core(5)                                                 File Formats Manual                                                 core(5)

       core - core dump file

       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 produced:

       •  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  de‐
          tails  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  getr‐
          limit(2)  and the documentation of the shell's ulimit command (limit in csh(1)).  However, RLIMIT_CORE will be ignored if
          the system is configured to pipe core dumps to a program.

       •  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 executable.)

       •  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)).  (How‐
          ever,   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 de‐
          scribed 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 em‐

       On systems that employ systemd(1) as the init framework, core dumps may instead be placed in a location determined  by  sys‐
       temd(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 char‐
       acter 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 re‐
       sulting core filename is 128 bytes (64 bytes before Linux 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 non‐
       zero, 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

       Since Linux 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  Linux  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  pro‐
       gram (or script) that is to be executed.

       Since  Linux  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  im‐
          mediately 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 root.

       •  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 /proc/pid.

       •  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 needed.

       •  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  ex‐
       ceeded, 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 wait‐
       ing 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 Linux 2.6.23, the Linux-specific /proc/pid/coredump_filter file can be used to control which memory segments are writ‐
       ten 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  map‐
       pings  of  the corresponding type are dumped; otherwise they are not dumped.  The bits in this file have the following mean‐

           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 CONFIG_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 check‐
       ing 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-core‐
       dump(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 signal number that caused the core dump, and the pathname of the executable 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  direc‐
       tory, 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 like:

           # 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 dump):

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

       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 mem‐
       ory  with another process by being created with the CLONE_VM flag of clone(2)) dumps core, then the process ID is always ap‐
       pended 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.)

       The program below can be used to demonstrate the use of the pipe syntax in the /proc/sys/kernel/core_pattern file.  The fol‐
       lowing 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 (size_t j = 0; j < argc; j++)
               fprintf(fp, "argc[%zu]=<%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);


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

Linux man-pages 6.03                                         2023-02-05                                                     core(5)