我有两个使用相同技巧和功能的程序，只有其中一个可以编译。
A）该代码可以编译，并且可以按预期工作：
#include <errno.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
/*
* int mallocs(T **restrict p, ptrdiff_t nmemb);
*/
#define mallocs(ptr, nmemb) ( \
{ \
ptrdiff_t nmemb_ = (nmemb); \
__auto_type ptr_ = (ptr); \
int err_; \
\
err_ = 0; \
if (ptr_ == NULL) { \
errno = EINVAL; \
err_ = EINVAL; \
goto ret_; \
} \
if (nmemb_ < 0) { \
*ptr_ = NULL; \
errno = EOVERFLOW; \
err_ = -EOVERFLOW; \
goto ret_; \
} \
if (nmemb_ > (PTRDIFF_MAX / (ptrdiff_t)sizeof(**ptr_))) { \
*ptr_ = NULL; \
errno = EOVERFLOW; \
err_ = EOVERFLOW; \
goto ret_; \
} \
\
*ptr_ = malloc(sizeof(**ptr_) * nmemb_); \
if (!(*ptr_)) \
err_ = ENOMEM; \
ret_: \
err_; \
} \
)
int main(void)
{
int *b1;
int **p;
int c = getchar();
p = &b1;
if (c == 'a')
p = 0;
printf("%c\n", c);
if (mallocs(p, 47))
goto err;
b1[4] = 52;
printf("Hi: %i\n", b1[4]);
free(b1);
return 0;
err:
perror(NULL);
exit(EXIT_FAILURE);
}
B）这个甚至不编译（错误如下所示）：
#include <assert.h>
#include <errno.h>
#include <stdio.h>
#define alx_same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
#define alx_static_assert_array(a) do \
{ \
static_assert(!alx_same_type((a), &(a)[0]), "Not an array!"); \
} while (0)
/*
* int alx_sbprintf(char buff[restrict], int *restrict written,
* const char *restrict format, ...);
*/
#define alx_sbprintf(buff, written, format, ...) ( \
{ \
__auto_type w_ = (written); \
int len_; \
int err_; \
\
alx_static_assert_array(buff); \
err_ = 0; \
\
len_ = snprintf(buff, sizeof(buff), format, ##__VA_ARGS__); \
if (w_ != NULL) \
*w_ = len_; \
\
if (len_ < 0) { \
err_ = -errno; \
goto ret_; \
} \
if ((unsigned)len_ >= sizeof(buff)) { \
if (w_ != NULL) \
*w_ = sizeof(buff) - 1; \
errno = ENOMEM; \
err_ = ENOMEM; \
goto ret_; \
} \
ret_: \
err_; \
} \
)
int main(void)
{
char b1[10];
char b2[BUFSIZ];
int w1;
int *w2 = NULL;
if (alx_sbprintf(b1, &w1, "testttt%i", 12))
printf("Error 1.1\n");
printf("b1: %s; w1 = %i\n", b1, w1);
if (alx_sbprintf(b2, w2, "test%s", "testtt"))
printf("Error 2.1\n");
printf("b2: %s; w2 = %p\n", b2, w2);
return 0;
}
错误：
$ gcc -std=gnu17 -Wall -Wextra -Werror main.c
main.c: In function ‘main’:
main.c:39:3: error: jump into statement expression
goto ret_; \
^~~~
main.c:70:6: note: in expansion of macro ‘alx_sbprintf’
if (alx_sbprintf(b2, w2, "test%s", "testtt"))
^~~~~~~~~~~~
main.c:48:1: note: label ‘ret_’ defined here
ret_: \
^~~~
main.c:66:6: note: in expansion of macro ‘alx_sbprintf’
if (alx_sbprintf(b1, &w1, "testttt%i", 12))
^~~~~~~~~~~~
main.c:46:3: error: jump into statement expression
goto ret_; \
^~~~
main.c:70:6: note: in expansion of macro ‘alx_sbprintf’
if (alx_sbprintf(b2, w2, "test%s", "testtt"))
^~~~~~~~~~~~
main.c:48:1: note: label ‘ret_’ defined here
ret_: \
^~~~
main.c:66:6: note: in expansion of macro ‘alx_sbprintf’
if (alx_sbprintf(b1, &w1, "testttt%i", 12))
^~~~~~~~~~~~
main.c:48:1: error: duplicate label ‘ret_’
ret_: \
^~~~
main.c:70:6: note: in expansion of macro ‘alx_sbprintf’
if (alx_sbprintf(b2, w2, "test%s", "testtt"))
^~~~~~~~~~~~
main.c:48:1: note: previous definition of ‘ret_’ was here
ret_: \
^~~~
main.c:66:6: note: in expansion of macro ‘alx_sbprintf’
if (alx_sbprintf(b1, &w1, "testttt%i", 12))
^~~~~~~~~~~~
为什么只有其中一个抛出该错误？
答案 0 :(得分：5)
GNU C确实禁止跳转到语句表达式，但是您的主要问题是扩展宏会导致ret_标签被复制。
您需要将此语句表达式与__label__ extension结合使用，以声明作用域本地标签：
#include <assert.h>
#include <errno.h>
#include <stdio.h>
#define alx_same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
#define alx_static_assert_array(a) do \
{ \
static_assert(!alx_same_type((a), &(a)[0]), "Not an array!"); \
} while (0)
/*
* int alx_sbprintf(char buff[restrict], int *restrict written,
* const char *restrict format, ...);
*/
#define alx_sbprintf(buff, written, format, ...) ( \
{ \
__label__ ret_; \
__auto_type w_ = (written); \
int len_; \
int err_; \
\
alx_static_assert_array(buff); \
err_ = 0; \
\
len_ = snprintf(buff, sizeof(buff), format, ##__VA_ARGS__); \
if (w_ != NULL) \
*w_ = len_; \
\
if (len_ < 0) { \
err_ = -errno; \
goto ret_; \
} \
if ((unsigned)len_ >= sizeof(buff)) { \
if (w_ != NULL) \
*w_ = sizeof(buff) - 1; \
errno = ENOMEM; \
err_ = ENOMEM; \
goto ret_; \
} \
ret_: \
err_; \
} \
)
int main(void)
{
char b1[10];
char b2[BUFSIZ];
int w1;
int *w2 = NULL;
if (alx_sbprintf(b1, &w1, "testttt%i", 12))
printf("Error 1.1\n");
printf("b1: %s; w1 = %i\n", b1, w1);
if (alx_sbprintf(b2, w2, "test%s", "testtt"))
printf("Error 2.1\n");
printf("b2: %s; w2 = %p\n", b2, w2);
return 0;
}
（我只复制了其余代码，但是添加了__label__ ret_;使代码得以编译。）
答案 1 :(得分：4)
在示例B中，您两次调用alx_sbprintf宏。这将导致ret_标签被定义两次，从而导致“重复标签”错误。
标签的作用域不是语句表达式，而是功能的范围。
我不确定在第一个示例中为什么没有出现相同的“跳转到语句表达式”错误。
当函数可以正常工作时，没有理由使用这样的GCC表达式语句。 （如果要将其放在头文件中，则可能是static inline。）