New String.Format for C - strrep2

Following on from the problems I had with my previous string.format C function, I decided to rewrite it today under heavy valgrind observation.

So I re-wrote it and now get the all clear from valgrind:

==25693== HEAP SUMMARY:
==25693== in use at exit: 0 bytes in 0 blocks
==25693== total heap usage: 43 allocs, 43 frees, 161 bytes allocated
==25693==
==25693== All heap blocks were freed -- no leaks are possible

/*
 * File:   strrep2.c
 * Author: Adam K Dean
 * Description: string replacement (replication of string.replace for C)
 *
 * Original created on 11 January 2011, 10:01
 * This version (2) created on 14 January 2011, 9:36
 */

// usage
int main()
{
    char str[] = "ab{0}def{1}hi{2}";
    char *rep = strrep2(str, 3, "c", "g", "j");

    printf("%s\n", rep);

    free(rep);

    return (EXIT_SUCCESS);
}

char *strrep2(char *format, int argc, char *argv, ...)
{
    char *args[argc];
    char *tmp_left, *tmp_right, *arg, *fstr;
    char *start, *end, *param;
    int l_len, r_len, f_sz, argn;

    va_list marker;
    va_start(marker, *argv);
    for(int i = 0; i < argc; i++)
    {
        args[i] = (char *)calloc(strlen(argv) + 1, sizeof(char));
        strcpy(args[i], argv);
        argv = va_arg(marker, char *);
    }
    va_end(marker);

    fstr = (char *)calloc(strlen(format) + 1, sizeof(char));
    strcpy(fstr, format);

    for(int c = 1; c < strlen(fstr) - 1; c++)
    {
        start = (char *)calloc(2, sizeof(char));
        param = (char *)calloc(2, sizeof(char));
        end = (char *)calloc(2, sizeof(char));
        strncpy(start, &fstr[c-1], 1);
        strncpy(param, &fstr[c], 1);
        strncpy(end, &fstr[c+1], 1);

        if (start[0] == '{' && end[0] == '}' && isdigit(param[0]))
        {
            argn = atoi(&param[0]);
            if (argn < 0 || argn >= argc) continue;

            arg = args[argn];

            l_len = c - 1;
            r_len = (strlen(fstr) - 3) - l_len;

            tmp_left = (char *)calloc(l_len + 1, sizeof(char));
            tmp_right = (char *)calloc(r_len + 1, sizeof(char));

            strncpy(tmp_left, fstr, l_len);
            strncpy(tmp_right, &fstr[c + 2], r_len);

            f_sz = l_len + r_len + strlen(arg);

            fstr = (char *)realloc(fstr, (f_sz + 1) * sizeof(char));

            strcpy(fstr, tmp_left);
            strcpy(&fstr[l_len], arg);
            strcpy(&fstr[l_len + strlen(arg)], tmp_right);

            free(tmp_left);
            free(tmp_right);
        }

        free(start);
        free(param);
        free(end);
    }

    // free args
    for(int i = 0; i < argc; i++) free(args[i]);

    return fstr;
}