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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. 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, 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 cre-
ated 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 sys-
tem.
* The directory in which the core dump file is to be created does not exist.
* RLIMIT_CORE or RLIMIT_FSIZE resource limits for a process are set to zero (see getr-
limit(2)).
* 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 descrip-
tion of the prctl(2) PR_SET_DUMPABLE operation, and the description of the
/proc/sys/fs/suid_dumpable file in proc(5).)
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 (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 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/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.
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 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 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.
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 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 only provided 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.
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_pat-
tern. (This is primarily useful when employing the 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
The source code of the program is as follows:
/* 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);
}
SEE ALSO
gdb(1), getrlimit(2), mmap(2), prctl(2), sigaction(2), elf(5), proc(5), pthreads(7), sig-
nal(7)
COLOPHON
This page is part of release 3.05 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2008-06-15 CORE(5)
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