Socket operation on non socket error in C - c

Question

I am new to socket programming... I tried this server side program
#define BUFLEN 512
#define MYPORT 3456
void errorp(char* msg)
{
perror(msg);
exit(1);
}
int main()
{
struct sockaddr_in server, client;
int sock;
int slen = sizeof(server);
int clen = sizeof(client);
char *recvbuf, senbuf[BUFLEN] = {'h','e','l','l','o'};
if((sock = socket(AF_INET, SOCK_DGRAM, 0) == -1))
errorp("Socket creation failed");
printf("To the client: %s, %s", senbuf, " World");
bzero(&server, sizeof(server));
server.sin_family = AF_INET;
server.sin_port = MYPORT;
server.sin_addr.s_addr = inet_addr("127.0.0.1");
if(bind(sock, (struct sockaddr*)&server, slen)==-1)
errorp("Socket Bind Failed");
if(recvfrom(sock, recvbuf, sizeof(recvbuf), 0, (struct sockaddr*) &client, &clen) == -1)
errorp("recv from error");
printf("From the client: %s", recvbuf);
if(sendto(sock, senbuf, sizeof(senbuf), 0, (struct sockaddr*) &client, sizeof(client)) == -1)
errorp("Error in sending");
printf("To the client: %s", senbuf);
close(sock);
return 0;
}
There are no compilation errors but the output is
Socket Bind Failed: Socket operation on non-socket
To the client: hello, World
Please help me figure out where the mistake is?
and help get rid of it

Answer 1

The error message says it all: The socket isn't a (valid) socket.
This should make you look at the code creating the socket:
if((sock = socket(AF_INET, SOCK_DGRAM, 0) == -1))
The code above 1st compares the result of the call to socket() to -1 and then assigns the result of the comparison to sock. So it's either 0 or 1. And the result of the call to socket() is lost.
The code shall look like this:
if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) == -1)
as == binds tighter then =.
BTW, having used a Yoda-Conditition would have avoided such kind of "typo":
if (-1 == (sock = socket(AF_INET, SOCK_DGRAM, 0)))
Also at least clen shall be of type socklen_t as its address is passed, to have a value written into it, which will fail miserably if the size of the expected socklen_t would be different from an int (which the code shown passes).

Answer 2

if((sock = socket(AF_INET, SOCK_DGRAM, 0) == -1))
// \__________________________________/
You have your brackets in the wrong place. It's setting sock to a true/false value because == is "more binding" than =. It should instead be:
if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) == -1)
// \_____________________________________/
which sets sock to the return value from socket() and then compares that to -1.
You also have no backing storage for recvbuf which means your recvfrom(), once it starts working, will almost certainly do something bad.