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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 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 sig-
       nals 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 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 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  opera-
          tion, 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 % spec-
       ifiers 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 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 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/kernel/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 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  pro-
       gram.  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 seg-
       ments  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 virtual DSO pages are always dumped, regard-
       less of the coredump_filter value.

       A child process created via fork(2) inherits its parents 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
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-08-26                           CORE(5)