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1#include <arpa/inet.h>   // for inet_pton
2
3static int is_ipv4_prefix(const char *host, size_t *prefix_len)
4{
5    size_t i = 0;
6    while (host[i] && ((host[i] >= '0' && host[i] <= '9') || host[i] == '.'))
7        i++;
8    if (i == 0)
9        return 0;
10    char buf[32];
11    if (i >= sizeof(buf)) // too long to be valid IPv4
12        return 0;
13    memcpy(buf, host, i);
14    buf[i] = '\0';
15    struct in_addr addr;
16    if (inet_pton(AF_INET, buf, &addr) == 1) {
17        *prefix_len = i;
18        return 1;
19    }
20    return 0;
21}