t.c

// How the hell does this work?
// gcc --std=c99 -Wall -pedantic t.c
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

int main(int argc, char *argv[]) {

    uint32_t num_ipd_intervals = atoi(argv[1]);
    uint32_t ipd_interval[num_ipd_intervals + 1];

    printf("num_ipd_intervals(argv): %u\n"
           "Sizeof ipd_interval: %lu\n", 
           num_ipd_intervals,
           (size_t) sizeof(ipd_interval));

    return(0);
}

https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html