(C) generating random data using /dev/urandom and writing it into file

Question

I'm pretty new to C, and I'm not sure what is wrong with this piece of code I have written. It is supposed to open (or create if needed) a file using a name specified and in that file write a bunch of signs taken from the /dev/urandom file. I need a precise amount of elements each one of a specified length. I need to be able to later edit that created file, but I wanted to focus on creating this generator part first. It doesn't show any errors when compiling or when I try to execute it, but still nothing happens - the file still doesn't exist anywhere. What's wrong with it?

int main(){

     void generate(char str[], int a, int b);

}


void generate(char str[], int a, int b){
    int n=0;
    char fname[128];
    strcpy(fname,str);
    strcpy(fname, ".txt");
    FILE *myFile = fopen(fname, "w");
    FILE *randomiser = fopen("/dev/urandom", "r");
   
    char bufor[a];
    size_t result = fread (bufor, b, a, randomiser);
    size_t end = fwrite (bufor, b, a, myFile);
   
    fclose(myFile);
    fclose(randomiser);
}

@edit change the tile as someone suggested and changed a code a bit since im still trying to work it out I forgot to mention that the whole point of this function is for it to be called in terminal as for example ./program generate data 100 100.

Answer 1

Here is the exact solution you asked for

There you go. I use open instead of fopen, read a certain amount of char out of /dev/urandom then write it to fd2 (the file created),

Be carefull the size of the buffer, be careful The NULL (\0) terminate the string

#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>

int generate(char **av)
{
    int fd1 = open("/dev/urandom", O_RDONLY);
    int fd2 = open(av[1], O_CREAT | O_RDWR, S_IRWXU);
    int size = atoi(av[2]);
    int row = atoi(av[3]);
    int i = -1;
    int j = -1;

    if (fd1 == -1 || fd2 == -1 || size <= 0 || row <= 0 )
        return (1);
    char buf[size];
    while (++i < row-1)
    {
        read(fd1, buf, size);
        buf[size -1] = '\0';
        write(fd2, buf, size -1);
        while (++j < size)
            buf[j] = '\0';
    }
    close(fd1);
    close(fd2);
    return (0);
}

int main(int ac, char **av)
{
    int s;

    if (ac < 4)
        return (1);
    generate(av);
    return (0);
}

for 10 * 100 char into file 'blabla' Use with :

$~> gcc prog.c -o program

$~> ./program blabla 100 10

Will create / open the file "blabla" read 100 from /dev/uramdom and wirte to blabla

To explain what you did wrong here look :

This is a declaration of a function

void generate(int i, char *a);

This is a call to a function

generate(25, "Blablabla");

sometime when you declare a function you also instanciate it aswell

int generate(int i, char *a)
{
    Operation;
    operation;
    return (0);
}

Declaration, and instanciation are different, let's assume i wrote funtion generate under function main in the above code example, function main would not know what is "generate" but if you declare it before main just like this it works

void generate(int i, char *a);

Answer 2

Here's what I made out of your source code with minimal changes

#include <stdio.h>

void generate(char basename[], size_t size);

int main(void) {
    generate("foobar", 42);
}

void generate(char basename[], size_t size) {
    char fname[128];
    sprintf(fname, "%s.txt", basename);

    FILE *myFile = fopen(fname, "w");
    FILE *randomiser = fopen("/dev/urandom", "r");

    char bufor[size];
    size_t result = fread(bufor, 1, size, randomiser);
    fwrite(bufor, 1, result, myFile);

    fclose(myFile);
    fclose(randomiser);
}

Answer 3

The problem is that your program never calls the generate() function, as @fredrik pointed out in comments. This:

     void generate(char str[], int a, int b);

is a function declaration, not a call. Although it is legal to put such a declaration inside a function, it is much more conventional to put it outside and preceding the function(s) that will be doing the calling. And in this case, you could avoid using a separate declaration at all by just moving the main() function to the end, after the definition of generate().

But that still doesn't get around the fact that you need to call the function if you want it to do anything. And to call it, you will need to provide arguments. For example,

generate("random_data", 10, 4);

From your description, you'll want to use arguments determined from evaluation of command-line parameters, but that aspect would be a separate question.