You have another module called "random" somewhere. Did you name your script "random.py"?

Upgrading pyopenssl with pip was not working as none of the commands related to to pip was working for me. By upgrading pyopenssl with easy_install, above problem can be solved.

sudo python -m easy_install --upgrade pyOpenSSL

credit @delimiter (Answer)

Your implementation of print_hexa is wrong. Random bytes may contain 0x00 and the function may stop there. Even worse thing is that if the random bytes don't contain 0x00, it will do out-of-range access to find 0x00.

To avoid this issue, the length to print should be given to print_hexa separately.

#include <openssl/rand.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>

uint8_t *generate_salt()
{
    uint8_t *res = malloc(16 * sizeof(uint8_t));
    RAND_bytes(res, 16);
    return res;
}

void print_hexa(uint8_t *message, int message_length)
{
    for (int i = 0; i < message_length; i++)
    {
        printf("%02x", message[i]);
    }
    printf(";\n");
}
int main()
{
    for (int i = 0; i < 20; i++)
    {
        print_hexa(generate_salt(), 16);
    }
}

Another choice is hard-coding the length as you did in the generate_salt function.

#include <openssl/rand.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>

uint8_t *generate_salt()
{
    uint8_t *res = malloc(16 * sizeof(uint8_t));
    RAND_bytes(res, 16);
    return res;
}

void print_hexa(uint8_t *message)
{
    for (int i = 0; i < 16; i++)
    {
        printf("%02x", message[i]);
    }
    printf(";\n");
}
int main()
{
    for (int i = 0; i < 20; i++)
    {
        print_hexa(generate_salt());
    }
}

Defining the length as macro is better than writing the magic number 16 directly.

#include <openssl/rand.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>

#define LENGTH 16

uint8_t *generate_salt()
{
    uint8_t *res = malloc(LENGTH * sizeof(uint8_t));
    RAND_bytes(res, LENGTH);
    return res;
}

void print_hexa(uint8_t *message)
{
    for (int i = 0; i < LENGTH; i++)
    {
        printf("%02x", message[i]);
    }
    printf(";\n");
}
int main()
{
    for (int i = 0; i < 20; i++)
    {
        print_hexa(generate_salt());
    }
}

Is there a file called random.py in the same directory as your program? What does this print? import random; print(random.__file__)?

int main()
{
    unsigned char key[128];
    Rand_bytes(key,128);
    return 0;
}

Try RAND_bytes:

int main()
{
    unsigned char key[128];
    int rc = RAND_bytes(key,sizeof(key));
    if(rc != 1)
        /* Handle failure */

    ...
    OPENSSL_cleanse(key,sizeof(key));
    return 0;
}

The OpenSSL docs are at RAND_bytes(3).