connect() function fails when passing sockaddr_in as argument - c

Question

I have been at this for an ungodly amount of time, so I really hope someone can provide me some keen insight as to what is going on.
I have the following main function:
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
int main(int argc, char **argv) {
char *serv_IP;
in_port_t serv_port;
int sock;
struct sockaddr_in serv_addr;
serv_IP = argv[1];
serv_port = atoi(argv[2]);
if ((sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
fprintf(stderr, "Failed to create TCP socket\r\n");
exit(1);
}
memset(&serv_addr, 0, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
if (inet_pton(AF_INET, serv_IP, &serv_addr.sin_addr.s_addr) == 0) {
fprintf(stderr, "Invalid IP address\r\n");
exit(1);
}
serv_addr.sin_port = htons(serv_port);
if (connect(sock, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
fprintf(stderr, "Failed to connect to serv\r\n");
exit(1);
}
else {
printf("You're connected!\n);
}
close(sock)
return 0;
}
Now, this code works just fine. However, what I want to do is to replace the call to connect() with a helper function call to something like this:
void function(int sock, struct sockaddr_in *serv_addr) {
if (connect(sock, (struct sockaddr *) serv_addr, sizeof(serv_addr)) < 0) {
printf("Server IP = %s\n", inet_ntoa(serv_addr->sin_addr));
printf("Server port = %d\n", ntohs(serv_addr->sin_port));
fprintf(stderr, "Failed to connect to server\r\n");
exit(1);
}
else {
// Do other stuff
}
}
I remove the call to connect() from main() and replace it with the function call:
function(sock, &serv_addr);
As soon as the function is called, the correct IP and port numbers are printed out, but I still fail to connect to my server. The only difference is, in my main function(), I preface serv_addr in the connect call with the & - i.e., connect(sock, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) - to reference its address, and I don't do that in the helper function because the address of serv_addr is already being passed as an argument - i.e., connect(sock, (struct sockaddr *) serv_addr, sizeof(serv_addr)). It makes no difference if I add the &, just in case you were wondering.
So, with the &serv_addr being passed to function() seemingly correctly, as verified by me being able to print out the correct IP and port numbers, why is it that I can connect in main() but not when I pass the serv_addr struct as an argument to another function and call connect() from there?
Thanks in advance for any help!

Answer 1

sizeof(serv_addr) returns 16 when serv_addr is declared as sockaddr_in, but returns 4 (in 32bit) or 8 (in 64bit) when declared as sockaddr_in*. It is too small either way, AF_INET needs 16. Had you looked at errno when connect() failed, it would have told you that you were passing an invalid parameter value.
You need to use sizeof(sockaddr_in), either directly:
void function(int sock, struct sockaddr_in *serv_addr)
{
if (connect(sock, (struct sockaddr *) serv_addr, sizeof(struct sockaddr_in)) < 0)
Or indirectly via sizeof(*serv_addr):
void function(int sock, struct sockaddr_in *serv_addr)
{
if (connect(sock, (struct sockaddr *) serv_addr, sizeof(*serv_addr)) < 0)

Answer 2

That did it! Thank you very much for your quick response!! I never would have thought about the return of sizeof() as a potential problem with returning different sizes for actual values vs. pointers to values. Totally makes sense, though. And I just read about the errno.h header as well as how to use it.
The exact line that fixed it was:
if (connect(sock, (struct sockaddr *) serv_addr, sizeof(struct sockaddr_in)) < 0)
I was right about not needing the '&' in front of serv_add. You also need the "struct" in sizeof() or else it returns sockaddr_in as an undeclared variable.
Anyways, thanks again.
Now I can finally move on with my code.