CODEWITHSUNDEEP
c
Music Backup
Music Backup

Reputation: 59

Argument is incompatible with prototype: struct in_addr

I am trying to print some output in my C program.

When I compile with cc, I get this error:

"whoisclient.c", line 70: argument #1 is incompatible with prototype: prototype: struct in_addr {union {..} S_un} : "/usr/include/arpa/inet.h", line 61 argument : pointer to char

At this part of my code:

 printf("Addresses\n");
      int j=0;
      while(hp ->h_addr_list[j]!=NULL){
        printf("hp_addr_list[%d] = %s\n", j, inet_ntoa(hp->h_addr_list[j]));
        j++;
      }

I have tried adding an asterisk or adding / removing hp-> but I am out of ideas. I don't fully understand the error message and what it is saying I am doing wrong.

Upvotes: 0

Views: 446

Answers (1)

dbush
dbush

Reputation: 223897

The inet_ntoa function takes a struct in_addr, which contains a numerical representation of an IPv4 address, and returns a pointer to a string contains a text representation of that IP address.

The h_addr_list field of a struct hostent is a pointer to an array of pointers to strings, each of which contains an IPv4 address. This is incompatible with what inet_ntoa is expecting.

Since you already have a string representation of an IP address and not a numerical representation, you don't need to pass it through inet_ntoa to convert it. Just pass the string directly to printf:

printf("hp_addr_list[%d] = %s\n", j, hp->h_addr_list[j]);

EDIT:

As mentioned in the comments, the h_addr_list member, which is defined as type char ** is actually an array of pointers to network addresses in network byte order. Such an element is the same as the contents of a struct in_addr. So the char * needs to be casted to a struct in_addr * and then dereferenced:

printf("hp_addr_list[%d] = %s\n", j, inet_ntoa(*((struct in_addr *)hp->h_addr_list[j])));

Upvotes: 3

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