C++: Classic communication exercise between server and client

Question

Good day.

As a computer science student, learning low-level C programming, i'm stucked in the "classic" practice exercise of writting a server-client communicating program.

The goal is to develop a server component which receives a command from a remote client component, and execute it as a local shell command; then, the command's output is send again to the client. Pretty simple.

My code send the command from the client, the server succesfully receive it, execute it and captures the output. But at this point, when the sayd server tries to reply with that output to the client ... something goes wrong and the client receives nothing. No clue if the problem is in the server part, or in the client counterpart.

Any idea? Thanks in advance!

Server:

struct sockaddr_in srvaddr, cliaddr;
memset(&srvaddr, 0, sizeof(srvaddr));
memset(&cliaddr, 0, sizeof(cliaddr));

int sk = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);

srvaddr.sin_family = AF_INET;
srvaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
srvaddr.sin_port = htons(42000);
bind(sk, (struct sockaddr*)&srvaddr, sizeof(srvaddr));
recvfrom(sk, recepcion, sizeof(recepcion), 0, (struct sockaddr*)&cliaddr, sizeof(cliaddr));

// [...] Portion of code with a Pipe pointing to a Fork which runs the command...

// And here is where, maybe, the communication is lost:
sendto(sk, recepcion, sizeof(recepcion), 0, (struct sockaddr*)&cliaddr, sizeof(cliaddr));

Client:

struct sockaddr_in srvaddr, cliaddr;

memset(&srvaddr, 0, sizeof(srvaddr));
memset(&cliaddr, 0, sizeof(cliaddr));

int sk = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);

cliaddr.sin_family = AF_INET;
cliaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
cliaddr.sin_port = htons(42001);

srvaddr.sin_family = AF_INET;
srvaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
srvaddr.sin_port = htons(42000);

// [...] Some other code catching the command from the argument paramenters:

sendto(sk, comando, strlen(comando), 0, (struct sockaddr*)&srvaddr, sizeof(srvaddr));

// And here's where the server reply should be, but theres nothing:
recvfrom(sk, buff, sizeof(buff), 0, (struct sockaddr*)&srvaddr, sizeof(srvaddr));

Say i print all the traces with: fprintf(stderr, ""); So, loosing the terminal's focus due to forking should not be an issue.

Bye and thanks!

Answer 1

The problem is most likely the recvfrom call. If you check the manual page you will see that the source-address length is a pointer. I don't know how you managed to get it to compile without errors or warnings.

You need to initialize the size to the actual size of the socket-address structure, pass a pointer to it, and the recvfrom function will fill in the actual size:

socklen_t cliaddrlen = sizeof(cliaddr);
recvfrom(sk, recepcion, sizeof(recepcion), 0,
         (struct sockaddr *) &cliaddr, &cliaddrlen);

Oh, and I do assume you check for errors in your actual code?