why is __STDC_WANT_LIB_EXT2__ needed in C?

Question

On cpp reference page for strdup, I notice it says,

As all functions from Dynamic Memory TR, strdup is only guaranteed to be available if __STDC_ALLOC_LIB__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT2__ to the integer constant 1 before including string.h.

I thought that what the function strdup do is just simply malloc a space and strcpy the source to that space, and I also found its implementation in glibc.

char * __strdup (const char *s)
{
  size_t len = strlen (s) + 1;
  void *new = malloc (len);

  if (new == NULL)
    return NULL;

  return (char *) memcpy (new, s, len);
}

There's nothing special in its implementation, so why shall I define __STDC_WANT_LIB_EXT2__ to integer 1 before using strdup (or any other Dynamic Memory TR)? What does defining __STDC_WANT_LIB_EXT2__ do here?

Answer 1

As the top of the page you linked clarifies, the strdup function was not officially a part of the C Standard Library until C23, which has not yet been finalized.

Before that, it was available as a "dynamic memory extension", a set of extensions to the C standard introduced in 2010. Compilers and implementations of the C Standard Library (such as the GNU C Standard Library) which implemented these extensions usually "hid" them behind macros, such as STDC_WANT_LIB_EXT2 in this case. See Feature Test Macros for more Glibc examples of those special feature macros.

---

strdup in particular has existed long before 2010. as part of the POSIX standard, which glibc implements, and as such you do not need to explicitly define the aforementioned macros if you are using the GNU toolchain.

See here for more examples of POSIX functions.