Question

I’m using GCC 4.6.0 (on an otherwise unidentified platform).

I am using the crypt() function to encrypt a password.

I have never used that function before so I checked out the main page:

man 3 crypt

And it says to include the unistd.h header.

However, when I did that, I got an implicit warning for the crypt function.

warning: implicit declaration of function ‘crypt’ [-Wimplicit-function-declaration]

I did a bit of searching and I found that you have to include the crypt.h. However, how come it doesn’t say that in the man page?

Answer 1

It also says #define _XOPEN_SOURCE (before including unistd.h) in my man page. So you should probably add it to expose the declaration of crypt.

EDIT

I just tried it. Including unistd.h and #define _XOPEN_SOURCE before it does the trick. Including it alone isn’t enough.

Using

gcc version 4.6.0 20110429
GNU C Library stable release version 2.13

Looking into unistd.h:

/* XPG4.2 specifies that prototypes for the encryption functions must
   be defined here.  */
#ifdef  __USE_XOPEN
/* Encrypt at most 8 characters from KEY using salt to perturb DES.  */
extern char *crypt (__const char *__key, __const char *__salt)
     __THROW __nonnull ((1, 2));