man5/core.5

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  25 .TH CORE 5 2014-01-24 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 core \- core dump file
  28 .SH DESCRIPTION
  29 The default action of certain signals is to cause a process to terminate
  30 and produce a
  31 .IR "core dump file" ,
  32 a disk file containing an image of the process's memory at
  33 the time of termination.
  34 This image can be used in a debugger (e.g.,
  35 .BR gdb (1))
  36 to inspect the state of the program at the time that it terminated.
  37 A list of the signals which cause a process to dump core can be found in
  38 .BR signal (7).
  39
  40 A process can set its soft
  41 .B RLIMIT_CORE
  42 resource limit to place an upper limit on the size of the core dump file
  43 that will be produced if it receives a "core dump" signal; see
  44 .BR getrlimit (2)
  45 for details.
  46
  47 There are various circumstances in which a core dump file is
  48 not produced:
  49 .IP * 3
  50 The process does not have permission to write the core file.
  51 (By default the core file is called
  52 .IR core ,
  53 and is created in the current working directory.
  54 See below for details on naming.)
  55 Writing the core file will fail if the directory in which
  56 it is to be created is nonwritable,
  57 or if a file with the same name exists and
  58 is not writable
  59 or is not a regular file
  60 (e.g., it is a directory or a symbolic link).
  61 .IP *
  62 A (writable, regular) file with the same name as would be used for the
  63 core dump already exists, but there is more than one hard link to that
  64 file.
  65 .IP *
  66 The filesystem where the core dump file would be created is full;
  67 or has run out of inodes; or is mounted read-only;
  68 or the user has reached their quota for the filesystem.
  69 .IP *
  70 The directory in which the core dump file is to be created does
  71 not exist.
  72 .IP *
  73 The
  74 .B RLIMIT_CORE
  75 (core file size) or
  76 .B RLIMIT_FSIZE
  77 (file size) resource limits for the process are set to zero; see
  78 .BR getrlimit (2)
  79 and the documentation of the shell's
  80 .I ulimit
  81 command
  82 .RI ( limit
  83 in
  84 .BR csh (1)).
  85 .IP *
  86 The binary being executed by the process does not have read
  87 permission enabled.
  88 .IP *
  89 The process is executing a set-user-ID (set-group-ID) program
  90 that is owned by a user (group) other than the real user (group)
  91 ID of the process.
  92 (However, see the description of the
  93 .BR prctl (2)
  94 .B PR_SET_DUMPABLE
  95 operation, and the description of the
  96 .I /proc/sys/fs/suid_dumpable
  97 .\" FIXME . Perhaps relocate discussion of /proc/sys/fs/suid_dumpable
  98 .\" and PR_SET_DUMPABLE to this page?
  99 file in
 100 .BR proc (5).)
 101 .IP *
 102 (Since Linux 3.7)
 103 .\" commit 046d662f481830e652ac34cd112249adde16452a
 104 The kernel was configured without the
 105 .BR CONFIG_COREDUMP
 106 option.
 107 .PP
 108 In addition,
 109 a core dump may exclude part of the address space of the process if the
 110 .BR madvise (2)
 111 .B MADV_DONTDUMP
 112 flag was employed.
 113 .SS Naming of core dump files
 114 By default, a core dump file is named
 115 .IR core ,
 116 but the
 117 .I /proc/sys/kernel/core_pattern
 118 file (since Linux 2.6 and 2.4.21)
 119 can be set to define a template that is used to name core dump files.
 120 The template can contain % specifiers which are substituted
 121 by the following values when a core file is created:
 122 .PP
 123 .RS 4
 124 .PD 0
 125 .TP 4
 126 %%
 127 a single % character
 128 .TP
 129 %p
 130 PID of dumped process (as seen in the current namespace)
 131 .TP
 132 .\" Added in git commit 65aafb1e7484b7434a0c1d4c593191ebe5776a2f
 133 %P
 134 global PID of dumped process (as seen in init's namespace)
 135 (since Linux 3.12)
 136 .TP
 137 %u
 138 (numeric) real UID of dumped process
 139 .TP
 140 %g
 141 (numeric) real GID of dumped process
 142 .TP
 143 %s
 144 number of signal causing dump
 145 .TP
 146 %t
 147 time of dump, expressed as seconds since the
 148 Epoch, 1970-01-01 00:00:00 +0000 (UTC)
 149 .TP
 150 %h
 151 hostname (same as \fInodename\fP returned by \fBuname\fP(2))
 152 .TP
 153 %e
 154 executable filename (without path prefix)
 155 .TP
 156 %E
 157 pathname of executable,
 158 with slashes (\(aq/\(aq) replaced by exclamation marks (\(aq!\(aq)
 159 (since Linux 3.0).
 160 .TP
 161 %c
 162 core file size soft resource limit of crashing process (since Linux 2.6.24)
 163 .PD
 164 .RE
 165 .PP
 166 A single % at the end of the template is dropped from the
 167 core filename, as is the combination of a % followed by any
 168 character other than those listed above.
 169 All other characters in the template become a literal
 170 part of the core filename.
 171 The template may include \(aq/\(aq characters, which are interpreted
 172 as delimiters for directory names.
 173 The maximum size of the resulting core filename is 128 bytes (64 bytes
 174 in kernels before 2.6.19).
 175 The default value in this file is "core".
 176 For backward compatibility, if
 177 .I /proc/sys/kernel/core_pattern
 178 does not include "%p" and
 179 .I /proc/sys/kernel/core_uses_pid
 180 (see below)
 181 is nonzero, then .PID will be appended to the core filename.
 182
 183 Since version 2.4, Linux has also provided
 184 a more primitive method of controlling
 185 the name of the core dump file.
 186 If the
 187 .I /proc/sys/kernel/core_uses_pid
 188 file contains the value 0, then a core dump file is simply named
 189 .IR core .
 190 If this file contains a nonzero value, then the core dump file includes
 191 the process ID in a name of the form
 192 .IR core.PID .
 193
 194 Since Linux 3.6,
 195 .\" 9520628e8ceb69fa9a4aee6b57f22675d9e1b709
 196 if
 197 .I /proc/sys/fs/suid_dumpable
 198 is set to 2 ("suidsafe"), the pattern must be either an absolute pathname
 199 (starting with a leading \(aq/\(aq character) or a pipe, as defined below.
 200 .SS Piping core dumps to a program
 201 Since kernel 2.6.19, Linux supports an alternate syntax for the
 202 .I /proc/sys/kernel/core_pattern
 203 file.
 204 If the first character of this file is a pipe symbol (\fB|\fP),
 205 then the remainder of the line is interpreted as a program to be
 206 executed.
 207 Instead of being written to a disk file, the core dump is given as
 208 standard input to the program.
 209 Note the following points:
 210 .IP * 3
 211 The program must be specified using an absolute pathname (or a
 212 pathname relative to the root directory, \fI/\fP),
 213 and must immediately follow the '|' character.
 214 .IP *
 215 The process created to run the program runs as user and group
 216 .IR root .
 217 .IP *
 218 Command-line arguments can be supplied to the
 219 program (since Linux 2.6.24),
 220 delimited by white space (up to a total line length of 128 bytes).
 221 .IP *
 222 The command-line arguments can include any of
 223 the % specifiers listed above.
 224 For example, to pass the PID of the process that is being dumped, specify
 225 .I %p
 226 in an argument.
 227 .SS Controlling which mappings are written to the core dump
 228 Since kernel 2.6.23, the Linux-specific
 229 .IR /proc/PID/coredump_filter
 230 file can be used to control which memory segments are written to the
 231 core dump file in the event that a core dump is performed for the
 232 process with the corresponding process ID.
 233
 234 The value in the file is a bit mask of memory mapping types (see
 235 .BR mmap (2)).
 236 If a bit is set in the mask, then memory mappings of the
 237 corresponding type are dumped; otherwise they are not dumped.
 238 The bits in this file have the following meanings:
 239 .PP
 240 .PD 0
 241 .RS 4
 242 .TP
 243 bit 0
 244 Dump anonymous private mappings.
 245 .TP
 246 bit 1
 247 Dump anonymous shared mappings.
 248 .TP
 249 bit 2
 250 Dump file-backed private mappings.
 251 .TP
 252 bit 3
 253 Dump file-backed shared mappings.
 254 .\" file-backed shared mappings of course also update the underlying
 255 .\" mapped file.
 256 .TP
 257 bit 4 (since Linux 2.6.24)
 258 Dump ELF headers.
 259 .TP
 260 bit 5 (since Linux 2.6.28)
 261 Dump private huge pages.
 262 .TP
 263 bit 6 (since Linux 2.6.28)
 264 Dump shared huge pages.
 265 .RE
 266 .PD
 267 .PP
 268 By default, the following bits are set: 0, 1, 4 (if the
 269 .B CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS
 270 kernel configuration option is enabled), and 5.
 271 The value of this file is displayed in hexadecimal.
 272 (The default value is thus displayed as 33.)
 273
 274 Memory-mapped I/O pages such as frame buffer are never dumped, and
 275 virtual DSO pages are always dumped, regardless of the
 276 .I coredump_filter
 277 value.
 278
 279 A child process created via
 280 .BR fork (2)
 281 inherits its parent's
 282 .I coredump_filter
 283 value;
 284 the
 285 .I coredump_filter
 286 value is preserved across an
 287 .BR execve (2).
 288
 289 It can be useful to set
 290 .I coredump_filter
 291 in the parent shell before running a program, for example:
 292
 293 .in +4n
 294 .nf
 295 .RB "$" " echo 0x7 > /proc/self/coredump_filter"
 296 .RB "$" " ./some_program"
 297 .fi
 298 .in
 299 .PP
 300 This file is provided only if the kernel was built with the
 301 .B CONFIG_ELF_CORE
 302 configuration option.
 303 .SH NOTES
 304 The
 305 .BR gdb (1)
 306 .I gcore
 307 command can be used to obtain a core dump of a running process.
 308
 309 In Linux versions up to and including 2.6.27,
 310 .\" Changed with commit 6409324b385f3f63a03645b4422e3be67348d922
 311 if a multithreaded process (or, more precisely, a process that
 312 shares its memory with another process by being created with the
 313 .B CLONE_VM
 314 flag of
 315 .BR clone (2))
 316 dumps core, then the process ID is always appended to the core filename,
 317 unless the process ID was already included elsewhere in the
 318 filename via a %p specification in
 319 .IR /proc/sys/kernel/core_pattern .
 320 (This is primarily useful when employing the obsolete
 321 LinuxThreads implementation,
 322 where each thread of a process has a different PID.)
 323 .\" Always including the PID in the name of the core file made
 324 .\" sense for LinuxThreads, where each thread had a unique PID,
 325 .\" but doesn't seem to serve any purpose with NPTL, where all the
 326 .\" threads in a process share the same PID (as POSIX.1 requires).
 327 .\" Probably the behavior is maintained so that applications using
 328 .\" LinuxThreads continue appending the PID (the kernel has no easy
 329 .\" way of telling which threading implementation the user-space
 330 .\" application is using). -- mtk, April 2006
 331 .SH EXAMPLE
 332 The program below can be used to demonstrate the use of the
 333 pipe syntax in the
 334 .I /proc/sys/kernel/core_pattern
 335 file.
 336 The following shell session demonstrates the use of this program
 337 (compiled to create an executable named
 338 .IR core_pattern_pipe_test ):
 339 .PP
 340 .in +4n
 341 .nf
 342 .RB "$" " cc \-o core_pattern_pipe_test core_pattern_pipe_test.c"
 343 .RB "$" " su"
 344 Password:
 345 .RB "#" " echo \(dq|$PWD/core_pattern_pipe_test %p \
 346 UID=%u GID=%g sig=%s\(dq > \e"
 347 .B "    /proc/sys/kernel/core_pattern"
 348 .RB "#" " exit"
 349 .RB "$" " sleep 100"
 350 .BR "^\e" "                     # type control-backslash"
 351 Quit (core dumped)
 352 .RB "$" " cat core.info"
 353 argc=5
 354 argc[0]=</home/mtk/core_pattern_pipe_test>
 355 argc[1]=<20575>
 356 argc[2]=<UID=1000>
 357 argc[3]=<GID=100>
 358 argc[4]=<sig=3>
 359 Total bytes in core dump: 282624
 360 .fi
 361 .in
 362 .SS Program source
 363 \&
 364 .nf
 365 /* core_pattern_pipe_test.c */
 366
 367 #define _GNU_SOURCE
 368 #include <sys/stat.h>
 369 #include <fcntl.h>
 370 #include <limits.h>
 371 #include <stdio.h>
 372 #include <stdlib.h>
 373 #include <unistd.h>
 374
 375 #define BUF_SIZE 1024
 376
 377 int
 378 main(int argc, char *argv[])
 379 {
 380     int tot, j;
 381     ssize_t nread;
 382     char buf[BUF_SIZE];
 383     FILE *fp;
 384     char cwd[PATH_MAX];
 385
 386     /* Change our current working directory to that of the
 387        crashing process */
 388
 389     snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv[1]);
 390     chdir(cwd);
 391
 392     /* Write output to file "core.info" in that directory */
 393
 394     fp = fopen("core.info", "w+");
 395     if (fp == NULL)
 396         exit(EXIT_FAILURE);
 397
 398     /* Display command\-line arguments given to core_pattern
 399        pipe program */
 400
 401     fprintf(fp, "argc=%d\\n", argc);
 402     for (j = 0; j < argc; j++)
 403         fprintf(fp, "argc[%d]=<%s>\\n", j, argv[j]);
 404
 405     /* Count bytes in standard input (the core dump) */
 406
 407     tot = 0;
 408     while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
 409         tot += nread;
 410     fprintf(fp, "Total bytes in core dump: %d\\n", tot);
 411
 412     exit(EXIT_SUCCESS);
 413 }
 414 .fi
 415 .SH SEE ALSO
 416 .BR bash (1),
 417 .BR gdb (1),
 418 .BR getrlimit (2),
 419 .BR mmap (2),
 420 .BR prctl (2),
 421 .BR sigaction (2),
 422 .BR elf (5),
 423 .BR proc (5),
 424 .BR pthreads (7),
 425 .BR signal (7)