Join a string array into one allocated buffer

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

/* Join 'count' strings with 'sep'; returns a malloc'd buffer or NULL. */
char *join_strings(const char *const *parts, size_t count, const char *sep)
{
    size_t total = 1, sep_len = strlen(sep);
    for (size_t i = 0; i < count; i++) {
        total += strlen(parts[i]) + (i > 0 ? sep_len : 0);
    }
    char *out = malloc(total);
    if (out == NULL) {
        return NULL;
    }
    char *cursor = out;
    for (size_t i = 0; i < count; i++) {
        if (i > 0) {
            memcpy(cursor, sep, sep_len);
            cursor += sep_len;
        }
        size_t part_len = strlen(parts[i]);
        memcpy(cursor, parts[i], part_len);
        cursor += part_len;
    }
    *cursor = '\0'; /* total already reserves the trailing NUL */
    return out;
}

int main(void)
{
    const char *words[] = {"alpha", "beta", "gamma"};
    char *line = join_strings(words, 3, ", ");
    if (line == NULL) return 1;
    puts(line);
    free(line);
    return 0;
}