__函数声明后的asm（“isoc99扫描”）

我在预处理的C代码中看到了以下代码。asm在函数声明之后做什么

extern int scanf (const char *__restrict __format, ...) __asm__ ("" "__isoc99_scanf");

---

显然，它使函数调用编译为“call uu isoc99_scanf”而不是“call scanf”。这是C/GCC标准语法吗？

我刚刚在

stdio common/isoc99\u scanf.C

下的

glibc

中找到了它的实现，如下所示

/* Copyright (C) 1991-2019 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
   <http://www.gnu.org/licenses/>.  */
#include <stdarg.h>
#include <stdio.h>
#include <libioP.h>
/* Read formatted input from stdin according to the format string FORMAT.  */
int
__isoc99_scanf (const char *format, ...)
{
  va_list arg;
  int done;
  va_start (arg, format);
  done = __vfscanf_internal (stdin, format, arg, SCANF_ISOC99_A);
  va_end (arg);
  return done;
}

 #if defined __USE_ISOC99 && !defined __USE_GNU \
     && (!defined __LDBL_COMPAT || !defined __REDIRECT) \
     && (defined __STRICT_ANSI__ || defined __USE_XOPEN2K)
 # ifdef __REDIRECT
 /* For strict ISO C99 or POSIX compliance disallow %as, %aS and %a[
    GNU extension which conflicts with valid %a followed by letter
    s, S or [.  */
 extern int __REDIRECT (fscanf, (FILE *__restrict __stream,
                                 __const char *__restrict __format, ...),
                        __isoc99_fscanf) __wur;
 extern int __REDIRECT (scanf, (__const char *__restrict __format, ...),
                        __isoc99_scanf) __wur;
 extern int __REDIRECT (sscanf, (__const char *__restrict __s,
                                 __const char *__restrict __format, ...),
                        __isoc99_sscanf) __THROW;
 # else
 extern int __isoc99_fscanf (FILE *__restrict __stream,
                             __const char *__restrict __format, ...) __wur;
 extern int __isoc99_scanf (__const char *__restrict __format, ...) __wur;
 extern int __isoc99_sscanf (__const char *__restrict __s,
                             __const char *__restrict __format, ...) __THROW;
 #  define fscanf __isoc99_fscanf
 #  define scanf __isoc99_scanf
 #  define sscanf __isoc99_sscanf
 # endif
 #endif

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你在手册里查过吗？这肯定是编译器特有的语法。类似的语法。@MartinRosenau:这正是为asm/链接设置符号名的GNU C语法。

“foo”

只是毫无意义的字符串连接，可能是C预处理器的结果。@MartinRosenau-您提供的链接指向一个相当旧的gcc文档版本（4.4）。这句话说的差不多，但可能更清楚一点。