and time
       of the dump, the PID, UID, and GID  of the dumping process, the sig‐
       nal number that caused the core dump, and the pathname of the exe‐
       cutable that was being run by the dumped process.  Various options to
       coredumpctl(1) allow a specified coredump file to be pulled from the
       systemd(1) location into a specified file.  For example, to extract
       the core dump for PID 2955 shown above to a file named core in the
       current directory, one could use:

           $ coredumpctl dump 2955 -o core

       For more extensive details, see the coredumpctl(1) manual page.

       To (persistently) disable the systemd(1) mechanism that archives core
       dumps, restoring to something more like traditional Linux behavior,
       one can set an override for the systemd(1) mechanism, using something

           # echo "kernel.core_pattern=core.%p" > \
           # /lib/systemd/systemd-sysctl

       It is also possible to temporarily (i.e., until the next reboot)
       change the core_pattern setting using a command such as the following
       (which causes the names of core dump files to include the executable
       name as well as the number of the signal which triggered the core

           # sysctl -w kernel.core_pattern="%e-%s.core"

NOTES         top

       The gdb(1) gcore command can be used to obtain a core dump of a
       running process.

       In Linux versions up to and including 2.6.27, if a multithreaded
       process (or, more precisely, a process that shares its memory with
       another process by being created with the CLONE_VM flag of clone(2))
       dumps core, then the process ID is always appended to the core
       filename, unless the process ID was already included elsewhere in the
       filename via a %p specification in /proc/sys/kernel/core_pattern.
       (This is primarily useful when employing the obsolete LinuxThreads
       implementation, where each thread of a process has a different PID.)

EXAMPLES         top

       The program below can be used to demonstrate the use of the pipe
       syntax in the /proc/sys/kernel/core_pattern file.  The following
       shell session demonstrates the use of this program (compiled to
       create an executable named core_pattern_pipe_test):

           $ cc -o core_pattern_pipe_test core_pattern_pipe_test.c
           $ su
           # echo "|$PWD/core_pattern_pipe_test %p UID=%u GID=%g sig=%s" > \
           # exit
           $ sleep 100
           ^\                     # type control-backslash
           Quit (core dumped)
           $ cat
           Total bytes in core dump: 282624

   Program source

       /* core_pattern_pipe_test.c */

       #define _GNU_SOURCE
       #include <sys/stat.h>
       #include <fcntl.h>
       #include <limits.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define BUF_SIZE 1024

       main(int argc, char *argv[])
           ssize_t nread, tot;
           char buf[BUF_SIZE];
           FILE *fp;
           char cwd[PATH_MAX];

           /* Change our current working directory to that of the
              crashing process */

           snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv[1]);

           /* Write output to file "" in that directory */

           fp = fopen("", "w+");
           if (fp == NULL)

           /* Display command-line arguments given to core_pattern
              pipe program */

           fprintf(fp, "argc=%d\n", argc);
           for (int j = 0; j < argc; j++)
               fprintf(fp, "argc[%d]=<%s>\n", j, argv[j]);

           /* Count bytes in standard input (the core dump) */

           tot = 0;
           while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
               tot += nread;
           fprintf(fp, "Total bytes in core dump: %zd\n", tot);


SEE ALSO         top

       bash(1), coredumpctl(1), gdb(1), getrlimit(2), mmap(2), prctl(2),
       sigaction(2), elf(5), proc(5), pthreads(7), signal(7),

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-11-01                          CORE(5)

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