NAME | DESCRIPTION | NOTES | EXAMPLE | SEE ALSO | COLOPHON

CORE(5)                   Linux Programmer's Manual                  CORE(5)

NAME         top

       core - core dump file

DESCRIPTION         top

       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 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
          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).)

       *  (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 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/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
           %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
               exclamation 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
       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.

       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_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 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 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 provided only if the kernel was built with the
       CONFIG_ELF_CORE configuration option.

NOTES         top

       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
       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 PID.)

EXAMPLE         top

       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         top

       bash(1), gdb(1), getrlimit(2), mmap(2), prctl(2), sigaction(2),
       elf(5), proc(5), pthreads(7), signal(7)

COLOPHON         top

       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 http://www.kernel.org/doc/man-pages/.

Linux                            2014-03-14                          CORE(5)