C language——convert ip address string into uint in linux

#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>

uint32_t ip2uint(const char *ip) {
    int a, b, c, d;
    uint32_t addr = 0;

    if (sscanf(ip, "%d.%d.%d.%d", &a, &b, &c, &d) != 4)
       return 0;

    addr = a << 24;
    addr |= b << 16;
    addr |= c << 8;
    addr |= d;
    return addr;
}

int main()
{
    printf("%u\n", ip2uint("0.0.0.0"));
    printf("%u\n", ip2uint("1.0.0.1"));
    printf("%u\n", ip2uint("222.202.55.66"));
    printf("%u\n", ip2uint("255.255.255.255"));

    FILE *fp1;
    char c;
    fp1= fopen ("ctrl.txt", "r");
    c = fgetc(fp1);
    printf("%c\n", c);
    fclose(fp1);
}

[root@localhost 桌面]# gcc tmp.c -o tmp
[root@localhost 桌面]# ./tmp
0
16777217
3737794370
4294967295
[root@localhost 桌面]#