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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 produced:

       *  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

       *  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, 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
           %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

   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 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

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

       This  file is provided only if the kernel was built with the CONFIG_ELF_CORE configuration


       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


       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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