little experiment on returning extra error information from functions in C

countdown.c

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

struct error {
  int errnum;
  const char *msg;
};

struct result_int {
  bool ok;
  int result;
  struct error error;
};

struct result_int count_down(int c) {
  if (c > 0) {
    return (struct result_int) {
      .ok = true,
      .result = c - 1
    };
  } else {
    return (struct result_int) {
      .ok = false,
      .error = (struct error) {
        .errnum = EINVAL,
        .msg = "counter must be strictly positive"
      }
    };
  }
}

int main(int argc, char *argv[]) {
  for (int argn = 1; argn < argc; argn++) {
    struct result_int result = count_down(atoi(argv[argn]));
    if (result.ok) {
      printf("%d\n", result.result);
    } else {
      error(EXIT_FAILURE, result.error.errnum, "%s", result.error.msg);
    }
  }
  return 0;
}

Makefile

.PHONY: all

CFLAGS := -std=c99 -Wall -Wextra -Wpedantic

all: countdown