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_midmagfield
is an a.out midmag number with a
COREMAGIC
magic number (see
a.out(5))
and flags from the following list:
#define CORE_CPU 1
#define CORE_DATA 2
#define CORE_STACK 4