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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 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, 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

       *  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.	  (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))
	   %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

       *  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.

       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.	 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.

       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);


       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 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2014-03-14			       CORE(5)

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