The system's notion of the current Greenwich time and the current time zone is set and returned by the call by the calls: #include <sys/time.h> settimeofday(tvp, tzp); struct timeval *tp; struct timezone *tzp; gettimeofday(tp, tzp); result struct timeval *tp; result struct timezone *tzp; where the structures are defined in <sys/time.h> as: ._f struct timeval { long tv_sec; /* seconds since Jan 1, 1970 */ long tv_usec; /* and microseconds */ }; struct timezone { int tz_minuteswest; /* of Greenwich */ int tz_dsttime; /* type of dst correction to apply */ }; The precision of the system clock is hardware dependent. Earlier versions of UNIX contained only a 1-second resolution version of this call, which remains as a library routine: time(tvsec) result long *tvsec; returning only the tv_sec field from the gettimeofday call. Interval time

The system provides each process with three interval timers, defined in <sys/time.h>: ._d #define ITIMER_REAL 0 /* real time intervals */ #define ITIMER_VIRTUAL 1 /* virtual time intervals */ #define ITIMER_PROF 2 /* user and system virtual time */ The ITIMER_REAL timer decrements in real time. It could be used by a library routine to maintain a wakeup service queue. A SIGALRM signal is delivered when this timer expires.

The ITIMER_VIRTUAL timer decrements in process virtual time. It runs only when the process is executing. A SIGVTALRM signal is delivered when it expires.

The ITIMER_PROF timer decrements both in process virtual time and when the system is running on behalf of the process. It is designed to be used by processes to statistically profile their execution. A SIGPROF signal is delivered when it expires.

A timer value is defined by the itimerval structure: ._f struct itimerval { struct timeval it_interval; /* timer interval */ struct timeval it_value; /* current value */ }; and a timer is set or read by the call: getitimer(which, value); int which; result struct itimerval *value; setitimer(which, value, ovalue); int which; struct itimerval *value; result struct itimerval *ovalue; The third argument to setitimer specifies an optional structure to receive the previous contents of the interval timer. A timer can be disabled by specifying a timer value of 0.

The system rounds argument timer intervals to be not less than the resolution of its clock. This clock resolution can be determined by loading a very small value into a timer and reading the timer back to see what value resulted.

The alarm system call of earlier versions of UNIX is provided as a library routine using the ITIMER_REAL timer. The process profiling facilities of earlier versions of UNIX remain because it is not always possible to guarantee the automatic restart of system calls after receipt of a signal. The profil call arranges for the kernel to begin gathering execution statistics for a process: profil(buf, bufsize, offset, scale); result char *buf; int bufsize, offset, scale; This begins sampling of the program counter, with statistics maintained in the user-provided buffer.