FreeBSD Manual Pages
CORE(5) File Formats Manual CORE(5) NAME core -- memory image file format SYNOPSIS #include <sys/param.h> DESCRIPTION A small number of signals which cause abnormal termination of a process also cause a record of the process's in-core state to be written to disk for later examination by one of the available debuggers. (See sigac- tion(2).) This memory image is written to a file named by default programname.core in the working directory; provided the terminated process had write permission in the directory, and provided the abnormality did not cause a system crash. (In this event, the decision to save the core file is arbitrary, see savecore(8).) The name of the file is controlled via the sysctl(8) variable kern.corefile. The contents of this variable describes a filename to store the core image to. This filename can be absolute, or relative (which will resolve to the current working directory of the program generating it). The following format specifiers may be used in the kern.corefile sysctl to insert additional information into the resulting core filename: %H Machine hostname. %I An index starting at zero until the sysctl debug.ncores is reached. This can be useful for limiting the number of corefiles generated by a particular process. %N process name. %P processes PID. %S signal during core. %U process UID. The name defaults to %N.core, yielding the traditional FreeBSD behaviour. The maximum size of a core file is limited by the RLIMIT_CORE setrlimit(2) limit. Files which would be larger than the limit are not created. With a large limit, a process that had mapped a very large, and perhaps sparsely populated, virtual memory region, could take a very long time to create core dumps. The system ignores all signals sent to a process writ- ing a core file, except SIGKILL which terminates the writing and causes im- mediate exit of the process. The behavior of SIGKILL can be disabled by setting tunable sysctl(8) variable kern.core_dump_can_intr to zero. By default, a process that changes user or group credentials whether real or effective will not create a corefile. This behaviour can be changed to generate a core dump by setting the sysctl(8) variable kern.sugid_coredump to 1. Corefiles can be compressed by the kernel if one of the following items are included in the kernel configuration file: options GZIO options ZSTDIO The following sysctl control core file compression: kern.compress_user_cores Enable compression of user cores. A value of 1 configures gzip(1) compres- sion, and a value of 2 configures zstd(1) compression. Compressed core files will have a suffix of `.gz' or `.zst' appended to their filenames de- pending on the selected format. kern.compress_user_cores_level Compression level. Defaults to 6. NOTES Corefiles are written with open file descriptor information as an ELF note. By default, file paths are packed to only use as much space as needed. However, file paths can change at any time, including during core dump, and this can result in truncated file descriptor data. All file descriptor information can be preserved by disabling packing. This potentially wastes up to PATH_MAX bytes per open fd. Packing is dis- abled with sysctl kern.coredump_pack_fileinfo=0. Similarly, corefiles are written with vmmap information as an ELF note, which contains file paths. By default, they are packed to only use as much space as needed. By the same mechanism as for the open files note, these paths can also change at any time and result in a truncated note. All vmmap information can be preserved by disabling packing. Like the file information, this potentially wastes up to PATH_MAX bytes per mapped ob- ject. Packing is disabled with sysctl kern.coredump_pack_vmmapinfo=0. EXAMPLES In order to store all core images in per-user private areas under /var/coredumps (assuming the appropriate subdirectories exist and are writable by users), the following sysctl(8) command can be used: sysctl kern.corefile=/var/coredumps/%U/%N.core SEE ALSO gdb(1)(ports/devel/gdb), gzip(1), kgdb(1)(ports/devel/gdb), setrlimit(2), sigaction(2), sysctl(8) HISTORY A core file format appeared in Version 1 AT&T UNIX. FreeBSD ports 15.1 July 17, 2025 CORE(5)
NAME | SYNOPSIS | DESCRIPTION | NOTES | EXAMPLES | SEE ALSO | HISTORY
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=core&sektion=5&manpath=FreeBSD+15.1-RELEASE+and+Ports>