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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
          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    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  (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  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.

       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, regardless 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

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