What's the C library function to generate random string?

Question

Is there a library function that creates a random string in the same way that mkstemp() creates a unique file name? What is it?

Answer 1

There's no standard function, but your OS might implement something. Have you considered searching through the manuals? Alternatively, this task is simple enough. I'd be tempted to use something like:

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

void rand_str(char *, size_t);

int main(void) {
    char str[] = { [41] = '\1' }; // make the last character non-zero so we can test based on it later
    rand_str(str, sizeof str - 1);
    assert(str[41] == '\0');      // test the correct insertion of string terminator
    puts(str);
}

void rand_str(char *dest, size_t length) {
    char charset[] = "0123456789"
                     "abcdefghijklmnopqrstuvwxyz"
                     "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    while (length-- > 0) {
        size_t index = (double) rand() / RAND_MAX * (sizeof charset - 1);
        *dest++ = charset[index];
    }
    *dest = '\0';
}

This has the neat benefit of working correctly on EBCDIC systems, and being able to accommodate virtually any character set. I haven't added any of the following characters into the character set, because it seems clear that you want strings that could be filenames:

":;?@[\]^_`{|}"

I figured many of those characters could be invalid in filenames on various OSes.

Answer 2

There's no build in API, you may use (on *x system) /dev/urandom like:

FILE *f = fopen( "/dev/urandom", "r");
if( !f) ...
fread( binary_string, string_length, f);
fclose(f);

Note that this will create binary data, not string data so you'll may have to filter it afterwards.

You may also use standard pseudorandom generator rand():

#include <time.h>
#include <stdlib.h>

// In main:
srand(time(NULL));
for( int i = 0; i < string_length; ++i){
    string[i] = '0' + rand()%72; // starting on '0', ending on '}'
}

And if you need really random string you need to google generating random sequence cryptography which is one of cryptography's difficult problems which still hasn't perfect solution :)