Ubuntu Manpage: core - core dump file

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 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
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 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
(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 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
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 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
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
       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.)

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);

           fclose(fp);
           exit(EXIT_SUCCESS);
       }

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/.

NAME

DESCRIPTION

NOTES

EXAMPLE

SEE ALSO

COLOPHON