strdup(): Confused about warnings ('implicit declaration', 'makes pointer...without a cast', memory leak)

Question

When I compile the short piece of code below (in which we define a string and then use strdup to make a copy), I get 3 warnings: 2 compiler warnings from GCC and 1 run-time warning/error from valgrind.

I suspect the memory leak error (reported by valgrind) is also related to my use of strdup, which is why I'm including the relevant output below.

What am I doing wrong? (I'm working my way through a C book and this is how strdup is used by the author.)

---

The code:

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

int main(int argc, char* argv[])
{
  char *string1 = "I love lamp";
  char *string2;

  string2 = strdup(string1);

  printf("Here's string 1: %s\n"
     "Here's string 2: %s\n",
     string1, string2);

  return 0;
}

---

The warnings/output:

dchaudh@dchaudhUbuntu:~/workspaceC/LearnCHW/Ex17_StructsPointers$ make test
cc -std=c99    test.c   -o test
test.c: In function ‘main’:
test.c:9:3: warning: implicit declaration of function ‘strdup’ [-Wimplicit-function-declaration]
   string2 = strdup(string1);
   ^
test.c:9:11: warning: assignment makes pointer from integer without a cast [enabled by default]
   string2 = strdup(string1);
           ^
dchaudh@dchaudhUbuntu:~/workspaceC/LearnCHW/Ex17_StructsPointers$ valgrind --track-origins=yes --leak-check=full ./test
==3122== Memcheck, a memory error detector
==3122== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==3122== Using Valgrind-3.10.0 and LibVEX; rerun with -h for copyright info
==3122== Command: ./test
==3122== 
Here's string 1: I love lamp
Here's string 2: I love lamp
==3122== 
==3122== HEAP SUMMARY:
==3122==     in use at exit: 12 bytes in 1 blocks
==3122==   total heap usage: 1 allocs, 0 frees, 12 bytes allocated
==3122== 
==3122== 12 bytes in 1 blocks are definitely lost in loss record 1 of 1
==3122==    at 0x4C2ABBD: malloc (vg_replace_malloc.c:296)
==3122==    by 0x4EBF2B9: strdup (strdup.c:42)
==3122==    by 0x4005A4: main (in /home/dchaudh/workspaceC/LearnCHW/Ex17_StructsPointers/test)
==3122== 
==3122== LEAK SUMMARY:
==3122==    definitely lost: 12 bytes in 1 blocks
==3122==    indirectly lost: 0 bytes in 0 blocks
==3122==      possibly lost: 0 bytes in 0 blocks
==3122==    still reachable: 0 bytes in 0 blocks
==3122==         suppressed: 0 bytes in 0 blocks
==3122== 
==3122== For counts of detected and suppressed errors, rerun with: -v
==3122== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

Answer 1

The C standard library does not have such function as strdup. Nevertheless this popular function is usually provided by standard library implementations as an extension. In GCC implementation this function is declared in <string.h>, which you do include.

However, when you compile your code with stricter standard settings, like -std=c99, the compiler hides non-standard function declarations made in standard library headers. This is what happened to strdup declaration in your case. The warning that you get is a typical warning that is issued when you attempt to call an undeclared function. Formally, this is an error from C99 point of view, but your compiler decided that a warning is sufficient in this case. If you remove the -std=c99 switch from the compiler's command line, the declaration of strdup will become visible and the code will compile without that warning.

More technically, specifying -std=c99 in the command line makes GCC to define __STRICT_ANSI__ macro, which causes all non-ANSI function declarations to "disappear" from the standard headers.

The function is still present in the library, which is why your code links properly. Note that it does not necessarily run properly, since the compiler assumed that strdup returned an int, when in reality it returns a pointer.

The valgrind report is just a consequence of memory leak. strdup allocates memory that you are supposed to free yourself when you no longer need it.