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:
In addition, a core dump may exclude part of the address space of the process if the madvise(2) MADV_DONTDUMP flag was employed.
On systems that employ systemd(1) as the init framework, core dumps may instead be placed in a location determined by systemd(1). See below for further details.
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.
Paths are interpreted according to the settings that are active for the crashing process. That means the crashing process's mount namespace (see mount_namespaces(7)), its current working directory (found via getcwd(2)), and its root directory (see chroot(2)).
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.
Since kernel 5.3.0, the pipe template is split on spaces into an argument list before the template parameters are expanded. In earlier kernels, the template parameters are expanded first and the resulting string is split on spaces into an argument list. This means that in earlier kernels executable names added by the %e and %E template parameters could get split into multiple arguments. So the core dump handler needs to put the executable names as the last argument and ensure it joins all parts of the executable name using spaces. Executable names with multiple spaces in them are not correctly represented in earlier kernels, meaning that the core dump handler needs to use mechanisms to find the executable name.
Instead of being written to a file, the core dump is given as standard input to the program. Note the following points:
Since Linux 2.6.32, the /proc/sys/kernel/core_pipe_limit can be used to defend against this possibility. The value in this file defines how many concurrent crashing processes may be piped to user-space programs in parallel. If this value is exceeded, then those crashing processes above this value are noted in the kernel log and their core dumps are skipped.
A value of 0 in this file is special. It indicates that unlimited processes may be captured in parallel, but that no waiting will take place (i.e., the collecting program is not guaranteed access to /proc/<crashing-PID>). The default value for this file is 0.
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:
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 (vdso(7)) 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.
$ cat /proc/sys/kernel/core_pattern |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %e
In this case, core dumps will be placed in the location configured for systemd-coredump(8), typically as lz4(1) compressed files in the directory /var/lib/systemd/coredump/. One can list the core dumps that have been recorded by systemd-coredump(8) using coredumpctl(1):
$ coredumpctl list | tail -5 Wed 2017-10-11 22:25:30 CEST 2748 1000 1000 3 present /usr/bin/sleep Thu 2017-10-12 06:29:10 CEST 2716 1000 1000 3 present /usr/bin/sleep Thu 2017-10-12 06:30:50 CEST 2767 1000 1000 3 present /usr/bin/sleep Thu 2017-10-12 06:37:40 CEST 2918 1000 1000 3 present /usr/bin/cat Thu 2017-10-12 08:13:07 CEST 2955 1000 1000 3 present /usr/bin/cat
The information shown for each core dump includes the date and time of the dump, the PID, UID, and GID of the dumping process, the signal number that caused the core dump, and the pathname of the executable that was being run by the dumped process. Various options to coredumpctl(1) allow a specified coredump file to be pulled from the systemd(1) location into a specified file. For example, to extract the core dump for PID 2955 shown above to a file named core in the current directory, one could use:
$ coredumpctl dump 2955 -o core
For more extensive details, see the coredumpctl(1) manual page.
To (persistently) disable the systemd(1) mechanism that archives core dumps, restoring to something more like traditional Linux behavior, one can set an override for the systemd(1) mechanism, using something like:
# echo "kernel.core_pattern=core.%p" > \ /etc/sysctl.d/50-coredump.conf # /lib/systemd/systemd-sysctl
It is also possible to temporarily (i.e., until the next reboot) change the core_pattern setting using a command such as the following (which causes the names of core dump files to include the executable name as well as the number of the signal which triggered the core dump):
# sysctl -w kernel.core_pattern="%e-%s.core"
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.)
$ 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 ha\ # 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
#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[])
{
ssize_t nread, tot;
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 (int 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: %zd\n", tot);
fclose(fp);
exit(EXIT_SUCCESS);
}