glibc的写工作如何? [英] How does glibc's write work?

问题描述

我试图用-nostdlib编译一个名为write的简单程序，但出现错误:

I tried to compile a simple program that called write with -nostdlib, but I got the error:

/path/to/file:3: undefined reference to `write'

我认为write是Unix上的东西，并且一直存在，但显然不是，事实证明libc具有write功能.我找到了源代码:

I thought write was a Unix thing and was always there, but apparently not, turns out libc has write function. I found the source code:

/* Write NBYTES of BUF to FD.  Return the number written, or -1.  */
ssize_t
__libc_write (int fd, const void *buf, size_t nbytes)
{
    if (nbytes == 0)
        return 0;
    if (fd < 0)
    {
        __set_errno (EBADF);
        return -1;
    }
    if (buf == NULL)
    {
        __set_errno (EINVAL);
        return -1;
    }

    __set_errno (ENOSYS);
    return -1;
}
libc_hidden_def (__libc_write)
stub_warning (write)

weak_alias (__libc_write, __write)
libc_hidden_weak (__write)
weak_alias (__libc_write, write)

这似乎是在设置errno. __libc_write如何写入文件描述符?

All this seems to be doing is setting errno. How does __libc_write write to a file descriptor?

推荐答案

您对C和unix有一些误解.

我认为write是Unix上的东西

I thought write was a Unix thing

write当然是系统调用，它是 POSIX 标准. POSIX是与unix(一个非常老的操作系统)兼容的标准.

Definitely, write is a syscall which is a part of the POSIX standard. POSIX is a standard that is compatible with unix (a very old operating system).

这意味着名为write的 syscall 存在于任何与Unix或POSIX兼容的操作系统(如Linux)中.但是，如果您没有C标准库的实现(例如Linux中的glibc或Android中的bionic)-您将如何进行系统调用(即要求OS的内核执行某项操作)? Syscall依赖于体系结构(SPARC，Intel，ARM，PowerPC等将具有不同的实现)-并由C标准库实现.如果您没有该库(例如，当您遵循-nostdlib时，您明确要求不使用C标准库)-您无法调用write，则该函数不存在.但是，您可以自己使用汇编来实现它.

This means that the syscall called write exists in any unix or POSIX-compliant (like Linux) operating system. However, if you don't have a an implementation of the C standard library (like glibc in Linux or bionic in Android) - how are you going to make a syscall (i.e. ask the kernel of the OS to do something)? Syscalls are architecture dependent (SPARC, Intel, ARM, PowerPC etc will have different implementations) - and are implemented by the C standard library. If you don't have one (e.g when you complie with -nostdlib you specifically ask not to have a C standard library) - you can't call write, the function doesn't exist. You can, however, implement it yourself using assembly.

您提供的代码是针对__libc_write的，与write不同.在glibc和gcc之间发生了一些伏都教，这与存根有关，我不完全理解.

The code you provided is for __libc_write, which is not the same as write. There's some voodoo that's going on between glibc and gcc here, something that has to do with stubs and I don't completely understand it.

简单的实现是这样的:

ssize_t write(int fd, const void *buf, size_t count)
{
    return __set_errno(syscall(__NR_write, fd, buf, count));
}

(在Android的libc实现bionic中，您可以在此处查看源代码 https://searchcode .com/codesearch/view/34924443/-看到它是在汇编中实现的

(In bionic, Android's libc implementation, you can see the sources here https://searchcode.com/codesearch/view/34924443/ - see that it is implemented in assembly)

syscall函数在glibc中(以及其他任何位置)以汇编形式实现:

The syscall function is implemented in assembly in glibc (and anywhere else): http://code.metager.de/source/xref/gnu/glibc/sysdeps/unix/sysv/linux/x86_64/syscall.S (for x86_64)

但是，如果在调用write时必须真正了解glibc中发生的事情，则有点复杂.据我了解，它调用_IO_new_file_write(已实现这里). _IO_new_file_write使用定义的write_not_cancel宏此处，

However, if you must really know what's happening in glibc when you call write - it's a bit complicated. As far as I understood it, it calls _IO_new_file_write (implemented here). _IO_new_file_write uses the write_not_cancel macro defined here which is a macro for INLINE_SYSCALL (write, 3, fd, buf, len), which is the implementation for calling a syscall and setting errno.

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