Understanding the Linux Kernel, 3rd Edition

System Calls Related to Timing Measurements

Several system calls allow User Mode processes to read and modify the time and date and to create timers. Let’s briefly review these and discuss how the kernel handles them.

The time( ) and gettimeofday( ) System Calls

Processes in User Mode can get the current time and date by means of several system calls:

time( )

Returns the number of elapsed seconds since midnight at the start of January 1, 1970 (UTC).

gettimeofday( )

Returns, in a data structure named timeval, the number of elapsed seconds since midnight of January 1, 1970 (UTC) and the number of elapsed microseconds in the last second (a second data structure named timezone is not currently used).

The time( ) system call is superseded by gettimeofday( ), but it is still included in Linux for backward compatibility. Another widely used function, ftime( ), which is no longer implemented as a system call, returns the number of elapsed seconds since midnight of January 1, 1970 (UTC) and the number of elapsed milliseconds in the last second.

The gettimeofday( ) system call is implemented by the sys_gettimeofday( ) function. To compute the current date and time of the day, this function invokes do_gettimeofday( ), which executes the following actions:

1. Acquires the xtime_lock seqlock for reading.

2. Determines the number of microseconds elapsed since the last timer interrupt by invoking the get_offset method of the cur_timer timer object:

       usec = cur_timer->getoffset( );

   As explained in the earlier ...