NAME | DESCRIPTION | NOTES | EXAMPLE | SEE ALSO | COLOPHON
CORE(5) Linux Programmer's Manual CORE(5)
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, 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 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 file system 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 file system.
* 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).)
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 (00:00h, 1 Jan
1970, UTC)
%h hostname (same as nodename returned by uname(2))
%e executable filename (without path prefix)
%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 non-zero, 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 non-zero value, then the
core dump file includes the process ID in a name of the form core.PID.
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 kernel
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.
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.
The default value of coredump_filter is 0x3; this reflects traditional Linux
behavior and means that only anonymous memory segments are dumped.
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 parents 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 only provided 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
process.
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 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
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
/* 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);
}
bash(1), gdb(1), getrlimit(2), mmap(2), prctl(2), sigaction(2), elf(5),
proc(5), pthreads(7), signal(7)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-08-26 CORE(5)