int array to a string in c

The most efficient way would be to iteratively snprintf each element into a buffer.

Here's a function that emulates Python's join().

#include <stdio.h>

#define ARRAY_LEN(x)         (sizeof(x) / sizeof(x[0]))

/**
 * Print an array of unsigned long long integers to a string.
 * Arguments:
 *   buf      Destination buffer
 *   buflen   Length of `buf'
 *   ar       Array of numbers to print
 *   arlen    Number of elements in `ar'
 *   sep      Separator string
 * Returns:
 *   The number of bytes printed
 */
int join_ull(char *buf, int buflen,
    unsigned long long ar[], int arlen, const char *sep)
{
    int i;
    char *p;
    const char *end = buf + buflen;

    /* While we have more elements to print, and have buffer space */
    for (i=0, p=buf; i<arlen && p<end; ++i) {
        p += snprintf(p, end-p, "%llu%s", ar[i],
             (i<arlen-1) ? sep : "");        /* Print separator if not last. */

        /* Note that p is advanced to the next place we want to print */
    }

    return p-buf;
}


int main(void)
{
    unsigned long long ar[] = {1, 2, 3};
    char buf[1024];

    join_ull(buf, sizeof(buf), ar, ARRAY_LEN(ar), " ");
    printf("Output: \"%s\"\n", buf);

    /* Test the buffer-too-small case */
    join_ull(buf, 4, ar, ARRAY_LEN(ar), " ");
    printf("Output: \"%s\"\n", buf);

    return 0;
}

Result:

Output: "1 2 3"
Output: "1 2"