Question

#include <stdio.h>
#include <stdlib.h>  
#include <uuid/uuid.h>

int main(void) {
    puts("!!!Hello World!!!"); /* prints !!!Hello World!!! */
    uuid_t uuid;
    int uuid_generate_time_safe(uuid);
    printf("%x",uuid);
    return EXIT_SUCCESS;
}

I just wonder why uuid is not 16 bytes long?
I use DEBUG to view the memory, It is indeed not 16 bytes.
And I use libpcap to develop my program, The uuid is not unique.

Answer 1

I just tried your program on my system, and uuid is 16 bytes long. But your program doesn’t display its size.

The line:

int uuid_generate_time_safe(uuid);

isn’t a call to the uuid_generate_time_safe function, it’s a declaration of that function with uuid as the (ignored) name of the single parameter. (And that kind of function declaration isn’t even valid as of the 1999 standard, which dropped the old “implicit int” rule.)

Your printf call:

printf("%x",uuid);

has undefined behavior; "%x" requires an argument of type unsigned int.

If you look in /usr/include/uuid/uuid.h, you’ll see that the definition of uuid_t is:

typedef unsigned char uuid_t[16];

The correct declaration of uuid_generate_time_safe (see man uuid_generate_time_safe) is:

int uuid_generate_time_safe(uuid_t out);

You don’t need that declaration in your own code; it’s provided by the #include <uuid/uuid.h>.
Because uuid_t is an array type, the parameter is really of type unsigned char*, which is why the function is seemingly able to modify its argument.

Here’s a more correct program that illustrates the use of the function:

#include <stdio.h>
#include <uuid/uuid.h>

int main(void) {
    uuid_t uuid;
    int result = uuid_generate_time_safe(uuid);
    printf("sizeof uuid = %d\n", (int)sizeof uuid);
    // or: printf("sizeof uuid = %zu\n", sizeof uuid);
    if (result == 0) {
        puts("uuid generated safely");
    }
    else {
        puts("uuid not generated safely");
    }
    for (size_t i = 0; i < sizeof uuid; i ++) {
        printf("%02x ", uuid[i]);
    }
    putchar('\n');
    return 0;
}

On my system, I got the following output:

sizeof uuid = 16
uuid not generated safely
26 9b fc b8 89 35 11 e1 96 30 00 13 20 db 0a c4

See the man page for information about why the “uuid not generated safely” message might appear.

Note that I had to install the uuid-dev package to be able to build and run this program.