Doug0855 Doug0855 - 8 days ago 6
C Question

Server only receiving null string from client?

So I have a basic client/server c++ program. Right now, when the client connects to the server I want the server sends a "Hello, world" message and the client to respond "Hello, server" just to make sure I am sending and relieving messages correctly.

When I run, the client receives the message from the server, but the server only receives a null string from the client.

Here is the code for the client

int main(int argc, char *argv[]) {
int sockfd, numbytes;
char buf[MAXDATASIZE];
struct addrinfo hints, *servinfo, *p;
int rv;
char s[INET6_ADDRSTRLEN];

if (argc != 2) {
fprintf(stderr,"usage: client hostname\n");
exit(1);
}

memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;

if ((rv = getaddrinfo(argv[1], PORT, &hints, &servinfo)) != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
return 1;
}

// loop through all the results and connect to the first we can
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror("client: socket");
continue;
}

if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
close(sockfd);
perror("client: connect");
continue;
}

break;
}

if (p == NULL) {
fprintf(stderr, "client: failed to connect\n");
return 2;
}

inet_ntop(p->ai_family, get_in_addr((struct sockaddr *)p->ai_addr),
s, sizeof s);
printf("client: connecting to %s\n", s);

freeaddrinfo(servinfo); // all done with this structure

if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {
perror("recv");
exit(1);
}

if (send(sockfd, "Hello, client!", 13, 0) == -1) {
perror("send");
}

buf[numbytes] = '\0';

printf("client: received '%s'\n",buf);

close(sockfd);

return 0;
}


and Here's the code for the server

int main(void)
{
int sockfd, new_fd, numbytes; // listen on sock_fd, new connection on new_fd
char buf[MAXDATASIZE];
struct addrinfo hints, *servinfo, *p;
struct sockaddr_storage their_addr; // connector's address information
socklen_t sin_size;
struct sigaction sa;
int yes=1;
char s[INET6_ADDRSTRLEN];
int rv;

memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE; // use my IP

if ((rv = getaddrinfo(NULL, PORT, &hints, &servinfo)) != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
return 1;
}

// loop through all the results and bind to the first we can
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror("server: socket");
continue;
}

if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes,
sizeof(int)) == -1) {
perror("setsockopt");
exit(1);
}

if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
close(sockfd);
perror("server: bind");
continue;
}

break;
}

freeaddrinfo(servinfo); // all done with this structure

if (p == NULL) {
fprintf(stderr, "server: failed to bind\n");
exit(1);
}

if (listen(sockfd, BACKLOG) == -1) {
perror("listen");
exit(1);
}

sa.sa_handler = sigchld_handler; // reap all dead processes
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART;
if (sigaction(SIGCHLD, &sa, NULL) == -1) {
perror("sigaction");
exit(1);
}

printf("server: waiting for connections...\n");

while(1) { // main accept() loop
sin_size = sizeof their_addr;
new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
if (new_fd == -1) {
perror("accept");
continue;
}

inet_ntop(their_addr.ss_family,
get_in_addr((struct sockaddr *)&their_addr),
s, sizeof s);
printf("server: got connection from %s\n", s);

if (!fork()) { // this is the child process
close(sockfd); // child doesn't need the listener
if (send(new_fd, "Hello, world!", 13, 0) == -1)
perror("send");
if (numbytes = recv(new_fd, &buf, MAXDATASIZE-1, 0) == -1) {
perror("recv");
exit(1);
}

buf[numbytes] = '\0';
printf("server: received '%s'\n",buf);
close(new_fd);
exit(0);
}
close(new_fd); // parent doesn't need this
}

return 0;
}

Answer

Now I figured it out:

if (numbytes = recv(new_fd, &buf, MAXDATASIZE-1, 0) == -1) {

recv(new_fd, &buf, MAXDATASIZE-1, 0) == -1 is 0 on success and 1 on error. And that gets assigned to numbytes. So, you want to add parenthesises like this:

if ((numbytes = recv(new_fd, &buf, MAXDATASIZE-1, 0)) == -1) {

Or you want to split it into two lines like this:

numbytes = recv(new_fd, &buf, MAXDATASIZE-1, 0);
if (numbytes == -1) {

Oh and next time plz decide for either C or C++, both are different languages.

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