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The operating system writes out a core file for a process when the process is terminated due to the receipt of certain signals. A core file is a disk copy of the contents of the process address space at the time the process received the signal, along with additional information about the state of the process. This information can be consumed by a debugger. Core files can also be generated by applying the gcore(1) utility to a running process.

Typically, core files are produced following abnormal termination of a process resulting from a bug in the corresponding application. Whatever the cause, the core file itself provides invaluable information to the programmer or support engineer to aid in diagnosing the problem. The core file can be inspected using a debugger such as dbx(1) or mdb(1) or by applying one of the proc(1) tools.

The operating system attempts to create up to two core files for each abnormally terminating process, using a global core file name pattern and a per-process core file name pattern. These patterns are expanded to determine the pathname of the resulting core files, and can be configured by coreadm(1M). By default, the global core file pattern is disabled and not used, and the per-process core file pattern is set to core. Therefore, by default, the operating system attempts to create a core file named core in the process’s current working directory.

A process will terminate and produce a core file whenever it receives one of the signals whose default disposition is to cause a core dump. The list of signals that result in generating a core file is shown in signal(3HEAD). Therefore, a process might not produce a core file if it has blocked or modified the behavior of the corresponding signal. Additionally, no core dump can be created under the following conditions:

	•		If normal file and directory access permissions prevent the creation or modification of the per-process core file pathname by the current process user and group ID. This test does not apply to the global core file pathname because the global core file is always written as the super-user.
	•		If the core file pattern expands to a pathname that contains intermediate directory components that do not exist. For example, if the global pattern is set to /var/core/%n/core.%p, and no directory /var/core/’uname -n’ has been created, no global core files will be produced.
	•		If the destination directory is part of a filesystem that is mounted read-only.
	•		If the resource limit RLIMIT_CORE has been set to 0 for the process. Refer to setrlimit(2) and ulimit(1) for more information on resource limits.
	•		If the core file name already exists in the destination directory and is not a regular file (that is, is a symlink, block or character special-file, and so forth).
	•		If the kernel cannot open the destination file O_EXCL, which can occur if same file is being created by another process simultaneously.
	•		If the process’s effective user ID is different from its real user ID or if its effective group ID is different from its real group ID. Similarly, set-user-ID and set-group-ID programs do not produce core files as this could potentially compromise system security. These processes can be explicitly granted permission to produce core files using coreadm(1M), at the risk of exposing secure information.

The core file contains all the process information pertinent to debugging: contents of hardware registers, process status, and process data. The format of a core file is object file specific.

For ELF executable programs (see a.out(4)), the core file generated is also an ELF file, containing ELF program and file headers. The e_type field in the file header has type ET_CORE. The program header contains an entry for every segment that was part of the process address space, including shared library segments. The contents of the writable segments are also part of the core image.

The program header of an ELF core file also contains entries for two NOTE segments, each containing several note entries as described below. The note entry header and core file note type (n_type) definitions are contained in <sys/elf.h>. The first NOTE segment exists for binary compatibility with old programs that deal with core files. It contains structures defined in <sys/old_procfs.h>. New programs should recognize and skip this NOTE segment, advancing instead to the new NOTE segment. The old NOTE segment will be deleted from core files in a future release.

The old NOTE segment contains the following entries. Each has entry name "CORE" and presents the contents of a system structure:
prpsinfo_t

n_type : NT_PRPSINFO. This entry contains information of interest to the ps(1) command, such as process status, CPU usage, "nice" value, controlling terminal, user-ID, process-ID, the name of the executable, and so forth. The prpsinfo_t structure is defined in <sys/old_procfs.h>.

char array

n_type: NT_PLATFORM. This entry contains a string describing the specific model of the hardware platform on which this core file was created. This information is the same as provided by sysinfo(2) when invoked with the command SI_PLATFORM.

auxv_t array

n_type: NT_AUXV. This entry contains the array of auxv_t structures that was passed by the operating system as startup information to the dynamic linker. Auxiliary vector information is defined in <sys/auxv.h>.

Following these entries, for each light-weight process (LWP) in the process, the old NOTE segment contains an entry with a prstatus_t structure, plus other optionally-present entries describing the LWP, as follows:
prstatus_t

n_type: NT_PRSTATUS. This structure contains things of interest to a debugger from the operating system, such as the general registers, signal dispositions, state, reason for stopping, process-ID, and so forth. The prstatus_t structure is defined in <sys/old_procfs.h>.

prfpregset_t

n_type: NT_PRFPREG. This entry is present only if the LWP used the floating-point hardware. It contains the floating-point registers. The prfpregset_t structure is defined in <sys/procfs_isa.h>.

gwindows_t

n_type: NT_GWINDOWS. This entry is present only on a SPARC machine and only if the system was unable to flush all of the register windows to the stack. It contains all of the unspilled register windows. The gwindows_t structure is defined in <sys/regset.h>.

prxregset_t

n_type: NT_PRXREG. This entry is present only if the machine has extra register state associated with it. It contains the extra register state. The prxregset_t structure is defined in <sys/procfs_isa.h>.

The new NOTE segment contains the following entries. Each has entry name "CORE" and presents the contents of a system structure:
psinfo_t

n_type: NT_PSINFO. This structure contains information of interest to the ps(1) command, such as process status, CPU usage, "nice" value, controlling terminal, user-ID, process-ID, the name of the executable, and so forth. The psinfo_t structure is defined in <sys/procfs.h>.

pstatus_t

n_type: NT_PSTATUS. This structure contains things of interest to a debugger from the operating system, such as pending signals, state, process-ID, and so forth. The pstatus_t structure is defined in <sys/procfs.h>.

char array

auxv_t array

struct utsname

n_type: NT_UTSNAME. This structure contains the system information that would have been returned to the process if it had performed a uname(2) system call prior to dumping core. The utsname structure is defined in <sys/utsname.h>.

prcred_t

n_type: NT_PRCRED. This structure contains the process credentials, including the real, saved, and effective user and group IDs. The prcred_t structure is defined in <sys/procfs.h>. Following the structure is an optional array of supplementary group IDs. The total number of supplementary group IDs is given by the pr_ngroups member of the prcred_t structure, and the structure includes space for one supplementary group. If pr_ngroups is greater than 1, there will be pr_ngroups - 1 gid_t items following the structure; otherwise, there will be no additional data.

Following these entries, for each LWP in the process, the new NOTE segment contains an entry with an lwpsinfo_t structure plus an entry with an lwpstatus_t structure, plus other optionally-present entries describing the LWP, as follows:
lwpsinfo_t

n_type: NT_LWPSINFO. This structure contains information of interest to the ps(1) command, such as LWP status, CPU usage, "nice" value, LWP-ID, and so forth. The lwpsinfo_t structure is defined in <sys/procfs.h>.

lwpstatus_t

n_type: NT_LWPSTATUS. This structure contains things of interest to a debugger from the operating system, such as the general registers, the floating point registers, state, reason for stopping, LWP-ID, and so forth. The lwpstatus_t structure is defined in <sys/procfs.h>.

gwindows_t

prxregset_t

asrset_t

n_type: NT_ASRS. This entry is present only on a SPARC V9 machine and only if the process is a 64-bit process. It contains the ancillary state registers for the LWP. The asrset_t structure is defined in <sys/regset.h>.

The size of the core file created by a process may be controlled by the user (see getrlimit(2)).

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