core(FP)

core -- core image file

Description

The UNIX system writes out a core image of a process when it is terminated due to the receipt of some signals. The core image is called core.procid and is written in the process's current directory (provided it can be; normal access controls apply). procid is the process identifier of the process receiving the signal. A process with an effective user ID different from the real user ID will not produce a core image.

The core image contains information that allows user-level tools to determine the cause of process termination and its state at termination. This information includes the contents of hardware registers, process status, and the writable segments of the process including its stack. The status information in a core file is divided into process-wide information and information specific to each Light Weight Process (LWP) in the address space.

For both COFF and ELF executable programs [see a.out(F)], the core file generated is an ELF file, consisting of the images of the process' data segments plus per-LWP status information. The e_type field in the file header has type ET_CORE.

A process is composed of several discrete areas of memory or segments. Each segment comprise one or more areas with identical permissions called regions. Each core file contains a program header table. There is a program header entry for every region of the process at termination. This includes all regions from the shared objects used by the process. If a region had read-only permission and was never written to during the process lifetime, then the contents of that region is not dumped to the core file. The p_filesz member of its program header will be zero. Each writable region of the process plus each region that was read-only at process termination but was writable at some point in the lifetime of the process is dumped to the core image.

A core file contains a NOTE section containing information common to the process as a whole and an additional NOTE section for each LWP existing in the process at termination. Each of these sections has an accompanying program header entry. The NOTE section containing information common to the process as a whole contains up to four entries. Each entry has the name CORE and presents the contents of a system structure:

prstatus_t: The entry containing this structure has a NOTE type of CF_T_PRSTATUS. This structure contains things of interest to a debugger from the operating system's u-area, such as system call trace masks, signal dispositions, process ID and so forth. This information is common to all LWPs in the process but this entry will also contain information on the LWP that encountered the signal that resulted in the process' termination. The structure is defined in <sys/procfs.h>.
prpsinfo_t: The entry containing this structure has a NOTE type of CF_T_PRPSINFO. It contains information of interest to the ps(C) command that is common to all LWPs in the process, such as process status, cpu usage, ``nice'' value,
controlling terminal, user ID, process ID, the name of the executable and so forth. The structure is defined in <sys/procfs.h>.
prcred_t: The entry containing this structure has a NOTE type of CF_T_PRCRED. This entry contains the credential information for the terminated process. The structure is defined in <sys/procfs.h>.
struct utsname: The entry containing this structure has a NOTE type of CF_T_UTSNAME. It contains information about the system on which the process was running when it terminated. The structure is defined in <sys/utsname.h>.

Each LWP-specific NOTE section contains up to two entries, each named CORE. The entries contain the following system structures:

lwpstatus_t: The entry containing this structure has a NOTE type of CF_T_LWPSTATUS. This entry contains things of interest to a debugger from the LWP's u-area, such as the processor registers, signal mask, state, reason for stopping and others. The structure is defined in <sys/procfs.h>.
lwpsinfo_t: The entry containing this structure has a NOTE type of CF_T_LWPSINFO. This entry contains things of interest to the ps command, such as flags, status, priority, cpu usage. The structure is defined in <sys/procfs.h>.

The definitions for NOTE types are contained in the header file <sys/core.h>.

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

References

a.out(F), crash(ADM), debug(C), Intro(ELF), getrlimit(S), setuid(S), signal(M)