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NAME

       core - core dump file

DESCRIPTION

       The  default  action  of  certain signals is to cause a process to terminate 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 produced:

       *  The  process  does  not  have permission to write the core file.  (By default, the core
          file is called core or core.pid, 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 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 csh(1)).

       *  The binary being executed by the process does not have read permission 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/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  executable filename (without path prefix)
           %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.  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 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 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.

   /proc/sys/kernel/core_pipe_limit
       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 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 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 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 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 override for the systemd(1) mechanism,
       using something like:

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

NOTES

       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.)

EXAMPLE

       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
           Password:
           # echo "|$PWD/core_pattern_pipe_test %p UID=%u GID=%g sig=%s" > \
               /proc/sys/kernel/core_pattern
           # exit
           $ sleep 100
           ^\                     # type control-backslash
           Quit (core dumped)
           $ cat core.info
           argc=5
           argc[0]=</home/mtk/core_pattern_pipe_test>
           argc[1]=<20575>
           argc[2]=<UID=1000>
           argc[3]=<GID=100>
           argc[4]=<sig=3>
           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

       int
       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]);
           chdir(cwd);

           /* Write output to file "core.info" in that directory */

           fp = fopen("core.info", "w+");
           if (fp == NULL)
               exit(EXIT_FAILURE);

           /* 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);

           fclose(fp);
           exit(EXIT_SUCCESS);
       }

SEE ALSO

       bash(1), coredumpctl(1), gdb(1), getrlimit(2), mmap(2),  prctl(2),  sigaction(2),  elf(5),
       proc(5), pthreads(7), signal(7), systemd-coredump(8)

COLOPHON

       This  page  is  part of release 4.16 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 https://www.kernel.org/doc/man-pages/.