Understanding the Linux Kernel, Second 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( ), ftime( ), 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).

ftime( )

Returns, in a data structure of type timeb, the number of elapsed seconds since midnight of January 1, 1970 (UTC) and the number of elapsed milliseconds in the last second.

gettimeofday( )

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

The former system calls are superseded by gettimeofday( ), but they are still included in Linux for backward compatibility. We don’t discuss them further.

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. Disables the interrupts and acquires the xtime_lock read/write spin lock for reading.

2. Gets the number of microseconds elapsed in the last second by using the function whose address is stored in do_gettimeoffset:

       usec = do_gettimeoffset( );

   If the CPU has a Time Stamp Counter, the do_fast_gettimeoffset( ...