man5/core.5

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  23 .TH CORE 5 2012-07-16 "Linux" "Linux Programmer's Manual"
  24 .SH NAME
  25 core \- core dump file
  26 .SH DESCRIPTION
  27 The default action of certain signals is to cause a process to terminate
  28 and produce a
  29 .IR "core dump file" ,
  30 a disk file containing an image of the process's memory at
  31 the time of termination.
  32 This image can be used in a debugger (e.g.,
  33 .BR gdb (1))
  34 to inspect the state of the program at the time that it terminated.
  35 A list of the signals which cause a process to dump core can be found in
  36 .BR signal (7).
  37
  38 A process can set its soft
  39 .B RLIMIT_CORE
  40 resource limit to place an upper limit on the size of the core dump file
  41 that will be produced if it receives a "core dump" signal; see
  42 .BR getrlimit (2)
  43 for details.
  44
  45 There are various circumstances in which a core dump file is
  46 not produced:
  47 .IP * 3
  48 The process does not have permission to write the core file.
  49 (By default the core file is called
  50 .IR core ,
  51 and is created in the current working directory.
  52 See below for details on naming.)
  53 Writing the core file will fail if the directory in which
  54 it is to be created is nonwritable,
  55 or if a file with the same name exists and
  56 is not writable
  57 or is not a regular file
  58 (e.g., it is a directory or a symbolic link).
  59 .IP *
  60 A (writable, regular) file with the same name as would be used for the
  61 core dump already exists, but there is more than one hard link to that
  62 file.
  63 .IP *
  64 The file system where the core dump file would be created is full;
  65 or has run out of inodes; or is mounted read-only;
  66 or the user has reached their quota for the file system.
  67 .IP *
  68 The directory in which the core dump file is to be created does
  69 not exist.
  70 .IP *
  71 The
  72 .B RLIMIT_CORE
  73 (core file size) or
  74 .B RLIMIT_FSIZE
  75 (file size) resource limits for the process are set to zero; see
  76 .BR getrlimit (2)
  77 and the documentation of the shell's
  78 .I ulimit
  79 command
  80 .RI ( limit
  81 in
  82 .BR csh (1)).
  83 .IP *
  84 The binary being executed by the process does not have read
  85 permission enabled.
  86 .IP *
  87 The process is executing a set-user-ID (set-group-ID) program
  88 that is owned by a user (group) other than the real user (group)
  89 ID of the process.
  90 (However, see the description of the
  91 .BR prctl (2)
  92 .B PR_SET_DUMPABLE
  93 operation, and the description of the
  94 .I /proc/sys/fs/suid_dumpable
  95 .\" FIXME . Perhaps relocate discussion of /proc/sys/fs/suid_dumpable
  96 .\" and PR_SET_DUMPABLE to this page?
  97 file in
  98 .BR proc (5).)
  99 .PP
 100 In addition,
 101 a core dump may exclude part of the address space of the process if the
 102 .BR madvise (2)
 103 .B MADV_DONTDUMP
 104 flag was employed.
 105 .SS Naming of core dump files
 106 By default, a core dump file is named
 107 .IR core ,
 108 but the
 109 .I /proc/sys/kernel/core_pattern
 110 file (since Linux 2.6 and 2.4.21)
 111 can be set to define a template that is used to name core dump files.
 112 The template can contain % specifiers which are substituted
 113 by the following values when a core file is created:
 114 .PP
 115 .RS 4
 116 .PD 0
 117 .TP 4
 118 %%
 119 a single % character
 120 .TP
 121 %p
 122 PID of dumped process
 123 .TP
 124 %u
 125 (numeric) real UID of dumped process
 126 .TP
 127 %g
 128 (numeric) real GID of dumped process
 129 .TP
 130 %s
 131 number of signal causing dump
 132 .TP
 133 %t
 134 time of dump, expressed as seconds since the
 135 Epoch, 1970-01-01 00:00:00 +0000 (UTC)
 136 .TP
 137 %h
 138 hostname (same as \fInodename\fP returned by \fBuname\fP(2))
 139 .TP
 140 %e
 141 executable filename (without path prefix)
 142 .TP
 143 %E
 144 pathname of executable,
 145 with slashes (\(aq/\(aq) replaced by exclamation marks (\(aq!\(aq).
 146 .TP
 147 %c
 148 core file size soft resource limit of crashing process (since Linux 2.6.24)
 149 .PD
 150 .RE
 151 .PP
 152 A single % at the end of the template is dropped from the
 153 core filename, as is the combination of a % followed by any
 154 character other than those listed above.
 155 All other characters in the template become a literal
 156 part of the core filename.
 157 The template may include \(aq/\(aq characters, which are interpreted
 158 as delimiters for directory names.
 159 The maximum size of the resulting core filename is 128 bytes (64 bytes
 160 in kernels before 2.6.19).
 161 The default value in this file is "core".
 162 For backward compatibility, if
 163 .I /proc/sys/kernel/core_pattern
 164 does not include "%p" and
 165 .I /proc/sys/kernel/core_uses_pid
 166 (see below)
 167 is nonzero, then .PID will be appended to the core filename.
 168
 169 Since version 2.4, Linux has also provided
 170 a more primitive method of controlling
 171 the name of the core dump file.
 172 If the
 173 .I /proc/sys/kernel/core_uses_pid
 174 file contains the value 0, then a core dump file is simply named
 175 .IR core .
 176 If this file contains a nonzero value, then the core dump file includes
 177 the process ID in a name of the form
 178 .IR core.PID .
 179
 180 Since Linux 3.6,
 181 .\" 9520628e8ceb69fa9a4aee6b57f22675d9e1b709
 182 if
 183 .I /proc/sys/fs/suid_dumpable
 184 is set to 2 ("suidsafe"), the pattern must be either an absolute pathname
 185 (starting with a leading \(aq/\(aq character) or a pipe, as defined below.
 186
 187 .SS Piping core dumps to a program
 188 Since kernel 2.6.19, Linux supports an alternate syntax for the
 189 .I /proc/sys/kernel/core_pattern
 190 file.
 191 If the first character of this file is a pipe symbol (\fB|\fP),
 192 then the remainder of the line is interpreted as a program to be
 193 executed.
 194 Instead of being written to a disk file, the core dump is given as
 195 standard input to the program.
 196 Note the following points:
 197 .IP * 3
 198 The program must be specified using an absolute pathname (or a
 199 pathname relative to the root directory, \fI/\fP),
 200 and must immediately follow the '|' character.
 201 .IP *
 202 The process created to run the program runs as user and group
 203 .IR root .
 204 .IP *
 205 Command-line arguments can be supplied to the
 206 program (since kernel 2.6.24),
 207 delimited by white space (up to a total line length of 128 bytes).
 208 .IP *
 209 The command-line arguments can include any of
 210 the % specifiers listed above.
 211 For example, to pass the PID of the process that is being dumped, specify
 212 .I %p
 213 in an argument.
 214 .SS Controlling which mappings are written to the core dump
 215 Since kernel 2.6.23, the Linux-specific
 216 .IR /proc/PID/coredump_filter
 217 file can be used to control which memory segments are written to the
 218 core dump file in the event that a core dump is performed for the
 219 process with the corresponding process ID.
 220
 221 The value in the file is a bit mask of memory mapping types (see
 222 .BR mmap (2)).
 223 If a bit is set in the mask, then memory mappings of the
 224 corresponding type are dumped; otherwise they are not dumped.
 225 The bits in this file have the following meanings:
 226 .PP
 227 .PD 0
 228 .RS 4
 229 .TP
 230 bit 0
 231 Dump anonymous private mappings.
 232 .TP
 233 bit 1
 234 Dump anonymous shared mappings.
 235 .TP
 236 bit 2
 237 Dump file-backed private mappings.
 238 .TP
 239 bit 3
 240 Dump file-backed shared mappings.
 241 .\" file-backed shared mappings of course also update the underlying
 242 .\" mapped file.
 243 .TP
 244 bit 4 (since Linux 2.6.24)
 245 Dump ELF headers.
 246 .TP
 247 bit 5 (since Linux 2.6.28)
 248 Dump private huge pages.
 249 .TP
 250 bit 6 (since Linux 2.6.28)
 251 Dump shared huge pages.
 252 .RE
 253 .PD
 254 .PP
 255 By default, the following bits are set: 0, 1, 4 (if the
 256 .B CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS
 257 kernel configuration option is enabled), and 5.
 258 The value of this file is displayed in hexadecimal.
 259 (The default value is thus displayed as 33.)
 260
 261 Memory-mapped I/O pages such as frame buffer are never dumped, and
 262 virtual DSO pages are always dumped, regardless of the
 263 .I coredump_filter
 264 value.
 265
 266 A child process created via
 267 .BR fork (2)
 268 inherits its parent's
 269 .I coredump_filter
 270 value;
 271 the
 272 .I coredump_filter
 273 value is preserved across an
 274 .BR execve (2).
 275
 276 It can be useful to set
 277 .I coredump_filter
 278 in the parent shell before running a program, for example:
 279
 280 .in +4n
 281 .nf
 282 .RB "$" " echo 0x7 > /proc/self/coredump_filter"
 283 .RB "$" " ./some_program"
 284 .fi
 285 .in
 286 .PP
 287 This file is only provided if the kernel was built with the
 288 .B CONFIG_ELF_CORE
 289 configuration option.
 290 .SH NOTES
 291 The
 292 .BR gdb (1)
 293 .I gcore
 294 command can be used to obtain a core dump of a running process.
 295
 296 If a multithreaded process (or, more precisely, a process that
 297 shares its memory with another process by being created with the
 298 .B CLONE_VM
 299 flag of
 300 .BR clone (2))
 301 dumps core, then the process ID is always appended to the core filename,
 302 unless the process ID was already included elsewhere in the
 303 filename via a %p specification in
 304 .IR /proc/sys/kernel/core_pattern .
 305 (This is primarily useful when employing the LinuxThreads implementation,
 306 where each thread of a process has a different PID.)
 307 .\" Always including the PID in the name of the core file made
 308 .\" sense for LinuxThreads, where each thread had a unique PID,
 309 .\" but doesn't seem to serve any purpose with NPTL, where all the
 310 .\" threads in a process share the same PID (as POSIX.1 requires).
 311 .\" Probably the behavior is maintained so that applications using
 312 .\" LinuxThreads continue appending the PID (the kernel has no easy
 313 .\" way of telling which threading implementation the user-space
 314 .\" application is using). -- mtk, April 2006
 315 .SH EXAMPLE
 316 The program below can be used to demonstrate the use of the
 317 pipe syntax in the
 318 .I /proc/sys/kernel/core_pattern
 319 file.
 320 The following shell session demonstrates the use of this program
 321 (compiled to create an executable named
 322 .IR core_pattern_pipe_test ):
 323 .PP
 324 .in +4n
 325 .nf
 326 .RB "$" " cc \-o core_pattern_pipe_test core_pattern_pipe_test.c"
 327 .RB "$" " su"
 328 Password:
 329 .RB "#" " echo \(dq|$PWD/core_pattern_pipe_test %p \
 330 UID=%u GID=%g sig=%s\(dq > \e"
 331 .B "    /proc/sys/kernel/core_pattern"
 332 .RB "#" " exit"
 333 .RB "$" " sleep 100"
 334 .BR "^\e" "                     # type control-backslash"
 335 Quit (core dumped)
 336 .RB "$" " cat core.info"
 337 argc=5
 338 argc[0]=</home/mtk/core_pattern_pipe_test>
 339 argc[1]=<20575>
 340 argc[2]=<UID=1000>
 341 argc[3]=<GID=100>
 342 argc[4]=<sig=3>
 343 Total bytes in core dump: 282624
 344 .fi
 345 .in
 346 .SS Program source
 347 \&
 348 .nf
 349 /* core_pattern_pipe_test.c */
 350
 351 #define _GNU_SOURCE
 352 #include <sys/stat.h>
 353 #include <fcntl.h>
 354 #include <limits.h>
 355 #include <stdio.h>
 356 #include <stdlib.h>
 357 #include <unistd.h>
 358
 359 #define BUF_SIZE 1024
 360
 361 int
 362 main(int argc, char *argv[])
 363 {
 364     int tot, j;
 365     ssize_t nread;
 366     char buf[BUF_SIZE];
 367     FILE *fp;
 368     char cwd[PATH_MAX];
 369
 370     /* Change our current working directory to that of the
 371        crashing process */
 372
 373     snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv[1]);
 374     chdir(cwd);
 375
 376     /* Write output to file "core.info" in that directory */
 377
 378     fp = fopen("core.info", "w+");
 379     if (fp == NULL)
 380         exit(EXIT_FAILURE);
 381
 382     /* Display command\-line arguments given to core_pattern
 383        pipe program */
 384
 385     fprintf(fp, "argc=%d\\n", argc);
 386     for (j = 0; j < argc; j++)
 387         fprintf(fp, "argc[%d]=<%s>\\n", j, argv[j]);
 388
 389     /* Count bytes in standard input (the core dump) */
 390
 391     tot = 0;
 392     while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
 393         tot += nread;
 394     fprintf(fp, "Total bytes in core dump: %d\\n", tot);
 395
 396     exit(EXIT_SUCCESS);
 397 }
 398 .fi
 399 .SH SEE ALSO
 400 .BR bash (1),
 401 .BR gdb (1),
 402 .BR getrlimit (2),
 403 .BR mmap (2),
 404 .BR prctl (2),
 405 .BR sigaction (2),
 406 .BR elf (5),
 407 .BR proc (5),
 408 .BR pthreads (7),
 409 .BR signal (7)