Random String Generator for Serial - c

Question

I'm working on a programm that generates Serial Codes. I know there a several threads about this topic, but I wasn't able to solve my problem.
I want to create a string that looks like this:
3S6G-TXMS-RLP6-VE5N
I can only use uppercase letters and numbers and every group of characters (like 3S6G) can only have a max. of 2 numbers in it.
My real struggle is that I have to open the internet explorer and navigate it to a address that includes this serial. It should look like this one: https://www.mywebpage.com/serial/3S6G-TXMS-RLP6-VE5N.
I hope someone can help me, because I'm not able to find a solution by myself.
At the moment it doesn't run, when I start the program, it instantly crashes. Also I didn't implement the Internet Explorer yet.
My Code:
#include <stdio.h>
#include <windows.h>
#include <time.h>
static const char abc[]=
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"123456789";
char Str[80];
char Str2[80];
char Str3[80];
char Str4[80];
char Stra[80];
unsigned int i;
int StringLength = sizeof(abc) -1;
char genRandom() {
return abc[rand() % StringLength];
}
int main()
{
SetConsoleTitle("Serial Generator");
srand(time(NULL));
for(i = 0; i < 4; i++) {
sprintf(Str, "%s", genRandom());
}
srand(time(NULL)-1);
for(i = 0; i < 4; i++) {
sprintf(Str2, "%s", genRandom());
}
srand(time(NULL)-2);
for(i = 0; i < 4; i++) {
sprintf(Str3, "%s", genRandom());
}
srand(time(NULL)-3);
for(i = 0; i < 4; i++) {
sprintf(Str4, "%s", genRandom());
}
sprintf(Stra, "%s-%s-%s-%s", Str, Str2, Str3, Str4);
printf("Your serial: \n");
printf("%s \n", Stra);
return 0;
}
I'm new to C, so please apologize if I ask dump questions.

Answer 1

Caveat:
I moved you code to a Linux box to test it there. I was able to successfully compile the code by (a) removing the #include <windows.h> line and (b) by removing the SetConsoleTitle("Serial Generator");
The compiler I'm using is gcc 4.8.2
OK, several things I see here
(1) you do not need to call srand more than once. srand seeds the random number generator. Really, really simplistically think of a huge table of random numbers and every time you make a call to rand you get the next number. srand picks where in that table you start. If you call srand multiple times in very quick succession, there is a good chance you will always be getting the same number.
(2) Why are all of your string arrays sized to 80? From what I can see, you will be sticking in four characters and allowing for the null terminator, you only need 5 bytes.
(3) Third, pay attention to compiler warnings ... they are there for a reason. When I compile you code I get the following warnings (I compiled with -g -ansi -Wall -pedantic):
dummy.c:19:5: warning: implicit declaration of function ‘rand’ [-Wimplicit-function-declaration]
return abc[rand() % StringLength];
^
dummy.c: In function ‘main’:
dummy.c:27:5: warning: implicit declaration of function ‘srand’ [-Wimplicit-function-declaration]
srand(time(NULL));
^
dummy.c:29:13: warning: format ‘%s’ expects argument of type ‘char *’, but argument 3 has type ‘int’ [-Wformat=]
sprintf(Str, "%s", genRandom());
Both rand and srand are declared in <stdlib.h> and including this file got rid of the first two errors.
The next error (of which there are four of that form, I've only shown one), from taking the results of getRandom() which returns a char and then try to write it into the buffer Str1 using a string format specifier. Remember that a char data type is really an integer.
Additionally, sprintf(....) will overwrite what is in the buffer with the new string, so calling in a loop doesn't append strings, it will over write the contents. You can accomplish what you want with a loop that looks like this:
for(i = 0; i < 4; i++) {
Str2[i] = genRandom();
}
I suspect that this was the root cause of the error you initially reported.
(4) There is no reason for the variable i to have file scope, it should be moved inside main.
(5) You really don't need to use so many string variables. Consider this loop:
Stra[4] = Stra[9]=Stra[14] = '-'
for(i = 0; i < 4; i++) {
Stra[i] = genRandom();
Stra[5+i] = genRandom();
Stra[10+i] = genRandom();
Stra[15+i] = genRandom();
}
With all these changes and modifications, your program now looks like:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/*#include <windows.h>*/
#include <time.h>
static const char abc[]= "ABCDEFGHIJKLMNOPQRSTUVWXYZ123456789";
#define STRING_LEN 20
char Stra[STRING_LEN];
int StringLength = sizeof(abc) -1;
char genRandom() {
return abc[rand() % StringLength];
}
int main()
{
/* SetConsoleTitle("Serial Generator");*/
unsigned int i;
memset(Stra, '\0', STRING_LEN);
Stra[4] = Stra[9]=Stra[14] = '-';
for(i = 0; i < 4; i++) {
Stra[i] = genRandom();
Stra[5+i] = genRandom();
Stra[10+i] = genRandom();
Stra[15+i] = genRandom();
}
printf("Your serial: \n%s \n ", Stra);
return 0;
}
Running this program on my Linux system produces the following:
******#ubuntu:~$ gcc -ansi -pedantic -Wall dummy.c -o dummy
******#ubuntu:~$ ./dummy
Your serial:
II4U-LK6E-C6CN-FCMV
Finally, your code (and thus my answer) does not address the restriction of no more than 2 digits.