C qsort example. It takes a list of values from the command line and sorts them in ascending order.

qsort_ascending.c

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

void usage(const char *pProgramName) {
  printf("Usage: %s x y z ...\n", pProgramName);
  printf("Example: %s 818 -3 566 5 12 100 12 25\n", pProgramName);
  printf("  This program will sort numbers in ascending order.\n");
}

/***
 * Input: argc, argv, an optional array, and the array length
 * Output: total number of scanned items
 *
 * All scanned items are stored in the array if it is supplied.
 * This function can be run once to get the length of the array,
 * and a second time to actually populate the array.
 */
size_t parse_args(int argc, const char *argv[], int *pArray, size_t pLength) {
  size_t index = 0;
  // skip arv[0]
  for (int i = 1; i < argc; i++) {
    int value_scanned;
    int items_scanned = sscanf(argv[i], "%d", &value_scanned);
    if (NULL != pArray && index < pLength) {
      pArray[index] = value_scanned;
    }
    // EOF is negative
    if (0 < items_scanned) {
      index += items_scanned;
    }
  }
  // final index is the total number of scanned items
  return index;
}

void print_array(int *pArray, size_t pLength) {
  int i;
  for (i = 0 ; i < pLength - 1; i++) {
    printf("%d ", pArray[i]);
  }
  printf("%d\n", pArray[i]);
}

int qsort_compare(const void *pA, const void *pB) {
  return (*(int*)pA - *(int*)pB);
}

int main (int argc, const char *argv[]) {
  size_t length = parse_args(argc, argv, NULL, 0);
  int array[length];
  parse_args(argc, argv, array, length);

  if (length < 1) {
    usage(argv[0]);
    return 1;
  } else {
    qsort(array, length, sizeof(int), qsort_compare);
    print_array(array, length);
  }

  return 0;
}

~Makefile

qsort_ascending: qsort_ascending.c
	$(CC) -o $@ $^

test: qsort_ascending
	./qsort_ascending || true # pipe to ignore error
	./qsort_ascending 818 -3 566 5 12 100 12 25
	./qsort_ascending 0
	./qsort_ascending 1 -1 0 0 0 0 0

.PHONY: clean

clean:
	rm -f *.o *~ core qsort_ascending

Although this is a qsort() example, the parse_args() function took the most time to get right.