man5/core.5

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  25 .TH CORE 5 2015-12-05 "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
 205 .I %p
 206 and
 207 .I /proc/sys/kernel/core_uses_pid
 208 (see below)
 209 is nonzero, then .PID will be appended to the core filename.
 210
 211 Since version 2.4, Linux has also provided
 212 a more primitive method of controlling
 213 the name of the core dump file.
 214 If the
 215 .I /proc/sys/kernel/core_uses_pid
 216 file contains the value 0, then a core dump file is simply named
 217 .IR core .
 218 If this file contains a nonzero value, then the core dump file includes
 219 the process ID in a name of the form
 220 .IR core.PID .
 221
 222 Since Linux 3.6,
 223 .\" 9520628e8ceb69fa9a4aee6b57f22675d9e1b709
 224 if
 225 .I /proc/sys/fs/suid_dumpable
 226 is set to 2 ("suidsafe"), the pattern must be either an absolute pathname
 227 (starting with a leading \(aq/\(aq character) or a pipe, as defined below.
 228 .SS Piping core dumps to a program
 229 Since kernel 2.6.19, Linux supports an alternate syntax for the
 230 .I /proc/sys/kernel/core_pattern
 231 file.
 232 If the first character of this file is a pipe symbol (\fB|\fP),
 233 then the remainder of the line is interpreted as a program to be
 234 executed.
 235 Instead of being written to a disk file, the core dump is given as
 236 standard input to the program.
 237 Note the following points:
 238 .IP * 3
 239 The program must be specified using an absolute pathname (or a
 240 pathname relative to the root directory, \fI/\fP),
 241 and must immediately follow the '|' character.
 242 .IP *
 243 The process created to run the program runs as user and group
 244 .IR root .
 245 .IP *
 246 Command-line arguments can be supplied to the
 247 program (since Linux 2.6.24),
 248 delimited by white space (up to a total line length of 128 bytes).
 249 .IP *
 250 The command-line arguments can include any of
 251 the % specifiers listed above.
 252 For example, to pass the PID of the process that is being dumped, specify
 253 .I %p
 254 in an argument.
 255 .SS Controlling which mappings are written to the core dump
 256 Since kernel 2.6.23, the Linux-specific
 257 .IR /proc/PID/coredump_filter
 258 file can be used to control which memory segments are written to the
 259 core dump file in the event that a core dump is performed for the
 260 process with the corresponding process ID.
 261
 262 The value in the file is a bit mask of memory mapping types (see
 263 .BR mmap (2)).
 264 If a bit is set in the mask, then memory mappings of the
 265 corresponding type are dumped; otherwise they are not dumped.
 266 The bits in this file have the following meanings:
 267 .PP
 268 .PD 0
 269 .RS 4
 270 .TP
 271 bit 0
 272 Dump anonymous private mappings.
 273 .TP
 274 bit 1
 275 Dump anonymous shared mappings.
 276 .TP
 277 bit 2
 278 Dump file-backed private mappings.
 279 .TP
 280 bit 3
 281 Dump file-backed shared mappings.
 282 .\" file-backed shared mappings of course also update the underlying
 283 .\" mapped file.
 284 .TP
 285 bit 4 (since Linux 2.6.24)
 286 Dump ELF headers.
 287 .TP
 288 bit 5 (since Linux 2.6.28)
 289 Dump private huge pages.
 290 .TP
 291 bit 6 (since Linux 2.6.28)
 292 Dump shared huge pages.
 293 .TP
 294 bit 7 (since Linux 4.4)
 295 .\" commit ab27a8d04b32b6ee8c30c14c4afd1058e8addc82
 296 Dump private DAX pages.
 297 .TP
 298 bit 8 (since Linux 4.4)
 299 .\" commit ab27a8d04b32b6ee8c30c14c4afd1058e8addc82
 300 Dump shared DAX pages.
 301 .RE
 302 .PD
 303 .PP
 304 By default, the following bits are set: 0, 1, 4 (if the
 305 .B CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS
 306 kernel configuration option is enabled), and 5.
 307 This default can be modified at boot time using the
 308 .I coredump_filter
 309 boot option.
 310
 311 The value of this file is displayed in hexadecimal.
 312 (The default value is thus displayed as 33.)
 313
 314 Memory-mapped I/O pages such as frame buffer are never dumped, and
 315 virtual DSO pages are always dumped, regardless of the
 316 .I coredump_filter
 317 value.
 318
 319 A child process created via
 320 .BR fork (2)
 321 inherits its parent's
 322 .I coredump_filter
 323 value;
 324 the
 325 .I coredump_filter
 326 value is preserved across an
 327 .BR execve (2).
 328
 329 It can be useful to set
 330 .I coredump_filter
 331 in the parent shell before running a program, for example:
 332
 333 .in +4n
 334 .nf
 335 .RB "$" " echo 0x7 > /proc/self/coredump_filter"
 336 .RB "$" " ./some_program"
 337 .fi
 338 .in
 339 .PP
 340 This file is provided only if the kernel was built with the
 341 .B CONFIG_ELF_CORE
 342 configuration option.
 343 .SH NOTES
 344 The
 345 .BR gdb (1)
 346 .I gcore
 347 command can be used to obtain a core dump of a running process.
 348
 349 In Linux versions up to and including 2.6.27,
 350 .\" Changed with commit 6409324b385f3f63a03645b4422e3be67348d922
 351 if a multithreaded process (or, more precisely, a process that
 352 shares its memory with another process by being created with the
 353 .B CLONE_VM
 354 flag of
 355 .BR clone (2))
 356 dumps core, then the process ID is always appended to the core filename,
 357 unless the process ID was already included elsewhere in the
 358 filename via a
 359 .I %p
 360 specification in
 361 .IR /proc/sys/kernel/core_pattern .
 362 (This is primarily useful when employing the obsolete
 363 LinuxThreads implementation,
 364 where each thread of a process has a different PID.)
 365 .\" Always including the PID in the name of the core file made
 366 .\" sense for LinuxThreads, where each thread had a unique PID,
 367 .\" but doesn't seem to serve any purpose with NPTL, where all the
 368 .\" threads in a process share the same PID (as POSIX.1 requires).
 369 .\" Probably the behavior is maintained so that applications using
 370 .\" LinuxThreads continue appending the PID (the kernel has no easy
 371 .\" way of telling which threading implementation the user-space
 372 .\" application is using). -- mtk, April 2006
 373 .SH EXAMPLE
 374 The program below can be used to demonstrate the use of the
 375 pipe syntax in the
 376 .I /proc/sys/kernel/core_pattern
 377 file.
 378 The following shell session demonstrates the use of this program
 379 (compiled to create an executable named
 380 .IR core_pattern_pipe_test ):
 381 .PP
 382 .in +4n
 383 .nf
 384 .RB "$" " cc \-o core_pattern_pipe_test core_pattern_pipe_test.c"
 385 .RB "$" " su"
 386 Password:
 387 .RB "#" " echo \(dq|$PWD/core_pattern_pipe_test %p \
 388 UID=%u GID=%g sig=%s\(dq > \e"
 389 .B "    /proc/sys/kernel/core_pattern"
 390 .RB "#" " exit"
 391 .RB "$" " sleep 100"
 392 .BR "^\e" "                     # type control-backslash"
 393 Quit (core dumped)
 394 .RB "$" " cat core.info"
 395 argc=5
 396 argc[0]=</home/mtk/core_pattern_pipe_test>
 397 argc[1]=<20575>
 398 argc[2]=<UID=1000>
 399 argc[3]=<GID=100>
 400 argc[4]=<sig=3>
 401 Total bytes in core dump: 282624
 402 .fi
 403 .in
 404 .SS Program source
 405 \&
 406 .nf
 407 /* core_pattern_pipe_test.c */
 408
 409 #define _GNU_SOURCE
 410 #include <sys/stat.h>
 411 #include <fcntl.h>
 412 #include <limits.h>
 413 #include <stdio.h>
 414 #include <stdlib.h>
 415 #include <unistd.h>
 416
 417 #define BUF_SIZE 1024
 418
 419 int
 420 main(int argc, char *argv[])
 421 {
 422     int tot, j;
 423     ssize_t nread;
 424     char buf[BUF_SIZE];
 425     FILE *fp;
 426     char cwd[PATH_MAX];
 427
 428     /* Change our current working directory to that of the
 429        crashing process */
 430
 431     snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv[1]);
 432     chdir(cwd);
 433
 434     /* Write output to file "core.info" in that directory */
 435
 436     fp = fopen("core.info", "w+");
 437     if (fp == NULL)
 438         exit(EXIT_FAILURE);
 439
 440     /* Display command\-line arguments given to core_pattern
 441        pipe program */
 442
 443     fprintf(fp, "argc=%d\\n", argc);
 444     for (j = 0; j < argc; j++)
 445         fprintf(fp, "argc[%d]=<%s>\\n", j, argv[j]);
 446
 447     /* Count bytes in standard input (the core dump) */
 448
 449     tot = 0;
 450     while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
 451         tot += nread;
 452     fprintf(fp, "Total bytes in core dump: %d\\n", tot);
 453
 454     fclose(fp);
 455     exit(EXIT_SUCCESS);
 456 }
 457 .fi
 458 .SH SEE ALSO
 459 .BR bash (1),
 460 .BR gdb (1),
 461 .BR getrlimit (2),
 462 .BR mmap (2),
 463 .BR prctl (2),
 464 .BR sigaction (2),
 465 .BR elf (5),
 466 .BR proc (5),
 467 .BR pthreads (7),
 468 .BR signal (7)