core

Naming of core dump files

By default, a core dump file is named core, but the /proc/sys/kernel/core_pattern file (since Linux 2.6 and 2.4.21) can be set to define a template that is used to name core dump files. The template can contain % specifiers which are substituted by the following values when a core file is created:

A single % at the end of the template is dropped from the core filename, as is the combination of a % followed by any character other than those listed above. All other characters in the template become a literal part of the core filename. The template may include '/' characters, which are interpreted as delimiters for directory names. The maximum size of the resulting core filename is 128 bytes (64 bytes in kernels before 2.6.19). The default value in this file is "core". For backward compatibility, if /proc/sys/kernel/core_pattern does not include "%p" and /proc/sys/kernel/core_uses_pid (see below) is nonzero, then .PID will be appended to the core filename.

Since version 2.4, Linux has also provided a more primitive method of controlling the name of the core dump file. If the /proc/sys/kernel/core_uses_pid file contains the value 0, then a core dump file is simply named core. If this file contains a nonzero value, then the core dump file includes the process ID in a name of the form core.PID.

Since Linux 3.6, if /proc/sys/fs/suid_dumpable is set to 2 ("suidsafe"), the pattern must be either an absolute pathname (starting with a leading '/' character) or a pipe, as defined below.

Piping core dumps to a program

Since kernel 2.6.19, Linux supports an alternate syntax for the /proc/sys/kernel/core_pattern file. If the first character of this file is a pipe symbol (|), then the remainder of the line is interpreted as a program to be executed. Instead of being written to a disk file, the core dump is given as standard input to the program. Note the following points:

• The program must be specified using an absolute pathname (or a pathname relative to the root directory, /), and must immediately follow the '|' character.
• The process created to run the program runs as user and group root.
• Command-line arguments can be supplied to the program (since Linux 2.6.24), delimited by white space (up to a total line length of 128 bytes).
• The command-line arguments can include any of the % specifiers listed above. For example, to pass the PID of the process that is being dumped, specify %p in an argument.

Controlling which mappings are written to the core dump

Since kernel 2.6.23, the Linux-specific /proc/PID/coredump_filter file can be used to control which memory segments are written to the core dump file in the event that a core dump is performed for the process with the corresponding process ID.

The value in the file is a bit mask of memory mapping types (see mmap(2)). If a bit is set in the mask, then memory mappings of the corresponding type are dumped; otherwise they are not dumped. The bits in this file have the following meanings:

By default, the following bits are set: 0, 1, 4 (if the CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS kernel configuration option is enabled), and 5. The value of this file is displayed in hexadecimal. (The default value is thus displayed as 33.)

Memory-mapped I/O pages such as frame buffer are never dumped, and virtual DSO pages are always dumped, regardless of the coredump_filter value.

A child process created via fork(2) inherits its parent's coredump_filter value; the coredump_filter value is preserved across an execve(2).

It can be useful to set coredump_filter in the parent shell before running a program, for example:

$ echo 0x7 > /proc/self/coredump_filter
$ ./some_program

This file is provided only if the kernel was built with the CONFIG_ELF_CORE configuration option.

Program source

/* core_pattern_pipe_test.c */
#define _GNU_SOURCE
#include <sys/stat.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define BUF_SIZE 1024
int
main(int argc, char *argv[])
{
    int tot, j;
    ssize_t nread;
    char buf[BUF_SIZE];
    FILE *fp;
    char cwd[PATH_MAX];
    /* Change our current working directory to that of the
       crashing process */
    snprintf(cwd, PATH_MAX, "/proc/%s/cwd", argv[1]);
    chdir(cwd);
    /* Write output to file "core.info" in that directory */
    fp = fopen("core.info", "w+");
    if (fp == NULL)
        exit(EXIT_FAILURE);
    /* Display command-line arguments given to core_pattern
       pipe program */
    fprintf(fp, "argc=%d\n", argc);
    for (j = 0; j < argc; j++)
        fprintf(fp, "argc[%d]=<%s>\n", j, argv[j]);
    /* Count bytes in standard input (the core dump) */
    tot = 0;
    while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
        tot += nread;
    fprintf(fp, "Total bytes in core dump: %d\n", tot);
    exit(EXIT_SUCCESS);
}