bpf_num_possible_cpus

main.c

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

static inline unsigned int bpf_num_possible_cpus(void)
{
	static const char *fcpu = "/tmp/foo";
	unsigned int start, end, possible_cpus = 0;
	char buff[128];
	FILE *fp;
	int n, i, j = 0;

	fp = fopen(fcpu, "r");
	if (!fp) {
		printf("Failed to open %s: '%s'!\n", fcpu, strerror(errno));
		exit(1);
	}

	if (!fgets(buff, sizeof(buff), fp)) {
		printf("Failed to read %s!\n", fcpu);
		exit(1);
	}

  int len = strlen(buff);
	for (i = 0; i <= len; i++) {
		if (buff[i] == ',' || buff[i] == '\0') {
			buff[i] = '\0';
			n = sscanf(&buff[j], "%u-%u", &start, &end);
			if (n <= 0) {
				printf("Failed to retrieve # possible CPUs!\n");
				exit(1);
			} else if (n == 1) {
				end = start;
			}
			possible_cpus += end - start + 1;
			j = i + 1;
		}
	}

	fclose(fp);

	return possible_cpus;
}

int main()
{
    printf("num: %d\n", bpf_num_possible_cpus());

    return 0;
}

test.output

$ echo -n "" > /tmp/foo && ./main
Failed to read /tmp/foo!
$ echo "" > /tmp/foo && ./main
Failed to retrieve # possible CPUs!
$ echo "1" > /tmp/foo && ./main
num: 1
$ echo "1,3" > /tmp/foo && ./main
num: 2
$ echo "0,2-3" > /tmp/foo && ./main
num: 3
$ echo "0-1,3-4" > /tmp/foo && ./main
num: 4
$ echo "0-3" > /tmp/foo && ./main
num: 4