如何在linux上实现clock_gettime？

当我strace这样的：如何在linux上实现clock_gettime？

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

int main() 
{ 
    struct timespec ts; 

    fprintf(stderr, "start!\n"); 

    clock_gettime(CLOCK_REALTIME, &ts); 
    fprintf(stderr, "realtime %lu %lu\n", ts.tv_sec, ts.tv_nsec); 

    clock_gettime(CLOCK_MONOTONIC, &ts); 
    fprintf(stderr, "monotonic %lu %lu\n", ts.tv_sec, ts.tv_nsec); 

    return 0; 
}

我得到这个：

execve("/tmp/x", ["/tmp/x"], [/* 72 vars */]) = 0 
brk(NULL)        = 0x13d0000 
access("/etc/ld.so.nohwcap", F_OK)  = -1 ENOENT (No such file or directory) 
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fed9a0d1000 
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=126501, ...}) = 0 
mmap(NULL, 126501, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fed9a0b2000 
close(3)        = 0 
access("/etc/ld.so.nohwcap", F_OK)  = -1 ENOENT (No such file or directory) 
open("/lib/x86_64-linux-gnu/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\0P\t\2\0\0\0\0\0"..., 832) = 832 
fstat(3, {st_mode=S_IFREG|0755, st_size=1864888, ...}) = 0 
mmap(NULL, 3967488, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fed99ae5000 
mprotect(0x7fed99ca5000, 2093056, PROT_NONE) = 0 
mmap(0x7fed99ea4000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1bf000) = 0x7fed99ea4000 
mmap(0x7fed99eaa000, 14848, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fed99eaa000 
close(3)        = 0 
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fed9a0b1000 
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fed9a0b0000 
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fed9a0af000 
arch_prctl(ARCH_SET_FS, 0x7fed9a0b0700) = 0 
mprotect(0x7fed99ea4000, 16384, PROT_READ) = 0 
mprotect(0x600000, 4096, PROT_READ)  = 0 
mprotect(0x7fed9a0d3000, 4096, PROT_READ) = 0 
munmap(0x7fed9a0b2000, 126501)   = 0 
write(2, "start!\n", 7start! 
)     = 7 
write(2, "realtime 1488919932 97097045\n", 29realtime 1488919932 97097045 
) = 29 
write(2, "monotonic 258985 170149836\n", 27monotonic 258985 170149836 
) = 27 
exit_group(0)       = ? 
+++ exited with 0 +++

似乎有不被系统调用涉及制备任CLOCK_REALTIME或CLOCK_MONOTONIC值。这是如何实施的？我试图用汇编语言来介绍它，但是我一定没有注意到关键部分，因为我无法弄清楚它是如何完成的。

来源

2017-03-07 DepressedDaniel

这是一个虚拟的系统调用。也就是说，调用它不需要切换到内核模式 - 它在用户模式下执行以提高性能。如果您想知道ldd输出中的linux-vdso.so.1是什么意思，那么这些虚拟系统调用就是在这里实现的。您可以了解更多here。

来源

2017-03-07 21:12:49

如何在linux上实现clock_gettime？

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