c - in_addr_t variable as paramether of connect - c

Question

I have a variable of in_addr_t type and I would like to use connect() with the given ip. I'm therefore needing a (struct sockaddr *) variable as parameter for the connect(). How to insert use the in_addr_t variable instead?
in_addr_t var; // Given variable, not actually declared here ofc
int sockfd = socket(AF_INET, SOCK_STREAM, 0);
struct sockaddr_in *srvraddr = malloc(sizeof(struct sockaddr_in));
memset((void *) srvraddr, 0, sizeof(struct sockaddr_in));
srvraddr->sin_family = AF_INET;
srvraddr->sin_port = htons(PORT_A); // Big - little endian arch compatibility
srvraddr->sin_addr.s_addr = var; // Somehow assign var here? <<<<<
connect(sockfd, (struct sockaddr *) srvraddr, sizeof(struct sockaddr_in));
Whatever I try to do connect always returns me -1.
Using perror I get the following error: Connection refused.

Answer 1

How to insert use the in_addr_t variable instead?
You cannot use an in_addr_t with connect() instead of a struct sockaddr_in. connect() accepts varying address structure types, but not arbitrary ones. The correct structure type for an IPv4 address is the struct sockaddr_in that you're already using. You need to store an appropriate representation of the remote host address into that.
Evidently, you already have an in_addr_t that you assert represents the remote address, but are uncertain how to use it:
in_addr_t var; // Given variable, not actually declared here ofc
[...]
srvraddr->sin_addr.s_addr = var; // Somehow assign var here? <<<<<
What you present is already exactly what POSIX expects you to do, however. POSIX requires that the sin_addr member of a struct sockaddr_in be a structure having at minimum a s_addr member of type in_addr_t. Supposing that the in_addr_t you have is in fact a correct representation of the machine address to which you want to connect, assigning that value to sin_addr.s_addr of your address structure is just right. In principle, there could be more members of that struct, but in practice, implementations that want to be interoperable will not require you to set any other members. Most don't have other members at all.
Do note, however, that just because connect() receives the address structure via a pointer does not mean you need to use dynamic allocation. It would be a bit more idiomatic to do this:
int sockfd = socket(AF_INET, SOCK_STREAM, 0);
struct sockaddr_in srvraddr = { 0 };
srvraddr.sin_family = AF_INET;
srvraddr.sin_port = htons(PORT_A);
srvraddr.sin_addr.s_addr = var;
connect(sockfd, (struct sockaddr *) &srvraddr, sizeof(srvraddr));
Among other things, that saves you having to free the memory afterward.
Whatever I try to do connect always returns me -1. Using perror I get the following error: Connection refused.
As far as I can see, the code you've presented is fine. You may want to check how you are obtaining the in_addr_t value in the first place, and to verify the port number you are using. On the other hand, do not overlook the possibility that the problem is at the remote host: perhaps the port you are trying to connect to is just not open (to you).

Answer 2

This is my way of using connect() to get a connection to "example.com" on port 80:
#include <stdio.h>
#include <netdb.h>
#include <unistd.h>
int main(void)
{
int sockfd;
if ((sockfd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) == -1) {
fprintf(stderr, "socket");
return 0;
}
char domain[] = "example.com";
struct hostent *he;
he = gethostbyname (domain);
if (!he)
{
switch (h_errno)
{
case HOST_NOT_FOUND:
fputs ("The host was not found.\n", stderr);
break;
case NO_ADDRESS:
fputs ("The name is valid but it has no address.\n", stderr);
break;
case NO_RECOVERY:
fputs ("A non-recoverable name server error occurred.\n", stderr);
break;
case TRY_AGAIN:
fputs ("The name server is temporarily unavailable.", stderr);
break;
}
return 1;
}
if (he->h_length < 4)
return 1;
struct sockaddr_in srvraddr;
srvraddr.sin_len = sizeof(srvraddr);
srvraddr.sin_family = AF_INET;
srvraddr.sin_port = htons(80);
srvraddr.sin_addr = * (struct in_addr *) he->h_addr_list[0];
if (connect(sockfd, (struct sockaddr*)&srvraddr, srvraddr.sin_len) == -1) {
fprintf(stderr,"connect() failed\n");
return 1;
}
// Use conection here...
close(sockfd);
return 0;
}