任何人都可以了解gettimeofday的作品？ [英] Anyone can understand how gettimeofday works?

问题描述

gettimeofday的是本页面（仅搜索的x86-86系统调用 gettimeofday的在框中）：

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

int gettimeofday(struct timeval *tv, struct timezone *tz);

我认为disas应该很容易anough，只要prepare两个指针，并调用相关的系统调用。

但其disas是做多：

But its disas is doing much more:

(gdb) disas gettimeofday
Dump of assembler code for function gettimeofday:
0x00000034f408c2d0 <gettimeofday+0>: sub    $0x8,%rsp
0x00000034f408c2d4 <gettimeofday+4>: mov    $0xffffffffff600000,%rax
0x00000034f408c2db <gettimeofday+11>: callq  *%rax
0x00000034f408c2dd <gettimeofday+13>: cmp    $0xfffff001,%eax
0x00000034f408c2e2 <gettimeofday+18>: jae    0x34f408c2e9 <gettimeofday+25>
0x00000034f408c2e4 <gettimeofday+20>: add    $0x8,%rsp
0x00000034f408c2e8 <gettimeofday+24>: retq   
0x00000034f408c2e9 <gettimeofday+25>: mov    0x2c4cb8(%rip),%rcx        # 0x34f4350fa8 <free+3356736>
0x00000034f408c2f0 <gettimeofday+32>: xor    %edx,%edx
0x00000034f408c2f2 <gettimeofday+34>: sub    %rax,%rdx
0x00000034f408c2f5 <gettimeofday+37>: mov    %edx,%fs:(%rcx)
0x00000034f408c2f8 <gettimeofday+40>: or     $0xffffffffffffffff,%rax
0x00000034f408c2fc <gettimeofday+44>: jmp    0x34f408c2e4 <gettimeofday+20>
End of assembler dump. 

和我没有看到系统调用的。

任何人都了解它是如何工作的？

Anyone understands how it works?

推荐答案

函数gettimeofday（）在Linux上什么叫做 vsyscall 和/或 VDSO 。因此，你看到了两行：

gettimeofday() on Linux is what's called a vsyscall and/or vdso. Hence you see the two lines:

0x00000034f408c2d4 : mov    $0xffffffffff600000,%rax
0x00000034f408c2db : callq  *%rax

在您的拆卸。地址 0xffffffffff600000 为vsyscall页面（在x86_64）。

in your disassembly. The address 0xffffffffff600000 is the vsyscall page (on x86_64).

该机制的特定内核创建code页面映射到用户内存，让一些系统调用可以在没有用户/内核上下文切换的开销进行，而是作为普通的函数调用。实际的实现是<一个href=\"http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=arch/x86/vdso/vclock_gettime.c;hb=HEAD\">right这里。

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. The actual implementation is right here.

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