Cdtjkyj

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
   
   /* One-way hash PHRASE, returning a string suitable for storage in the
   
      user database.  SALT selects the one-way function to use, and
   
      ensures that no two users' hashes are the same, even if they use
   
      the same passphrase.  The return value points to static storage
   
      which will be overwritten by the next call to crypt.  */
   
   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

   
   
   

   /* One-way hash PHRASE, returning a string suitable for storage in the
   
   user database.  SALT selects the one-way function to use, and
   
   ensures that no two users' hashes are the same, even if they use
   
   the same passphrase.  The return value points to static storage
   
   which will be overwritten by the next call to crypt.  */
   
   
   
   extern char *crypt (const char *__phrase, const char *__salt)                                                                                          
   
     __THROW __nonnull ((1, 2));
   
   

   
   

ℹ This question is different from my question here where I'm merely asking about compilation with the unistd.h version

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.