writing formatted data of unknown length to a string (C programming)

Question

The following C function:

int sprintf ( char * str, const char * format, ... );

writes formatted data to a string. The size of the array pa

Answer 1

I threw this together as an exercise for myself based on DigitalRoss' suggestion. Feel free to merge this with his answer if you have the rep (I don't).

#include 
#include 
#include 
#include 

char* createRandstr(void);

int main(void)
{
    char* mystr;
    char* randomString;
    size_t len;

    /* Makes a random string of A's */
    randomString = createRandstr();

    /* 1st pass gets needed size */
    len = (size_t)snprintf(NULL, 0, "random string -->%s<--\n", randomString);
    mystr = malloc(len);

    /* Safely write to mystr with known length 'len' */
    snprintf(mystr, len, "random string -->%s<--\n", randomString);
    puts(mystr);

    free(mystr);
    free(randomString);

    return 0;
}

char* createRandstr(void)
{
    char*  randstr;
    size_t randlen;
    unsigned int i;

    srand((unsigned int)time((time_t*)NULL)); /* Seed rand() */
    randlen = (size_t)rand() % 50; /* Limit to max of 49 chars */
    randstr = malloc(randlen);

    for (i=0; i < randlen; i++)
    {
        randstr[i] = 'A';
    }
    randstr[randlen - 1] = '\0';

    return randstr;
}