times - get process times
#include <sys/times.h>
clock_t times(struct tms *buf);
times() stores the current process times in the
struct tms that
buf points to. The
struct tms is as defined in
<sys/times.h>:
struct tms {
clock_t tms_utime; /* user time */
clock_t tms_stime; /* system time */
clock_t tms_cutime; /* user time of children */
clock_t tms_cstime; /* system time of children */
};
The
tms_utime field contains the CPU time spent executing instructions of
the calling process. The
tms_stime field contains the CPU time spent
executing inside the kernel while performing tasks on behalf of the calling
process.
The
tms_cutime field contains the sum of the
tms_utime and
tms_cutime values for all waited-for terminated children. The
tms_cstime field contains the sum of the
tms_stime and
tms_cstime values for all waited-for terminated children.
Times for terminated children (and their descendants) are added in at the moment
wait(2) or
waitpid(2) returns their process ID. In particular,
times of grandchildren that the children did not wait for are never seen.
All times reported are in clock ticks.
times() returns the number of clock ticks that have elapsed since an
arbitrary point in the past. The return value may overflow the possible range
of type
clock_t. On error,
(clock_t) -1 is returned, and
errno is set appropriately.
- EFAULT
- tms points outside the process's address space.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
The number of clock ticks per second can be obtained using:
sysconf(_SC_CLK_TCK);
In POSIX.1-1996 the symbol
CLK_TCK (defined in
<time.h>) is
mentioned as obsolescent. It is obsolete now.
In Linux kernel versions before 2.6.9, if the disposition of
SIGCHLD is
set to
SIG_IGN, then the times of terminated children are automatically
included in the
tms_cstime and
tms_cutime fields, although
POSIX.1-2001 says that this should happen only if the calling process
wait(2)s on its children. This nonconformance is rectified in Linux
2.6.9 and later.
On Linux, the
buf argument can be specified as NULL, with the result that
times() just returns a function result. However, POSIX does not specify
this behavior, and most other UNIX implementations require a non-NULL value
for
buf.
Note that
clock(3) also returns a value of type
clock_t, but this
value is measured in units of
CLOCKS_PER_SEC, not the clock ticks used
by
times().
On Linux, the "arbitrary point in the past" from which the return
value of
times() is measured has varied across kernel versions. On
Linux 2.4 and earlier, this point is the moment the system was booted. Since
Linux 2.6, this point is
(2^32/HZ) - 300 seconds before system boot
time. This variability across kernel versions (and across UNIX
implementations), combined with the fact that the returned value may overflow
the range of
clock_t, means that a portable application would be wise
to avoid using this value. To measure changes in elapsed time, use
clock_gettime(2) instead.
SVr1-3 returns
long and the struct members are of type
time_t
although they store clock ticks, not seconds since the Epoch. V7 used
long for the struct members, because it had no type
time_t yet.
A limitation of the Linux system call conventions on some architectures (notably
i386) means that on Linux 2.6 there is a small time window (41 seconds) soon
after boot when
times() can return -1, falsely indicating that an error
occurred. The same problem can occur when the return value wraps past the
maximum value that can be stored in
clock_t.
time(1),
getrusage(2),
wait(2),
clock(3),
sysconf(3),
time(7)