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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.  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, 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 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  file  system 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 file system.

       *  The  directory in which the core dump file is to be created does not
          exist.

       *  RLIMIT_CORE or RLIMIT_FSIZE resource limits for a process are set to
          zero (see getrlimit(2)).

       *  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).)

   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
           %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  (00:00h,
               1 Jan 1970, UTC)
           %h  hostname (same as nodename returned by uname(2))
           %e  executable filename (without path prefix)
           %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  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  non-
       zero, 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 non-zero  value,  then  the
       core dump file includes the process ID in a name of the form core.PID.

   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  kernel
          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.

       The default value of coredump_filter is 0x3; this reflects  traditional
       Linux  behavior  and  means  that  only  anonymous  memory segments are
       dumped.

       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 parents  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 only provided 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.

       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  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 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

       The source code of the program is as follows:

       /* 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);

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       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 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-06-15                           CORE(5)