I noticed some programmers use unsigned long for tv_sec and tv_usec [when they copy them or operate with them] of timeval while they are defined as simply long.
Though it does make me wonder why they were defined like that when time usually goes forward.
Using long int for those variables will work until year 2038, and after that the tv_sec will overflow on machines where long is 4bytes.
The <sys/time.h> header shall define the timeval structure that includes at least the following members:
time_t tv_sec Seconds.
suseconds_t tv_usec Microseconds.
You should notice that time_t type is used instead of long, but which is also a 32bit representation on some systems while there are even 64bit representations on other systems. In order to avoid overflow, time_t will probably be changed to an unsigned 32bit integer or a 64bit one.
That is why some are using unsigned long, as it will stop the overflow until year 2100+. You should use the time_t type instead, and you won't need to think about how long your program is supposed to run for in the future.