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CORE(5)                             Linux Programmer's 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 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 getr‐
       limit(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 will fail if the directory in which it is to be created  is  non‐
          writable,  or if a file with the same name exists and is not writable or is not a regu‐
          lar 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  filesys‐

       *  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.  (However, see the descrip‐
          tion   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_COREDUMP option.

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

   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))
           %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
           %u  (numeric) real UID of dumped process

       A single % at the end of the template is dropped from the core filename, as is the  combi‐
       nation  of a % followed by any character other than those listed above.  All other charac‐
       ters 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/ker‐
       nel/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/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_pat‐
       tern  file.   If the first character of this file is a pipe symbol (|), then the remainder
       of the line is interpreted 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

       *  The program must be specified using an absolute pathname (or a pathname 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 exam‐
          ple, 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 con‐
       trol 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.

       By default, the following bits are set: 0, 1, 4 (if the CONFIG_CORE_DUMP_DEFAULT_ELF_HEAD‐
       ERS kernel configuration option is enabled), and 5.  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 core‐
       dump_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

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

       This file is provided only if the kernel was built with the CONFIG_ELF_CORE  configuration

       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 pre‐
       cisely, 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

       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 core.info
           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 "core.info" in that directory */

           fp = fopen("core.info", "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);


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

       This page is part of release 3.74 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                                       2014-08-19                                    CORE(5)

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