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       core - core dump file


       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

       *  The  process  does  not have permission to write the core file.  (By
          default, the core file is called core or, where pid is  the
          ID  of  the  process that dumped 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

       *  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

       *  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,  or  the  process is executing a program that has file
          capabilities (see capabilities(7)).  (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))
           %i  TID  of  thread  that  triggered  core dump, as seen in the PID
               namespace in which the thread resides (since Linux 3.18)
           %I  TID of thread that triggered core dump, as seen in the  initial
               PID namespace (since Linux 3.18)
           %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

       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

       *  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.
           bit 7 (since Linux 4.4)
                  Dump private DAX pages.
           bit 8 (since Linux 4.4)
                  Dump shared DAX 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.  This default can be modified at boot time using the
       coredump_filter boot option.

       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

       A   child   process   created   via   fork(2)   inherits  its  parent's
       coredump_filter value; the coredump_filter value is preserved across an

       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.


       The gdb(1) gcore command can be used to obtain a core dump of a running

       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.)


       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
           # echo "|$PWD/core_pattern_pipe_test %p UID=%u GID=%g sig=%s" > \
           # exit
           $ sleep 100
           ^\                     # type control-backslash
           Quit (core dumped)
           $ cat
           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

       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]);

           /* Write output to file "" in that directory */

           fp = fopen("", "w+");
           if (fp == NULL)

           /* 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);



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


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