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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 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 getr‐
       limit(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 core.pid, 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  non‐
          writable,  or if a file with the same name exists and is not writable or is not a regu‐
          lar 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  filesys‐
          tem.

       *  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 descrip‐
          tion  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 mad‐
       vise(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 combi‐
       nation of a % followed by any character other than those listed above.  All other  charac‐
       ters 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/ker‐
       nel/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_pat‐
       tern 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 exam‐
          ple, 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 con‐
       trol 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_HEAD‐
       ERS  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 core‐
       dump_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
       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 pre‐
       cisely, 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
       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]=
           argc[1]=<20575>
           argc[2]=
           argc[3]=
           argc[4]=
           Total bytes in core dump: 282624

   Program source

       /* core_pattern_pipe_test.c */

       #define _GNU_SOURCE
       #include 
       #include 
       #include 
       #include 
       #include 
       #include 

       #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);

           fclose(fp);
           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 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2015-12-05                                    CORE(5)

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