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Linux 2.6 - man page for core (linux section 5)

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, 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 cre-
	  ated 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 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
	   %p  PID of dumped process
	   %u  (numeric) real UID of dumped process
	   %g  (numeric) real GID of dumped process
	   %s  number of signal causing dump
	   %t  time  of  dump,	expressed  as  seconds since the Epoch, 1970-01-01 00:00:00 +0000
	   %h  hostname (same as nodename returned by uname(2))
	   %e  executable filename (without path prefix)
	   %E  pathname of executable, with slashes ('/') replaced by exclamation marks ('!').
	   %c  core file size soft resource limit of crashing process (since Linux 2.6.24)

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

Linux					    2013-06-08					  CORE(5)

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