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, 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. (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
%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, 1970-01-01 00:00:00 +0000 (UTC)
%h hostname (same as nodename returned by uname(2))
%e executable filename (without path prefix)
%E pathname of executable, with slashes ('/') replaced by exclamation marks ('!').
%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
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.

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. 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);

           exit(EXIT_SUCCESS);
       }

COLOPHON

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

NAME

DESCRIPTION

NOTES

EXAMPLE

SEE ALSO

COLOPHON