man5/core.5

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  25 .TH CORE 5 2015-02-21 "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 or
  54 .IR core.pid ,
  55 where
  56 .I pid
  57 is the ID of the process that dumped core,
  58 and is created in the current working directory.
  59 See below for details on naming.)
  60 Writing the core file will fail if the directory in which
  61 it is to be created is nonwritable,
  62 or if a file with the same name exists and
  63 is not writable
  64 or is not a regular file
  65 (e.g., it is a directory or a symbolic link).
  66 .IP *
  67 A (writable, regular) file with the same name as would be used for the
  68 core dump already exists, but there is more than one hard link to that
  69 file.
  70 .IP *
  71 The filesystem where the core dump file would be created is full;
  72 or has run out of inodes; or is mounted read-only;
  73 or the user has reached their quota for the filesystem.
  74 .IP *
  75 The directory in which the core dump file is to be created does
  76 not exist.
  77 .IP *
  78 The
  79 .B RLIMIT_CORE
  80 (core file size) or
  81 .B RLIMIT_FSIZE
  82 (file size) resource limits for the process are set to zero; see
  83 .BR getrlimit (2)
  84 and the documentation of the shell's
  85 .I ulimit
  86 command
  87 .RI ( limit
  88 in
  89 .BR csh (1)).
  90 .IP *
  91 The binary being executed by the process does not have read
  92 permission enabled.
  93 .IP *
  94 The process is executing a set-user-ID (set-group-ID) program
  95 that is owned by a user (group) other than the real user (group)
  96 ID of the process,
  97 or the process is executing a program that has file capabilities (see
  98 .BR capabilities (7)).
  99 (However, see the description of the
 100 .BR prctl (2)
 101 .B PR_SET_DUMPABLE
 102 operation, and the description of the
 103 .I /proc/sys/fs/suid_dumpable
 104 .\" FIXME . Perhaps relocate discussion of /proc/sys/fs/suid_dumpable
 105 .\" and PR_SET_DUMPABLE to this page?
 106 file in
 107 .BR proc (5).)
 108 .IP *
 109 (Since Linux 3.7)
 110 .\" commit 046d662f481830e652ac34cd112249adde16452a
 111 The kernel was configured without the
 112 .BR CONFIG_COREDUMP
 113 option.
 114 .PP
 115 In addition,
 116 a core dump may exclude part of the address space of the process if the
 117 .BR madvise (2)
 118 .B MADV_DONTDUMP
 119 flag was employed.
 120 .SS Naming of core dump files
 121 By default, a core dump file is named
 122 .IR core ,
 123 but the
 124 .I /proc/sys/kernel/core_pattern
 125 file (since Linux 2.6 and 2.4.21)
 126 can be set to define a template that is used to name core dump files.
 127 The template can contain % specifiers which are substituted
 128 by the following values when a core file is created:
 129 .PP
 130 .RS 4
 131 .PD 0
 132 .TP 4
 133 %%
 134 a single % character
 135 .TP
 136 %c
 137 core file size soft resource limit of crashing process (since Linux 2.6.24)
 138 .TP
 139 %d
 140 .\" Added in git commit 12a2b4b2241e318b4f6df31228e4272d2c2968a1
 141 dump mode\(emsame as value returned by
 142 .BR prctl (2)
 143 .B PR_GET_DUMPABLE
 144 (since Linux 3.7)
 145 .TP
 146 %e
 147 executable filename (without path prefix)
 148 .TP
 149 %E
 150 pathname of executable,
 151 with slashes (\(aq/\(aq) replaced by exclamation marks (\(aq!\(aq)
 152 (since Linux 3.0).
 153 .TP
 154 %g
 155 (numeric) real GID of dumped process
 156 .TP
 157 %h
 158 hostname (same as \fInodename\fP returned by \fBuname\fP(2))
 159 .TP
 160 %i
 161 TID of thread that triggered core dump,
 162 as seen in the PID namespace in which the thread resides
 163 .\" commit b03023ecbdb76c1dec86b41ed80b123c22783220
 164 (since Linux 3.18)
 165 .TP
 166 %I
 167 TID of thread that triggered core dump, as seen in the initial PID namespace
 168 .\" commit b03023ecbdb76c1dec86b41ed80b123c22783220
 169 (since Linux 3.18)
 170 .TP
 171 %p
 172 PID of dumped process,
 173 as seen in the PID namespace in which the process resides
 174 .TP
 175 %P
 176 .\" Added in git commit 65aafb1e7484b7434a0c1d4c593191ebe5776a2f
 177 PID of dumped process, as seen in the initial PID namespace
 178 (since Linux 3.12)
 179 .TP
 180 %s
 181 number of signal causing dump
 182 .TP
 183 %t
 184 time of dump, expressed as seconds since the
 185 Epoch, 1970-01-01 00:00:00 +0000 (UTC)
 186 .TP
 187 %u
 188 (numeric) real UID of dumped process
 189 .PD
 190 .RE
 191 .PP
 192 A single % at the end of the template is dropped from the
 193 core filename, as is the combination of a % followed by any
 194 character other than those listed above.
 195 All other characters in the template become a literal
 196 part of the core filename.
 197 The template may include \(aq/\(aq characters, which are interpreted
 198 as delimiters for directory names.
 199 The maximum size of the resulting core filename is 128 bytes (64 bytes
 200 in kernels before 2.6.19).
 201 The default value in this file is "core".
 202 For backward compatibility, if
 203 .I /proc/sys/kernel/core_pattern
 204 does not include "%p" and
 205 .I /proc/sys/kernel/core_uses_pid
 206 (see below)
 207 is nonzero, then .PID will be appended to the core filename.
 208
 209 Since version 2.4, Linux has also provided
 210 a more primitive method of controlling
 211 the name of the core dump file.
 212 If the
 213 .I /proc/sys/kernel/core_uses_pid
 214 file contains the value 0, then a core dump file is simply named
 215 .IR core .
 216 If this file contains a nonzero value, then the core dump file includes
 217 the process ID in a name of the form
 218 .IR core.PID .
 219
 220 Since Linux 3.6,
 221 .\" 9520628e8ceb69fa9a4aee6b57f22675d9e1b709
 222 if
 223 .I /proc/sys/fs/suid_dumpable
 224 is set to 2 ("suidsafe"), the pattern must be either an absolute pathname
 225 (starting with a leading \(aq/\(aq character) or a pipe, as defined below.
 226 .SS Piping core dumps to a program
 227 Since kernel 2.6.19, Linux supports an alternate syntax for the
 228 .I /proc/sys/kernel/core_pattern
 229 file.
 230 If the first character of this file is a pipe symbol (\fB|\fP),
 231 then the remainder of the line is interpreted as a program to be
 232 executed.
 233 Instead of being written to a disk file, the core dump is given as
 234 standard input to the program.
 235 Note the following points:
 236 .IP * 3
 237 The program must be specified using an absolute pathname (or a
 238 pathname relative to the root directory, \fI/\fP),
 239 and must immediately follow the '|' character.
 240 .IP *
 241 The process created to run the program runs as user and group
 242 .IR root .
 243 .IP *
 244 Command-line arguments can be supplied to the
 245 program (since Linux 2.6.24),
 246 delimited by white space (up to a total line length of 128 bytes).
 247 .IP *
 248 The command-line arguments can include any of
 249 the % specifiers listed above.
 250 For example, to pass the PID of the process that is being dumped, specify
 251 .I %p
 252 in an argument.
 253 .SS Controlling which mappings are written to the core dump
 254 Since kernel 2.6.23, the Linux-specific
 255 .IR /proc/PID/coredump_filter
 256 file can be used to control which memory segments are written to the
 257 core dump file in the event that a core dump is performed for the
 258 process with the corresponding process ID.
 259
 260 The value in the file is a bit mask of memory mapping types (see
 261 .BR mmap (2)).
 262 If a bit is set in the mask, then memory mappings of the
 263 corresponding type are dumped; otherwise they are not dumped.
 264 The bits in this file have the following meanings:
 265 .PP
 266 .PD 0
 267 .RS 4
 268 .TP
 269 bit 0
 270 Dump anonymous private mappings.
 271 .TP
 272 bit 1
 273 Dump anonymous shared mappings.
 274 .TP
 275 bit 2
 276 Dump file-backed private mappings.
 277 .TP
 278 bit 3
 279 Dump file-backed shared mappings.
 280 .\" file-backed shared mappings of course also update the underlying
 281 .\" mapped file.
 282 .TP
 283 bit 4 (since Linux 2.6.24)
 284 Dump ELF headers.
 285 .TP
 286 bit 5 (since Linux 2.6.28)
 287 Dump private huge pages.
 288 .TP
 289 bit 6 (since Linux 2.6.28)
 290 Dump shared huge pages.
 291 .RE
 292 .PD
 293 .PP
 294 By default, the following bits are set: 0, 1, 4 (if the
 295 .B CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS
 296 kernel configuration option is enabled), and 5.
 297 The value of this file is displayed in hexadecimal.
 298 (The default value is thus displayed as 33.)
 299
 300 Memory-mapped I/O pages such as frame buffer are never dumped, and
 301 virtual DSO pages are always dumped, regardless of the
 302 .I coredump_filter
 303 value.
 304
 305 A child process created via
 306 .BR fork (2)
 307 inherits its parent's
 308 .I coredump_filter
 309 value;
 310 the
 311 .I coredump_filter
 312 value is preserved across an
 313 .BR execve (2).
 314
 315 It can be useful to set
 316 .I coredump_filter
 317 in the parent shell before running a program, for example:
 318
 319 .in +4n
 320 .nf
 321 .RB "$" " echo 0x7 > /proc/self/coredump_filter"
 322 .RB "$" " ./some_program"
 323 .fi
 324 .in
 325 .PP
 326 This file is provided only if the kernel was built with the
 327 .B CONFIG_ELF_CORE
 328 configuration option.
 329 .SH NOTES
 330 The
 331 .BR gdb (1)
 332 .I gcore
 333 command can be used to obtain a core dump of a running process.
 334
 335 In Linux versions up to and including 2.6.27,
 336 .\" Changed with commit 6409324b385f3f63a03645b4422e3be67348d922
 337 if a multithreaded process (or, more precisely, a process that
 338 shares its memory with another process by being created with the
 339 .B CLONE_VM
 340 flag of
 341 .BR clone (2))
 342 dumps core, then the process ID is always appended to the core filename,
 343 unless the process ID was already included elsewhere in the
 344 filename via a %p specification in
 345 .IR /proc/sys/kernel/core_pattern .
 346 (This is primarily useful when employing the obsolete
 347 LinuxThreads implementation,
 348 where each thread of a process has a different PID.)
 349 .\" Always including the PID in the name of the core file made
 350 .\" sense for LinuxThreads, where each thread had a unique PID,
 351 .\" but doesn't seem to serve any purpose with NPTL, where all the
 352 .\" threads in a process share the same PID (as POSIX.1 requires).
 353 .\" Probably the behavior is maintained so that applications using
 354 .\" LinuxThreads continue appending the PID (the kernel has no easy
 355 .\" way of telling which threading implementation the user-space
 356 .\" application is using). -- mtk, April 2006
 357 .SH EXAMPLE
 358 The program below can be used to demonstrate the use of the
 359 pipe syntax in the
 360 .I /proc/sys/kernel/core_pattern
 361 file.
 362 The following shell session demonstrates the use of this program
 363 (compiled to create an executable named
 364 .IR core_pattern_pipe_test ):
 365 .PP
 366 .in +4n
 367 .nf
 368 .RB "$" " cc \-o core_pattern_pipe_test core_pattern_pipe_test.c"
 369 .RB "$" " su"
 370 Password:
 371 .RB "#" " echo \(dq|$PWD/core_pattern_pipe_test %p \
 372 UID=%u GID=%g sig=%s\(dq > \e"
 373 .B "    /proc/sys/kernel/core_pattern"
 374 .RB "#" " exit"
 375 .RB "$" " sleep 100"
 376 .BR "^\e" "                     # type control-backslash"
 377 Quit (core dumped)
 378 .RB "$" " cat core.info"
 379 argc=5
 380 argc[0]=</home/mtk/core_pattern_pipe_test>
 381 argc[1]=<20575>
 382 argc[2]=<UID=1000>
 383 argc[3]=<GID=100>
 384 argc[4]=<sig=3>
 385 Total bytes in core dump: 282624
 386 .fi
 387 .in
 388 .SS Program source
 389 \&
 390 .nf
 391 /* core_pattern_pipe_test.c */
 392
 393 #define _GNU_SOURCE
 394 #include <sys/stat.h>
 395 #include <fcntl.h>
 396 #include <limits.h>
 397 #include <stdio.h>
 398 #include <stdlib.h>
 399 #include <unistd.h>
 400
 401 #define BUF_SIZE 1024
 402
 403 int
 404 main(int argc, char *argv[])
 405 {
 406     int tot, j;
 407     ssize_t nread;
 408     char buf[BUF_SIZE];
 409     FILE *fp;
 410     char cwd[PATH_MAX];
 411
 412     /* Change our current working directory to that of the
 413        crashing process */
 414
 415     snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv[1]);
 416     chdir(cwd);
 417
 418     /* Write output to file "core.info" in that directory */
 419
 420     fp = fopen("core.info", "w+");
 421     if (fp == NULL)
 422         exit(EXIT_FAILURE);
 423
 424     /* Display command\-line arguments given to core_pattern
 425        pipe program */
 426
 427     fprintf(fp, "argc=%d\\n", argc);
 428     for (j = 0; j < argc; j++)
 429         fprintf(fp, "argc[%d]=<%s>\\n", j, argv[j]);
 430
 431     /* Count bytes in standard input (the core dump) */
 432
 433     tot = 0;
 434     while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
 435         tot += nread;
 436     fprintf(fp, "Total bytes in core dump: %d\\n", tot);
 437
 438     fclose(fp);
 439     exit(EXIT_SUCCESS);
 440 }
 441 .fi
 442 .SH SEE ALSO
 443 .BR bash (1),
 444 .BR gdb (1),
 445 .BR getrlimit (2),
 446 .BR mmap (2),
 447 .BR prctl (2),
 448 .BR sigaction (2),
 449 .BR elf (5),
 450 .BR proc (5),
 451 .BR pthreads (7),
 452 .BR signal (7)