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.

       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
	  /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 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
	  /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).

   /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 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
       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
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       latest	 version    of	  this	  page,	   can	   be	  found	    at
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Linux				  2017-03-13			       CORE(5)