acct("/var/log/pacct");
When process accounting is enabled, the kernel writes a record to the accounting file as each process on the system terminates. This record contains information about the terminated process, and is defined in <sys/acct.h> as follows:
#define ACCT_COMM 16
typedef u_int16_t comp_t;
struct acct {
char ac_flag; /* Accounting flags */
u_int16_t ac_uid; /* Accounting user ID */
u_int16_t ac_gid; /* Accounting group ID */
u_int16_t ac_tty; /* Controlling terminal */
u_int32_t ac_btime; /* Process creation time
(seconds since the Epoch) */
comp_t ac_utime; /* User CPU time */
comp_t ac_stime; /* System CPU time */
comp_t ac_etime; /* Elapsed time */
comp_t ac_mem; /* Average memory usage (kB) */
comp_t ac_io; /* Characters transferred (unused) */
comp_t ac_rw; /* Blocks read or written (unused) */
comp_t ac_minflt; /* Minor page faults */
comp_t ac_majflt; /* Major page faults */
comp_t ac_swaps; /* Number of swaps (unused) */
u_int32_t ac_exitcode; /* Process termination status
(see wait(2)) */
char ac_comm[ACCT_COMM+1];
/* Command name (basename of last
executed command; null-terminated) */
char ac_pad[X]; /* padding bytes */
};
enum { /* Bits that may be set in ac_flag field */
AFORK = 0x01, /* Has executed fork, but no exec */
ASU = 0x02, /* Used superuser privileges */
ACORE = 0x08, /* Dumped core */
AXSIG = 0x10 /* Killed by a signal */
};
The comp_t data type is a floating-point value consisting of a 3-bit, base-8 exponent, and a 13-bit mantissa. A value, c, of this type can be converted to a (long) integer as follows:
v = (c & 0x1fff) << (((c >> 13) & 0x7) * 3);
The ac_utime, ac_stime, and ac_etime fields measure time in "clock ticks"; divide these values by sysconf(_SC_CLK_TCK) to convert them to seconds.
struct acct_v3 {
char ac_flag; /* Flags */
char ac_version; /* Always set to ACCT_VERSION (3) */
u_int16_t ac_tty; /* Controlling terminal */
u_int32_t ac_exitcode; /* Process termination status */
u_int32_t ac_uid; /* Real user ID */
u_int32_t ac_gid; /* Real group ID */
u_int32_t ac_pid; /* Process ID */
u_int32_t ac_ppid; /* Parent process ID */
u_int32_t ac_btime; /* Process creation time */
float ac_etime; /* Elapsed time */
comp_t ac_utime; /* User CPU time */
comp_t ac_stime; /* System time */
comp_t ac_mem; /* Average memory usage (kB) */
comp_t ac_io; /* Characters transferred (unused) */
comp_t ac_rw; /* Blocks read or written
(unused) */
comp_t ac_minflt; /* Minor page faults */
comp_t ac_majflt; /* Major page faults */
comp_t ac_swaps; /* Number of swaps (unused) */
char ac_comm[ACCT_COMM]; /* Command name */
};
In kernels up to and including 2.6.9, a separate accounting record is written for each thread created using the NPTL threading library; since Linux 2.6.10, a single accounting record is written for the entire process on termination of the last thread in the process.
The proc/sys/kernel/acct file, described in proc(5), defines settings that control the behavior of process accounting when disk space runs low.