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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
* 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
* 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
* 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
* The binary being executed by the process does not have read
* 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).)
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
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
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 kernel
2.6.24), delimited by white space (up to a total line length of 128
* 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.
The default value of coredump_filter is 0x3; this reflects traditional
Linux behavior and means that only anonymous memory segments are
Memory-mapped I/O pages such as frame buffer are never dumped, and
virtual DSO pages are always dumped, regardless of the coredump_filter
A child process created via fork(2) inherits its parents
coredump_filter value; the coredump_filter value is preserved across an
It can be useful to set coredump_filter in the parent shell before
running a program, for example:
$ echo 0x7 > /proc/self/coredump_filter
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
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
$ cc -o core_pattern_pipe_test core_pattern_pipe_test.c
# echo '|$PWD/core_pattern_pipe_test %p UID=%u GID=%g sig=%s' > \
$ sleep 100
^\ # type control-backslash
Quit (core dumped)
$ cat core.info
Total bytes in core dump: 282624
/* core_pattern_pipe_test.c */
#define BUF_SIZE 1024
main(int argc, char *argv)
int tot, j;
/* Change our current working directory to that of the
crashing process */
snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv);
/* Write output to file "core.info" in that directory */
fp = fopen("core.info", "w+");
if (fp == NULL)
/* 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);
bash(1), gdb(1), getrlimit(2), mmap(2), prctl(2), sigaction(2), elf(5),
proc(5), pthreads(7), signal(7)
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