1CORE(5) Linux Programmer's Manual CORE(5)
2
6 core - core dump file
7
9 The default action of certain signals is to cause a process to termi‐
10 nate and produce a core dump file, a disk file containing an image of
11 the process's memory at the time of termination. This image can be
12 used in a debugger (e.g., gdb(1)) to inspect the state of the program
13 at the time that it terminated. A list of the signals which cause a
14 process to dump core can be found in signal(7).
15 A process can set its soft RLIMIT_CORE resource limit to place an upper
17 limit on the size of the core dump file that will be produced if it
18 receives a "core dump" signal; see getrlimit(2) for details.
19 There are various circumstances in which a core dump file is not pro‐
21 duced:
22 * The process does not have permission to write the core file. (By
24 default the core file is called core, and is created in the current
25 working directory. See below for details on naming.) Writing the
26 core file will fail if the directory in which it is to be created is
27 nonwritable, or if a file with the same name exists and is not
28 writable or is not a regular file (e.g., it is a directory or a sym‐
29 bolic link).
30 * A (writable, regular) file with the same name as would be used for
32 the core dump already exists, but there is more than one hard link
33 to that file.
34 * The file system where the core dump file would be created is full;
36 or has run out of inodes; or is mounted read-only; or the user has
37 reached their quota for the file system.
38 * The directory in which the core dump file is to be created does not
40 exist.
41 * The RLIMIT_CORE (core file size) or RLIMIT_FSIZE (file size)
43 resource limits for the process are set to zero; see getrlimit(2)
44 and the documentation of the shell's ulimit command (limit in
45 csh(1)).
46 * The binary being executed by the process does not have read permis‐
48 sion enabled.
49 * The process is executing a set-user-ID (set-group-ID) program that
51 is owned by a user (group) other than the real user (group) ID of
52 the process. (However, see the description of the prctl(2)
53 PR_SET_DUMPABLE operation, and the description of the
54 /proc/sys/fs/suid_dumpable file in proc(5).)
55 * (Since Linux 3.7) The kernel was configured without the CONFIG_CORE‐
57 DUMP option.
58 In addition, a core dump may exclude part of the address space of the
60 process if the madvise(2) MADV_DONTDUMP flag was employed.
61 Naming of core dump files
63 By default, a core dump file is named core, but the /proc/sys/ker‐
64 nel/core_pattern file (since Linux 2.6 and 2.4.21) can be set to define
65 a template that is used to name core dump files. The template can con‐
66 tain % specifiers which are substituted by the following values when a
67 core file is created:
68 %% a single % character
70 %p PID of dumped process
71 %u (numeric) real UID of dumped process
72 %g (numeric) real GID of dumped process
73 %s number of signal causing dump
74 %t time of dump, expressed as seconds since the Epoch, 1970-01-01
75 00:00:00 +0000 (UTC)
76 %h hostname (same as nodename returned by uname(2))
77 %e executable filename (without path prefix)
78 %E pathname of executable, with slashes ('/') replaced by exclama‐
79 tion marks ('!').
80 %c core file size soft resource limit of crashing process (since
81 Linux 2.6.24)
82 A single % at the end of the template is dropped from the core file‐
84 name, as is the combination of a % followed by any character other than
85 those listed above. All other characters in the template become a lit‐
86 eral part of the core filename. The template may include '/' charac‐
87 ters, which are interpreted as delimiters for directory names. The
88 maximum size of the resulting core filename is 128 bytes (64 bytes in
89 kernels before 2.6.19). The default value in this file is "core". For
90 backward compatibility, if /proc/sys/kernel/core_pattern does not
91 include "%p" and /proc/sys/kernel/core_uses_pid (see below) is nonzero,
92 then .PID will be appended to the core filename.
93 Since version 2.4, Linux has also provided a more primitive method of
95 controlling the name of the core dump file. If the /proc/sys/ker‐
96 nel/core_uses_pid file contains the value 0, then a core dump file is
97 simply named core. If this file contains a nonzero value, then the
98 core dump file includes the process ID in a name of the form core.PID.
99 Since Linux 3.6, if /proc/sys/fs/suid_dumpable is set to 2 ("suid‐
101 safe"), the pattern must be either an absolute pathname (starting with
102 a leading '/' character) or a pipe, as defined below.
103 Piping core dumps to a program
105 Since kernel 2.6.19, Linux supports an alternate syntax for the
106 /proc/sys/kernel/core_pattern file. If the first character of this
107 file is a pipe symbol (|), then the remainder of the line is inter‐
108 preted as a program to be executed. Instead of being written to a disk
109 file, the core dump is given as standard input to the program. Note
110 the following points:
111 * The program must be specified using an absolute pathname (or a path‐
113 name relative to the root directory, /), and must immediately follow
114 the '|' character.
115 * The process created to run the program runs as user and group root.
117 * Command-line arguments can be supplied to the program (since Linux
119 2.6.24), delimited by white space (up to a total line length of 128
120 bytes).
121 * The command-line arguments can include any of the % specifiers
123 listed above. For example, to pass the PID of the process that is
124 being dumped, specify %p in an argument.
125 Controlling which mappings are written to the core dump
127 Since kernel 2.6.23, the Linux-specific /proc/PID/coredump_filter file
128 can be used to control which memory segments are written to the core
129 dump file in the event that a core dump is performed for the process
130 with the corresponding process ID.
131 The value in the file is a bit mask of memory mapping types (see
133 mmap(2)). If a bit is set in the mask, then memory mappings of the
134 corresponding type are dumped; otherwise they are not dumped. The bits
135 in this file have the following meanings:
136 bit 0 Dump anonymous private mappings.
138 bit 1 Dump anonymous shared mappings.
139 bit 2 Dump file-backed private mappings.
140 bit 3 Dump file-backed shared mappings.
141 bit 4 (since Linux 2.6.24)
142 Dump ELF headers.
143 bit 5 (since Linux 2.6.28)
144 Dump private huge pages.
145 bit 6 (since Linux 2.6.28)
146 Dump shared huge pages.
147 By default, the following bits are set: 0, 1, 4 (if the CON‐
149 FIG_CORE_DUMP_DEFAULT_ELF_HEADERS kernel configuration option is
150 enabled), and 5. The value of this file is displayed in hexadecimal.
151 (The default value is thus displayed as 33.)
152 Memory-mapped I/O pages such as frame buffer are never dumped, and vir‐
154 tual DSO pages are always dumped, regardless of the coredump_filter
155 value.
156 A child process created via fork(2) inherits its parent's coredump_fil‐
158 ter value; the coredump_filter value is preserved across an execve(2).
159 It can be useful to set coredump_filter in the parent shell before run‐
161 ning a program, for example:
162 $ echo 0x7 > /proc/self/coredump_filter
164 $ ./some_program
165 This file is provided only if the kernel was built with the CON‐
167 FIG_ELF_CORE configuration option.
168
170 The gdb(1) gcore command can be used to obtain a core dump of a running
171 process.
172 In Linux versions up to and including 2.6.27, if a multithreaded
174 process (or, more precisely, a process that shares its memory with
175 another process by being created with the CLONE_VM flag of clone(2))
176 dumps core, then the process ID is always appended to the core file‐
177 name, unless the process ID was already included elsewhere in the file‐
178 name via a %p specification in /proc/sys/kernel/core_pattern. (This is
179 primarily useful when employing the obsolete LinuxThreads implementa‐
180 tion, where each thread of a process has a different PID.)
181
183 The program below can be used to demonstrate the use of the pipe syntax
184 in the /proc/sys/kernel/core_pattern file. The following shell session
185 demonstrates the use of this program (compiled to create an executable
186 named core_pattern_pipe_test):
187 $ cc -o core_pattern_pipe_test core_pattern_pipe_test.c
189 $ su
190 Password:
191 # echo "|$PWD/core_pattern_pipe_test %p UID=%u GID=%g sig=%s" > \
192 /proc/sys/kernel/core_pattern
193 # exit
194 $ sleep 100
195 ^\ # type control-backslash
196 Quit (core dumped)
197 $ cat core.info
198 argc=5
199 argc[0]=</home/mtk/core_pattern_pipe_test>
200 argc[1]=<20575>
201 argc[2]=<UID=1000>
202 argc[3]=<GID=100>
203 argc[4]=<sig=3>
204 Total bytes in core dump: 282624
205 Program source
207 /* core_pattern_pipe_test.c */
209 #define _GNU_SOURCE
211 #include <sys/stat.h>
212 #include <fcntl.h>
213 #include <limits.h>
214 #include <stdio.h>
215 #include <stdlib.h>
216 #include <unistd.h>
217 #define BUF_SIZE 1024
219 int
221 main(int argc, char *argv[])
222 {
223 int tot, j;
224 ssize_t nread;
225 char buf[BUF_SIZE];
226 FILE *fp;
227 char cwd[PATH_MAX];
228 /* Change our current working directory to that of the
230 crashing process */
231 snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv[1]);
233 chdir(cwd);
234 /* Write output to file "core.info" in that directory */
236 fp = fopen("core.info", "w+");
238 if (fp == NULL)
239 exit(EXIT_FAILURE);
240 /* Display command-line arguments given to core_pattern
242 pipe program */
243 fprintf(fp, "argc=%d\n", argc);
245 for (j = 0; j < argc; j++)
246 fprintf(fp, "argc[%d]=<%s>\n", j, argv[j]);
247 /* Count bytes in standard input (the core dump) */
249 tot = 0;
251 while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
252 tot += nread;
253 fprintf(fp, "Total bytes in core dump: %d\n", tot);
254 exit(EXIT_SUCCESS);
256 }
257
259 bash(1), gdb(1), getrlimit(2), mmap(2), prctl(2), sigaction(2), elf(5),
260 proc(5), pthreads(7), signal(7)
261
263 This page is part of release 3.53 of the Linux man-pages project. A
264 description of the project, and information about reporting bugs, can
265 be found at http://www.kernel.org/doc/man-pages/.
266Linux 2013-06-08 CORE(5)