How does the gettimeofday syscall wor‍k?

Question

gettimeofday is a syscall of x86-86 according to this page(just search gettimeofday in the box):

int gettimeofday(struct timeval *t         


        

Answer 1

Syscalls generally create a lot of overhead, and given the abundance of gettimeofday() calls, one would prefer not to use a syscall. To that end, Linux kernel may map one or two special areas into each program, called vdso and vsyscall. Your implementation of gettimeofday() seems to be using vsyscall:

mov $0xffffffffff600000,%rax

This is the standard address of vsyscall map. Try cat /proc/self/maps to see that mapping. The idea behind vsyscall is that kernel provides fast user-space implementations of some functions, and libc just calls them.

Read this nice article for more details.