Windows下asprintf隐式声明警告排查：本地正常同事环境报错

我开发了一个将PHP代码转换为可运行C文件的程序，用于调用PHP函数。该程序在我的Windows 10系统上运行正常，但在同事的Windows 10系统上运行失败。由于Windows原生不支持asprintf函数，我在头文件asprintf.h中自行实现了该函数。

asprintf.h 代码 #

#ifndef ASPRINTF_H
#define ASPRINTF_H

#if defined(__GNUC__) && ! defined(_GNU_SOURCE)
#define _GNU_SOURCE /* needed for (v)asprintf, affects '#include <stdio.h>' */
#endif
#include <stdio.h>  /* needed for vsnprintf    */
#include <stdlib.h> /* needed for malloc, free */
#include <stdarg.h> /* needed for va_*         */

/*
 * vscprintf:
 * MSVC implements this as _vscprintf, thus we just 'symlink' it here
 * GNU-C-compatible compilers do not implement this, thus we implement it here
 */
#ifdef _MSC_VER
#define vscprintf _vscprintf
#endif

#ifdef __GNUC__
int vscprintf(const char *format, va_list ap)
{
    va_list ap_copy;
    va_copy(ap_copy, ap);
    int retval = vsnprintf(NULL, 0, format, ap_copy);
    va_end(ap_copy);
    return retval;
}
#endif

/*
 * asprintf, vasprintf:
 * MSVC does not implement these, thus we implement them here
 * GNU-C-compatible compilers implement these with the same names, thus we
 * don't have to do anything
 */
#ifdef _MSC_VER
int vasprintf(char **strp, const char *format, va_list ap)
{
    int len = vscprintf(format, ap);
    if (len == -1)
        return -1;
    char *str = (char*)malloc((size_t) len + 1);
    if (!str)
        return -1;
    int retval = vsnprintf(str, len + 1, format, ap);
    if (retval == -1) {
        free(str);
        return -1;
    }
    *strp = str;
    return retval;
}

int asprintf(char **strp, const char *format, ...)
{
    va_list ap;
    va_start(ap, format);
    int retval = vasprintf(strp, format, ap);
    va_end(ap);
    return retval;
}
#endif

#endif // ASPRINTF_H

main.c 代码 #

#include "asprintf.h"

#include <stdio.h>
#include <stdlib.h>

extern char _binary_script_php_start;
extern char _binary_script_php_end;

int main(int argc, char *argv[]) {
    // EXTRACT OUR RESOURCE OBJECT INTO /tmp/test.php
    char *p = &_binary_script_php_start;
    FILE *fp = fopen("/tmp/test.php","wb");
    while ( p != &_binary_script_php_end ) {
        fputc(*p++,fp);
    }
    fclose(fp);
    // NOW READ IN OUR STANDARD ARGUMENTS AND LAUNCH OUR COMMAND
    int i = 1;
    char *cmd = "php /tmp/test.php";
    char *s = NULL;
    asprintf(&s, "%s",cmd);
    for(i = 1; i < argc; i++) {
        asprintf(&s, "%s \"%s\"",s,argv[i]);
    }
    // concatf("%s",cmd);
    // for(i = 1; i < argc; i++) {
    //     concatf("%s \"%s\"",s,argv[i]);
    // }
    // concatf(&s, "%s",cmd);
    // for(i = 1; i < argc; i++) {
    //     concatf(&s, "%s \"%s\"",s,argv[i]);
    // }
    system(s);
    free(s);
    unlink("/tmp/test.php"); // comment me out for debugging if you want
}

错误复现与疑问 #

我通过创建新头文件并修改函数名的方式复现了同事的错误：同事那边收到的警告是 warning: implicit declaration of function 'asprintf' [-Wimplicit-function-declaration]，而我修改后收到类似的 warning: implicit declaration of function 'concatf' 警告。

我清楚函数需要在main函数前声明，且已在头文件中完成；也知道需要定义_GNU_SOURCE，这一点在头文件中也已处理。我想知道是否存在未遵守的规则，以及为何该程序在我的环境中正常运行，在同事环境中却报错？

编译运行步骤 #

• 创建名为script.php的PHP文件
• 执行以下命令：

  ld -r -b binary script.php data.o
  gcc main.c data.o -o runme
  

• 正常情况下，执行./runme即可运行该PHP文件

内容的提问来源于stack exchange，提问作者Sammy