gettime syscall

TheLinuxQuestioner · 03-01-2017, 11:36 AM

Hello,
I wanted to know whats the Implementation behind 'gettimeofday' syscall.

I made this testprogram:

Code:

#include <sys/time.h>
#include <stdio.h>

int main()
{
        struct timeval tv;
        while(1)
        {
                gettimeofday(&tv, NULL);
                printf("Zeit: %d s %d us\n", tv.tv_sec, tv.tv_usec);
                usleep(1000000);
        }
}

And called with strace ./time.

Code:

developer@linux-da23:~/development/trunk/projects/macs77> strace ./time
execve("./time", ["./time"], [/* 96 vars */]) = 0
brk(0)                                  = 0x16da000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4c119a7000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=158421, ...}) = 0
mmap(NULL, 158421, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f4c11980000
close(3)                                = 0
open("/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\10\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1925280, ...}) = 0
mmap(NULL, 3811872, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f4c113e5000
mprotect(0x7f4c1157f000, 2093056, PROT_NONE) = 0
mmap(0x7f4c1177e000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x199000) = 0x7f4c1177e000
mmap(0x7f4c11784000, 14880, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f4c11784000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4c1197f000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4c1197e000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4c1197d000
arch_prctl(ARCH_SET_FS, 0x7f4c1197e700) = 0
mprotect(0x7f4c1177e000, 16384, PROT_READ) = 0
mprotect(0x600000, 4096, PROT_READ)     = 0
mprotect(0x7f4c119a8000, 4096, PROT_READ) = 0
munmap(0x7f4c11980000, 158421)          = 0
fstat(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 0), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4c119a6000
write(1, "Zeit: 1488388237 s 962911 us\n", 29Zeit: 1488388237 s 962911 us
) = 29
nanosleep({1, 0}, NULL)                 = 0
write(1, "Zeit: 1488388238 s 963343 us\n", 29Zeit: 1488388238 s 963343 us
) = 29

I wonder why I dont see the call of 'gettimeofday' syscall.

I did a little research, could this be the reason?:

gettimeofday() on Linux is what's called a vsyscall and/or vdso. The mechanism maps a specific kernel-created code page into user memory, so that a few "syscalls" can be made without the overhead of a user/kernel context switch, but rather as "ordinary" function call.

NoStressHQ · 03-01-2017, 11:42 AM

Have you tried... https://github.com/lattera/glibc/sea...q=gettimeofday ??