CORE(5)                    Linux Programmer's Manual                   CORE(5)
NAME
       core - core dump file

DESCRIPTION
       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-
       duced:

       *  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 will fail 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
          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  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).)

       *  (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.

   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.

       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 a program 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 process created to run the program runs as user and group root.

       *  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 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.

   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
       value.

       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.

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 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.)

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

COLOPHON
       This  page  is  part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-12-05                           CORE(5)