strdup error on g++ with c++0x

Question

I have some C++0x code. I was able to reproduce it below. The code below works fine without -std=c++0x however i need it for my real code.

How do i include strdup in C++0x? with gcc 4.5.2

note i am using mingw. i tried including cstdlib, cstring, string.h and tried using std::. No luck.

>g++ -std=c++0x a.cpp
a.cpp: In function 'int main()':
a.cpp:4:11: error: 'strdup' was not declared in this scope

code:

#include <string.h>
int main()
{
    strdup("");
    return 0;
}

Solution 1

-std=gnu++0x (instead of -std=c++0x) does the trick for me; -D_GNU_SOURCE didn't work (I tried with a cross-compiler, but perhaps it works with other kinds of g++).

It appears that the default (no -std=... passed) is "GNU C++" and not "strict standard C++", so the flag for "don't change anything except for upgrading to C++11" is -std=gnu++0x, not -std=c++0x; the latter means "upgrade to C++11 and be stricter than by default".

Solution 2

strdup may not be included in the library you are linking against (you mentioned mingw). I'm not sure if it's in c++0x or not; I know it's not in earlier versions of C/C++ standards.

It's a very simple function, and you could just include it in your program (though it's not legal to call it simply "strdup" since all names beginning with "str" and a lowercase letter are reserved for implementation extensions.)

char *my_strdup(const char *str) {
    size_t len = strlen(str);
    char *x = (char *)malloc(len+1); /* 1 for the null terminator */
    if(!x) return NULL; /* malloc could not allocate memory */
    memcpy(x,str,len+1); /* copy the string into the new buffer */
    return x;
}

Solution 3

This page explains that strdup is conforming, among others, to the POSIX and BSD standards, and that GNU extensions implement it. Maybe if you compile your code with "-D_GNU_SOURCE" it works?

EDIT: just to expand a bit, you probably do not need anything else than including cstring on a POSIX system. But you are using GCC on Windows, which is not POSIX, so you need the extra definition to enable strdup.

Comments

[GManNickG]

Works on gcc 4.5.1. Try #include <cstring>` and using std::strdup, that's the "C++ way". (Still isn't an answer, though, since that should be valid too, IIRC.)

[Admin]

@GMan: I modified my question. I tried it and no luck :(. As a temp solution i put extern C _CRTIMP char* __cdecl __MINGW_NOTHROW strdup (const char*) __MINGW_ATTRIB_MALLOC; in my headers. It works with that.

[Admin]

its reserved yet its nonstandard!?

[Ben]

malloc may need a cast to char * (stackoverflow.com/questions/5099669/…)

[Random832]

@Steve That's what I get for coding it as C because it's normally a C function. Yes, it needs a cast in C++.

[coanor]

strdup not in ANSI C, #define feature test macro may not be the best choice, some other strdup-like may need other feature test (_GNU_SOURCE, _BSD_SOURCE, ...). Just implement it.

[einpoklum]

That's an MSVC flag.