Question

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

int main()
{    
    char *s;
    s = strdup("foo");
    printf("%s\n", s);
    getchar();
    return 0;
}

Looks pretty harmless, doesn’t it ?
But my IDE, which is Dev-C++, gives my the following warning:
warning: assignment makes pointer from integer without a cast

The warning disappears if you would change the code like this:

char *s;
s = (char*)strdup("foo");

Can anyone help me explain this?

Answer 1

You’re using Dev-C++, but strdup is not part of the C or C++ standard, it’s a POSIX function. You need to define the proper (according to your IDE’s documentation) preprocessor symbols in order for strdup to be declared by the header file … this is necessary in order for the header file not to pollute the name space when included into conforming C or C++ source files.

For a simple portable alternative, consider

char* mystrdup(const char* s)
{
    char* p = malloc(strlen(s)+1);
    if (p) strcpy(p, s);
    return p;
}

Or, if you know strdup is actually in the library, you can copy its declaration from string.h into your own source file or header … or use the simpler declaration from the man page:

char *strdup(const char *s);