NAME
core —
memory image file format
SYNOPSIS
#include
<sys/param.h>
#include <sys/core.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 sigaction(2)). This memory image is written to a file named 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 maximum size of a programname.core file is limited by setrlimit(2). Files which would be larger than the limit are not created.
The programname.core file consists of the
u-area, whose size (in pages) is defined by the
UPAGES manifest in the
<machine/param.h> file. The
u-area starts with a user structure as given in
<sys/user.h>. The remainder
of the programname.core file consists of the data
pages followed by the stack pages of the process image. The amount of data
space image in the programname.core file is given
(in pages) by the variable u_dsize in the u-area. The
amount of stack image in the core file is given (in pages) by the variable
u_ssize in the u-area. The size of a
“page” is given by the constant
PAGE_SIZE, defined in
<machine/param.h>. The
user structure is defined as:
struct user {
struct pcb u_pcb;
struct pstats u_stats;
/*
* Remaining fields only for core dump and/or ptrace--
* not valid at other times!
*/
struct kinfo_proc u_kproc;
struct md_coredump u_md;
};
md_coredump is defined in the header file
<machine/pcb.h>.
The on-disk core file consists of a header followed by a number of segments. Each segment is preceded by a coreseg structure giving the segment's type, the virtual address where the bits resided in process address space and the size of the segment.
The core header specifies the lengths of the core header itself and each of the following core segment headers to allow for any machine dependent alignment requirements.
struct coreseg {
u_int32_t c_midmag; /* magic, id, flags */
u_long c_addr; /* Virtual address of segment */
u_long c_size; /* Size of this segment */
};
struct core {
u_int32_t c_midmag; /* magic, id, flags */
u_int16_t c_hdrsize; /* Size of this header (machdep algn) */
u_int16_t c_seghdrsize; /* Size of a segment header */
u_int32_t c_nseg; /* # of core segments */
char c_name[MAXCOMLEN+1]; /* Copy of p->p_comm */
u_int32_t c_signo; /* Killing signal */
u_long c_ucode; /* Hmm ? */
u_long c_cpusize; /* Size of machine dependent segment */
u_long c_tsize; /* Size of traditional text segment */
u_long c_dsize; /* Size of traditional data segment */
u_long c_ssize; /* Size of traditional stack segment */
};
The core structure's c_midmag field is an
a.out-style midmag number with a COREMAGIC magic
number. and flags from the following list:
#define CORE_CPU 1 #define CORE_DATA 2 #define CORE_STACK 4
SEE ALSO
HISTORY
A core file format appeared in
Version 3 AT&T UNIX.
CAVEATS
Programs which are started with either the set-user-ID or
set-group-ID bits set, or which change their UID or GID after starting, will
normally not dump core. This is to prevent sensitive information from
inadvertently ending up on disk. This behaviour can be changed (for
debugging purposes) by changing the
kern.nosuidcoredump
sysctl(3) variable to the right settings.