Background

crypt has two definitions, from the docs,

1. 
   

   One of them uses unistd.h

   
   
   

   #define _XOPEN_SOURCE /* See feature_test_macros(7) */
   #include <unistd.h>
   

   
   
   

   This is defined as

   
   
   

   #ifdef __USE_MISC
   
   extern char *crypt (const char *__key, const char *__salt)
    __THROW __nonnull ((1, 2));
   #endif
   

   
   


2. 
   

   One of them uses GNU crypt.h

   
   
   

   #define _GNU_SOURCE /* See feature_test_macros(7) */
   #include <crypt.h>
   

   
   
   

   This is defined as

   
   
   

   extern char *crypt (const char *__phrase, const char *__salt) 
    __THROW __nonnull ((1, 2));
   

   
   

Problem

When I compile with the definition in the first example (unistd.h)

#define _XOPEN_SOURCE
#include <unistd.h> 
#include <stdio.h> 

int main() 
 
 printf("%s", crypt("foobar", "sa")); 

I'm getting the error

In function ‘main’:
warning: implicit declaration of function ‘crypt’; did you mean ‘chroot’? [-Wimplicit-function-declaration]
 printf("%s", crypt("foobar", "sa"));
 ^~~~~
 chroot
warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘int’ [-Wformat=]
 printf("%s", crypt("foobar", "sa"));
 ~^ ~~~~~~~~~~~~~~~~~~~~~
 %d

Out of desperation, I've tried adding this.

#define _XOPEN_VERSION 700
#define _POSIX_VERSION 200809L
#define __USE_MISC 1

On Ubuntu Trusty 14.04 I believe this works fine use the unistd.h declaration, which makes all the more confusing.

These are two declarations, not two definitions. The content of the header file has no impact on which definition of a function is included: this is determined by the linker. There is only one definition of the crypt function in the Unix C standard library, it's whatever the symbol crypt points to in libcrypt.a or libcrypt.so (depending on whether you link statically or dynamically).

Since the two declarations are compatible, it doesn't matter which header a program gets its through. It's also fine if both declarations are processed: a program can contain any number of declarations for a function as long as they're compatible. Without going into the exact details (refer to the C language specification), two function declarations are compatible if they have the same return type, the same number of arguments and the same type for each argument. The name of the argument given in a declaration is not significant.

You should ignore unistd.h. That declaration is there for POSIX compatibility but GLIB has removed the definition for crypt when they migrated to libxcrypt to develop those features in isolation of glibc. This means you can still get ABI-compatibility but must link in the crypt. You can read more about that here

Availability in glibc

The crypt (), encrypt (), and setkey () functions are part of the POSIX.1-2008 XSI Options Group for Encryption and are optional. If the interfaces are not available then the symbolic constant _XOPEN_CRYPT is either not defined or defined to -1, and can be checked at runtime with sysconf ().

This may be the case if the downstream distribution has switched from glibc crypt to libxcrypt. When recompiling applications in such distributions the
user must detect if _XOPEN_CRPYT is not available and include crypt.h for the function prototypes; otherwise libxcrypt is a ABI compatible drop-in replacement.

As for why even if you link the definition isn't pulled down with

#define _XOPEN_VERSION 700
#define _POSIX_VERSION 200809L
#define __USE_MISC 1
#define _XOPEN_SOURCE

Well there are a few things that happening there

1. The only header that matters in unistd.h is __USE_MISC.


2. 
   unistd.h includes features.h which undefs the __USE_ macros to start with a "clean slate"
   


3. The only way to define __USE_MISC is to have defined _GNU_SOURCE.

If you want to pull in the declaration of crypt.h from unistd.h you must define _GNU_SOURCE! Those two examples cited from man page of crypt.h do the same thing and whether you include unistd.h or crypt.h you MUST also define _GNU_SOURCE in contemporary versions of glibc.

So you can either do,

#define _GNU_SOURCE
#include <unistd.h>

Or,

#define _GNU_SOURCE
#include <crypt.h>

But because you must link against crypt (-lcrypt) I would suggest using the declaration in crypt.h for sanity.