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NAME

       core - core dump file

DESCRIPTION

       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
       getrlimit(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
          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 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 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.

       *  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
          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/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
               (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 filename, as is the
       combination of a % followed by any character other than those  listed  above.   All  other
       characters  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/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/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.

   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 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 points:

       *  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 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.
           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_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 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
       coredump_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
       example:

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

       This  file is only provided if the kernel was built with the CONFIG_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
       implementation, 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);

           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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       pages/.