用实时路径功能投射问题（C编程） [英] casting issue with realpath function (c programming)

问题描述

当我编译以下代码时：

When I compile the following code:

#define _POSIX_C_SOURCE 200112L
#define _ISOC99_SOURCE
#define __EXTENSIONS__

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

int
main(int argc, char *argv[])
{
    char *symlinkpath = argv[1];
    char actualpath [PATH_MAX];
    char *ptr;
    ptr = realpath(symlinkpath, actualpath);
    printf("%s\n", ptr);
}

我在包含调用实际路径函数的行上收到警告，说：

I get a warning on the line that contains the call to the realpath function, saying:

warning: assignment makes pointer from integer without a cast

有人知道这是怎么回事？我正在运行Ubuntu Linux 9.04

Anybody know what's up? I'm running Ubuntu Linux 9.04

推荐答案

这很简单。 Glibc将realpath（）视为GNU扩展，而不是POSIX。所以，添加这一行：

This is very simple. Glibc treats realpath() as a GNU extension, not POSIX. So, add this line:

#define _GNU_SOURCE

...在包含stdlib.h之前，这样它的原型和已知要返回 char * 。否则，gcc将假定它返回默认类型 int 。除非定义 _GNU_SOURCE ，否则不会看到stdlib.h中的原型。

... prior to including stdlib.h so that it is prototyped and known to to return char *. Otherwise, gcc is going to assume it returns the default type of int. The prototype in stdlib.h is not seen unless _GNU_SOURCE is defined.

以下是符合要求的，没有警告，通过-Wall通过：

The following complies fine without warnings with -Wall passed:

#include <stdio.h>
#include <limits.h>

#define _GNU_SOURCE
#include <stdlib.h>

int
main(int argc, char *argv[])
{
    char *symlinkpath = argv[1];
    char actualpath [PATH_MAX];
    char *ptr;
    ptr = realpath(symlinkpath, actualpath);
    printf("%s\n", ptr);

    return 0;
}

您将看到与其他常用扩展名（如asprintf（））类似的行为。它值得看一看/ usr / include /中的内容，看看这个宏究竟打开了多少，以及它发生了什么变化。

You will see similar behavior with other popular extensions such as asprintf(). Its worth a look at /usr/include/ to see exactly how much that macro turns on and what it changes.

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