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

       core - core dump file

       The  default  action of certain signals is to cause a process to termi-
       nate and produce a core dump file, a disk 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  pro-

       *  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
          nonwritable, 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 sym-
          bolic 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

       *  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  permis-
          sion enabled.

       *  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/ker-
          nel/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_CORE-
          DUMP 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/ker-
       nel/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 con-
       tain % 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  executable filename (without path prefix)
           %E  pathname of executable, with slashes ('/') replaced by exclama-
               tion 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 file-
       name, as is the combination of a % followed by any character other than
       those listed above.  All other characters in the template become a lit-
       eral part of the core filename.  The template may include  '/'  charac-
       ters,  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/ker-
       nel/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 ("suid-
       safe"), 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 inter-
       preted as the command-line for a user-space program (or script) that is
       to be executed.  Instead of being written to a disk 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 path-
          name 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 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

       *  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 uti-
          lize 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  direc-
          tory  of  the dumping process by employing the value provided by the
          %P specifier to change to the location of the  dumping  process  via

       *  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

       *  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

       A value of 0 in this file is special.  It indicates that unlimited pro-
       cesses may be captured in parallel, but that no waiting will take place
       (i.e., the collecting program is not guaranteed access to /proc/<crash-
       ing-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 CON-
       FIG_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 vir-
       tual DSO pages are always dumped,  regardless  of  the  coredump_filter

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

       It can be useful to set coredump_filter in the parent shell before run-
       ning 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 direc-
       tory /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  signal
       number  that  caused  the core dump, and the pathname of the executable
       that was being run by the dumped process.   Various  options  to  core-
       dumpctl(1)  allow  a specified coredump file to be pulled from the sys-
       temd(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 disable the systemd(1) mechanism that archives core dumps, restoring
       to something more like traditional Linux behavior, one can set an over-
       ride for the systemd(1) mechanism, using something like:

         # echo "kernel.core_pattern=core.%p" > /etc/sysctl.d/50-coredump.conf
         # /lib/systemd/systemd-sysctl

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

       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  file-
       name, unless the process ID was already included elsewhere in the file-
       name via a %p specification in /proc/sys/kernel/core_pattern.  (This is
       primarily  useful  when employing the obsolete LinuxThreads implementa-
       tion, 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 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[])
           int tot, j;
           ssize_t nread;
           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 (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: %d\n", tot);


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Linux                             2017-09-15                           CORE(5)
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