man5/core.5

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  23 .TH CORE 5 2008-06-15 "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 A list of the signals which cause a process to dump core can be found in
  33 .BR signal (7).
  34
  35 A process can set its soft
  36 .B RLIMIT_CORE
  37 resource limit to place an upper limit on the size of the core dump file
  38 that will be produced if it receives a "core dump" signal; see
  39 .BR getrlimit (2)
  40 for details.
  41
  42 There are various circumstances in which a core dump file is
  43 not produced:
  44 .IP * 3
  45 The process does not have permission to write the core file.
  46 (By default the core file is called
  47 .IR core ,
  48 and is created in the current working directory.
  49 See below for details on naming.)
  50 Writing the core file will fail if the directory in which
  51 it is to be created is non-writable,
  52 or if a file with the same name exists and
  53 is not writable
  54 or is not a regular file
  55 (e.g., it is a directory or a symbolic link).
  56 .IP *
  57 A (writable, regular) file with the same name as would be used for the
  58 core dump already exists, but there is more than one hard link to that
  59 file.
  60 .IP *
  61 The file system where the core dump file would be created is full;
  62 or has run out of inodes; or is mounted read-only;
  63 or the user has reached their quota for the file system.
  64 .IP *
  65 The directory in which the core dump file is to be created does
  66 not exist.
  67 .IP *
  68 .B RLIMIT_CORE
  69 or
  70 .B RLIMIT_FSIZE
  71 resource limits for a process are set to zero (see
  72 .BR getrlimit (2)).
  73 .IP *
  74 The binary being executed by the process does not have read
  75 permission enabled.
  76 .IP *
  77 The process is executing a set-user-ID (set-group-ID) program
  78 that is owned by a user (group) other than the real user (group)
  79 ID of the process.
  80 (However, see the description of the
  81 .BR prctl (2)
  82 .B PR_SET_DUMPABLE
  83 operation, and the description of the
  84 .I /proc/sys/fs/suid_dumpable
  85 file in
  86 .BR proc (5).)
  87 .SS Naming of core dump files
  88 By default, a core dump file is named
  89 .IR core ,
  90 but the
  91 .I /proc/sys/kernel/core_pattern
  92 file (since Linux 2.6 and 2.4.21)
  93 can be set to define a template that is used to name core dump files.
  94 The template can contain % specifiers which are substituted
  95 by the following values when a core file is created:
  96 .PP
  97 .RS 4
  98 .PD 0
  99 .TP 4
 100 %%
 101 a single % character
 102 .TP
 103 %p
 104 PID of dumped process
 105 .TP
 106 %u
 107 (numeric) real UID of dumped process
 108 .TP
 109 %g
 110 (numeric) real GID of dumped process
 111 .TP
 112 %s
 113 number of signal causing dump
 114 .TP
 115 %t
 116 time of dump, expressed as seconds since the Epoch (00:00h, 1\ Jan 1970, UTC)
 117 .TP
 118 %h
 119 hostname (same as \fInodename\fP returned by \fBuname\fP(2))
 120 .TP
 121 %e
 122 executable filename (without path prefix)
 123 .TP
 124 %c
 125 core file size soft resource limit of crashing process (since Linux 2.6.24)
 126 .PD
 127 .RE
 128 .PP
 129 A single % at the end of the template is dropped from the
 130 core filename, as is the combination of a % followed by any
 131 character other than those listed above.
 132 All other characters in the template become a literal
 133 part of the core filename.
 134 The template may include \(aq/\(aq characters, which are interpreted
 135 as delimiters for directory names.
 136 The maximum size of the resulting core filename is 128 bytes (64 bytes
 137 in kernels before 2.6.19).
 138 The default value in this file is "core".
 139 For backward compatibility, if
 140 .I /proc/sys/kernel/core_pattern
 141 does not include "%p" and
 142 .I /proc/sys/kernel/core_uses_pid
 143 (see below)
 144 is non-zero, then .PID will be appended to the core filename.
 145
 146 Since version 2.4, Linux has also provided
 147 a more primitive method of controlling
 148 the name of the core dump file.
 149 If the
 150 .I /proc/sys/kernel/core_uses_pid
 151 file contains the value 0, then a core dump file is simply named
 152 .IR core .
 153 If this file contains a non-zero value, then the core dump file includes
 154 the process ID in a name of the form
 155 .IR core.PID .
 156 .SS Piping core dumps to a program
 157 Since kernel 2.6.19, Linux supports an alternate syntax for the
 158 .I /proc/sys/kernel/core_pattern
 159 file.
 160 If the first character of this file is a pipe symbol (\fB|\fP),
 161 then the remainder of the line is interpreted as a program to be
 162 executed.
 163 Instead of being written to a disk file, the core dump is given as
 164 standard input to the program.
 165 Note the following points:
 166 .IP * 3
 167 The program must be specified using an absolute pathname (or a
 168 pathname relative to the root directory, \fI/\fP),
 169 and must immediately follow the '|' character.
 170 .IP *
 171 The process created to run the program runs as user and group
 172 .IR root .
 173 .IP *
 174 Command-line arguments can be supplied to the
 175 program (since kernel 2.6.24),
 176 delimited by white space (up to a total line length of 128 bytes).
 177 .IP *
 178 The command-line arguments can include any of
 179 the % specifiers listed above.
 180 For example, to pass the PID of the process that is being dumped, specify
 181 .I %p
 182 in an argument.
 183 .SS Controlling which mappings are written to the core dump
 184 Since kernel 2.6.23, the Linux-specific
 185 .IR /proc/PID/coredump_filter
 186 file can be used to control which memory segments are written to the
 187 core dump file in the event that a core dump is performed for the
 188 process with the corresponding process ID.
 189
 190 The value in the file is a bit mask of memory mapping types (see
 191 .BR mmap (2)).
 192 If a bit is set in the mask, then memory mappings of the
 193 corresponding type are dumped; otherwise they are not dumped.
 194 The bits in this file have the following meanings:
 195 .PP
 196 .PD 0
 197 .RS 4
 198 .TP
 199 bit 0
 200 Dump anonymous private mappings.
 201 .TP
 202 bit 1
 203 Dump anonymous shared mappings.
 204 .TP
 205 bit 2
 206 Dump file-backed private mappings.
 207 .TP
 208 bit 3
 209 Dump file-backed shared mappings.
 210 .\" file-backed shared mappings of course also update the underlying
 211 .\" mapped file.
 212 .RE
 213 .PD
 214 .PP
 215 The default value of
 216 .I coredump_filter
 217 is 0x3;
 218 this reflects traditional Linux behavior and means that
 219 only anonymous memory segments are dumped.
 220
 221 Memory-mapped I/O pages such as frame buffer are never dumped, and
 222 virtual DSO pages are always dumped, regardless of the
 223 .I coredump_filter
 224 value.
 225
 226 A child process created via
 227 .BR fork (2)
 228 inherits its parents
 229 .I coredump_filter
 230 value;
 231 the
 232 .I coredump_filter
 233 value is preserved across an
 234 .BR execve (2).
 235
 236 It can be useful to set
 237 .I coredump_filter
 238 in the parent shell before running a program, for example:
 239
 240 .in +4n
 241 .nf
 242 $ echo 0x7 > /proc/self/coredump_filter
 243 $ ./some_program
 244 .fi
 245 .in
 246 .PP
 247 This file is only provided if the kernel was built with the
 248 CONFIG_ELF_CORE configuration option.
 249 .SH NOTES
 250 The
 251 .BR gdb (1)
 252 .I gcore
 253 command can be used to obtain a core dump of a running process.
 254
 255 If a multithreaded process (or, more precisely, a process that
 256 shares its memory with another process by being created with the
 257 .B CLONE_VM
 258 flag of
 259 .BR clone (2))
 260 dumps core, then the process ID is always appended to the core filename,
 261 unless the process ID was already included elsewhere in the
 262 filename via a %p specification in
 263 .IR /proc/sys/kernel/core_pattern .
 264 (This is primarily useful when employing the LinuxThreads implementation,
 265 where each thread of a process has a different PID.)
 266 .\" Always including the PID in the name of the core file made
 267 .\" sense for LinuxThreads, where each thread had a unique PID,
 268 .\" but doesn't seem to serve any purpose with NPTL, where all the
 269 .\" threads in a process share the same PID (as POSIX.1 requires).
 270 .\" Probably the behavior is maintained so that applications using
 271 .\" LinuxThreads continue appending the PID (the kernel has no easy
 272 .\" way of telling which threading implementation the userspace
 273 .\" application is using). -- mtk, April 2006
 274 .SH EXAMPLE
 275 The program below can be used to demonstrate the use of the
 276 pipe syntax in the
 277 .I /proc/sys/kernel/core_pattern
 278 file.
 279 The following shell session demonstrates the use of this program
 280 (compiled to create an executable named
 281 .IR core_pattern_pipe_test ):
 282 .PP
 283 .in +4n
 284 .nf
 285 $ cc -o core_pattern_pipe_test core_pattern_pipe_test.c
 286 $ su
 287 Password:
 288 # echo "|$PWD/core_pattern_pipe_test %p UID=%u GID=%g sig=%s" > \\
 289     /proc/sys/kernel/core_pattern
 290 # exit
 291 $ sleep 100
 292 .I type control-backslash
 293 Quit (core dumped)
 294 $ cat core.info
 295 argc=5
 296 argc[0]=</home/mtk/core_pattern_pipe_test>
 297 argc[1]=<20575>
 298 argc[2]=<UID=1000>
 299 argc[3]=<GID=100>
 300 argc[4]=<sig=3>
 301 Total bytes in core dump: 282624
 302 .fi
 303 .in
 304 .PP
 305 The source code of the program is as follows:
 306 .PP
 307 .nf
 308 /* core_pattern_pipe_test.c */
 309
 310 #define _GNU_SOURCE
 311 #include <sys/stat.h>
 312 #include <fcntl.h>
 313 #include <limits.h>
 314 #include <stdio.h>
 315 #include <stdlib.h>
 316 #include <unistd.h>
 317
 318 #define BUF_SIZE 1024
 319
 320 int
 321 main(int argc, char *argv[])
 322 {
 323     int tot, j;
 324     ssize_t nread;
 325     char buf[BUF_SIZE];
 326     FILE *fp;
 327     char cwd[PATH_MAX];
 328
 329     /* Change our current working directory to that of the
 330        crashing process */
 331
 332     snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv[1]);
 333     chdir(cwd);
 334
 335     /* Write output to file "core.info" in that directory */
 336
 337     fp = fopen("core.info", "w+");
 338     if (fp == NULL)
 339         exit(EXIT_FAILURE);
 340
 341     /* Display command\-line arguments given to core_pattern
 342        pipe program */
 343
 344     fprintf(fp, "argc=%d\\n", argc);
 345     for (j = 0; j < argc; j++)
 346         fprintf(fp, "argc[%d]=<%s>\\n", j, argv[j]);
 347
 348     /* Count bytes in standard input (the core dump) */
 349
 350     tot = 0;
 351     while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
 352         tot += nread;
 353     fprintf(fp, "Total bytes in core dump: %d\\n", tot);
 354
 355     exit(EXIT_SUCCESS);
 356 }
 357 .fi
 358 .SH SEE ALSO
 359 .BR gdb (1),
 360 .BR getrlimit (2),
 361 .BR prctl (2),
 362 .BR sigaction (2),
 363 .BR elf (5),
 364 .BR proc (5),
 365 .BR pthreads (7),
 366 .BR signal (7)