Unable to copy ip address from string to sockaddr_in

I am trying to copy an IP address from a string to struct sockaddr_in but somehow there's some error popping up.

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <string.h>

int main(void)
{
    char *buf = "128.10.25.102:30015";
    char ip[30];
    struct sockaddr_in s1_addport;
    int i = 0;

    while (buf[i] != ':') i++;
    strncpy(ip, &buf[0], i);
    ip[strlen(ip)] = '\0';

    printf("ip: %s, strlen:%zu\n",ip,strlen(ip));
    inet_aton(ip,&s1_addport.sin_addr);
    printf("Server IP: %s\n",inet_ntoa(s1_addport.sin_addr));
    return 0;
}

The output of the above code shows:

ip: 128.10.25.102, strlen:13  
Server IP: 0.0.0.0

There is some error in copying the value from ip string which I am unable to figure out. What could be the reason?

This is the problem in your pastebin code:

ip[strlen(ip)]='\0';

(Trying to append null terminator but using strlen, which itself depends on a null terminator to be present).

Here is the fix:

....
while(buf[i]!=':')
    i++;
strncpy(ip,&buf[0],i);
ip[i]='\0';
puts(ip);
....