errno 和 strerror函数

今天写了一个很简单的代码，编译时没啥错误和警告（主要编译选项没开启警告），然后运行时居然 segmentation fault，把我给看傻了，代码如下：

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

int main()
{
    int fd = open("readme",  O_RDWR);
    if(fd < 0)
    {
        printf("open fail: %s\n", strerror(errno));
        return fd;
    }

    return 0;
}

代码简单吧，就把 errno 对应的字符串打印出来，结果就 Segmentation fault 了，搞得我以为用法不对呢，我还去 man 了一个 strerror() 函数的用法：

用 %s 打印 char * 类型，是对的啊，怎么会这样呢？搞得我还去看了源码，顺便学习了一下 errno，原因后面再讲到。先看一下函数 strerror() 的源码：

/* Copyright (C) 1991-2022 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.
   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.
   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */
#include <string.h>
#include <locale/localeinfo.h>
char *
strerror (int errnum)
{
  return __strerror_l (errnum, __libc_tsd_get (locale_t, LOCALE));
}

再看 __strerror_l 函数的源码：

/* Return a string describing the errno code in ERRNUM.  */
char *
__strerror_l (int errnum, locale_t loc)
{
  int saved_errno = errno;
  char *err = (char *) __get_errlist (errnum);
  if (__glibc_unlikely (err == NULL))
    {
      struct tls_internal_t *tls_internal = __glibc_tls_internal ();
      free (tls_internal->strerror_l_buf);
      if (__asprintf (&tls_internal->strerror_l_buf, "%s%d",
		      translate ("Unknown error ", loc), errnum) == -1)
	tls_internal->strerror_l_buf = NULL;
      err = tls_internal->strerror_l_buf;
    }
  else
    err = (char *) translate (err, loc);
  __set_errno (saved_errno);
  return err;
}

接着往下看 __get_errlist() 这个函数：

#ifndef ERR_MAP
# define ERR_MAP(n) n
#endif
const char *const _sys_errlist_internal[] =
  {
#define _S(n, str)         [ERR_MAP(n)] = str,
#include <errlist.h>
#undef _S
  };
const char *
__get_errlist (int errnum)
{
  int mapped = ERR_MAP (errnum);
  if (mapped >= 0 && mapped < array_length (_sys_errlist_internal))
    return _sys_errlist_internal[mapped];
  return NULL;
}

最后就是直接返回对应的指针数组元素值，这个指针数组初始化方式可以参考上一篇文件这里。定义了宏 #define _S(n, str) [ERR_MAP(n)] = str 其实就是 _S(n,str) 被替换为 [n]=str，即为元素初始化，而在 errlist.h 头文件里使用了该宏：

_S(0, N_("Success"))
#ifdef EPERM
/*
TRANS Only the owner of the file (or other resource)
TRANS or processes with special privileges can perform the operation. */
_S(EPERM, N_("Operation not permitted"))
#endif
#ifdef ENOENT
/*
TRANS This is a ``file doesn't exist'' error
TRANS for ordinary files that are referenced in contexts where they are
TRANS expected to already exist. */
_S(ENOENT, N_("No such file or directory"))
#endif
#ifdef ESRCH
/*
TRANS No process matches the specified process ID. */
_S(ESRCH, N_("No such process"))
#endif
#ifdef EINTR

/*往下还有很多，这里就不完整展现了*/

而这些 EPERM、ENOENT、ESRCH 等待，就是 errno 的值，这里初始化了 errno 对应的错误字符串，所以经过预编译，_sys_errlist_internal 数组的值应该类似这样：

const char *const _sys_errlist_internal[] =
{
    [0] = "Success",
    [1] = "Operation not permitted",
    [2] = "No such file or directory",
    [3] = "No such process",
    [4] = "Interrupted system call",
    /*.....*/
};

/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
#ifndef _ASM_GENERIC_ERRNO_BASE_H
#define _ASM_GENERIC_ERRNO_BASE_H
#define	EPERM		 1	/* Operation not permitted */
#define	ENOENT		 2	/* No such file or directory */
#define	ESRCH		 3	/* No such process */
#define	EINTR		 4	/* Interrupted system call */
#define	EIO		 5	/* I/O error */
#define	ENXIO		 6	/* No such device or address */
#define	E2BIG		 7	/* Argument list too long */
#define	ENOEXEC		 8	/* Exec format error */
#define	EBADF		 9	/* Bad file number */
#define	ECHILD		10	/* No child processes */
#define	EAGAIN		11	/* Try again */
#define	ENOMEM		12	/* Out of memory */
#define	EACCES		13	/* Permission denied */
#define	EFAULT		14	/* Bad address */
#define	ENOTBLK		15	/* Block device required */
#define	EBUSY		16	/* Device or resource busy */
#define	EEXIST		17	/* File exists */
#define	EXDEV		18	/* Cross-device link */
#define	ENODEV		19	/* No such device */
#define	ENOTDIR		20	/* Not a directory */
#define	EISDIR		21	/* Is a directory */
#define	EINVAL		22	/* Invalid argument */
#define	ENFILE		23	/* File table overflow */
#define	EMFILE		24	/* Too many open files */
#define	ENOTTY		25	/* Not a typewriter */
#define	ETXTBSY		26	/* Text file busy */
#define	EFBIG		27	/* File too large */
#define	ENOSPC		28	/* No space left on device */
#define	ESPIPE		29	/* Illegal seek */
#define	EROFS		30	/* Read-only file system */
#define	EMLINK		31	/* Too many links */
#define	EPIPE		32	/* Broken pipe */
#define	EDOM		33	/* Math argument out of domain of func */
#define	ERANGE		34	/* Math result not representable */
#endif

到这里我们已经知道函数 strerror() 是怎么实现的了，而且也知道函数使用方法也是对的，但为什么会出现 Segmentation fault 呢？原因是没包含头文件，导致 strerror（）函数变成隐式声明了，而隐式声明的函数它的默认返回值是 int 类型（没找到C语言关于这点的说明文档，有同学知道的话烦告知一下），所以用 %s 打印 int 类型导致 segmentation fault 了。所以改用 %d 来打印还会出现段错误吗？

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

const char *const _sys_errlist_internal[] =
{
    [0] = "Success",
    [1] = "Operation not permitted",
    [2] = "No such file or directory",
    [3] = "No such process",
    [4] = "Interrupted system call",
    /*.....*/
};
int main()
{
    int fd = open("readme",  O_RDWR);
    if(fd < 0)
    {
        printf("open fail: %d\n", strerror(errno));
        return fd;
    }
    
    return 0;
}