C - Sending Modified String Over Sockets - c

Question

I'm trying to send a modified string over sockets. The goal is to take a string from the client, add something to it, then send it back. The strings are passed in command line arguments. Right now, I can receive the message from the client, but for some reason my recvfrom function is returning -1 which causes the client to get hung up and not receive the sendto from the server. It seems like the server is modifying the string fine, but I can't get it to receive correctly on the other side. I have print statements in my code for testing purposes. The command line arguments for the client are server name, port number, string. The command line arguments for the server are port number, string to concatenate. Below is my code:
headerFiles.h:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <errno.h>
#include <signal.h>
#include <unistd.h>
#include <string.h>
#include <arpa/inet.h>
#include <sys/wait.h>
server.c:
#include "headerFiles.h"
int main(int argc, char* argv[])
{
int s;
int len;
char buffer[256];
struct sockaddr_in servAddr;
struct sockaddr_in clntAddr;
int clntAddrLen;
int serverPort;
char catStringMeow[256];
serverPort = atoi(argv[1]);
strcpy(catStringMeow, argv[2]);
// Build local (server) socket address
memset(&servAddr, 0, sizeof(servAddr));
servAddr.sin_family = AF_INET;
servAddr.sin_port = htons(serverPort);
servAddr.sin_addr.s_addr = htonl(INADDR_ANY);
// Create socket
if((s = socket(PF_INET, SOCK_DGRAM, 0)) < 0)
{
perror("Error: socket failed!");
exit(1);
}
// Bind socket to local address and port
if((bind(s, (struct sockaddr*)&servAddr, sizeof(servAddr)) < 0))
{
perror("Error: bind failed!");
exit(1);
}
for(;;) // Runs forever
{
printf("buffer = %s\n", buffer);
printf("In for\n");
// Receive String
len = recvfrom(s, buffer, sizeof(buffer), 0, (struct sockaddr*)&clntAddr, &clntAddrLen);
printf("Received %d bytes\n", len);
printf("buffer = %s\n", buffer);
strcat(buffer, " ");
strcat(buffer, catStringMeow);
printf("New string = %s\n",buffer);
printf("buffer size = %d\n", (int)strlen(buffer));
len = (int)strlen(buffer);
// Send String
sendto(s, buffer, len, 0, (struct sockaddr*)&clntAddr, sizeof(clntAddr));
printf("Sent %d bytes\n", len);
}
}
client.c:
#include "headerFiles.h"
int main (int argc, char* argv[]) // Three arguments to be checked later
{
int s; // Socket descriptor
int len; // Length of string to be echoed
char* servName; // Server name
int servPort; // Server port
char* string; // String to be echoed
char buffer[256+1]; // Data buffer
struct sockaddr_in servAddr; // Server socket address
// Check and set program arguments
if(argc != 4)
{
printf("Error: three arguments are needed!\n");
exit(1);
}
servName = argv[1];
servPort = atoi(argv[2]);
string = argv[3];
// Build server socket address
memset(&servAddr, 0, sizeof(servAddr));
servAddr.sin_family = AF_INET;
inet_pton(AF_INET, servName, &servAddr.sin_addr);
servAddr.sin_port = htons(servPort);
// Create socket
if((s = socket(PF_INET, SOCK_DGRAM, 0)) < 0)
{
perror("Error: Socket failed!");
exit(1);
}
// Send echo string
len = sendto(s, string, strlen(string), 0, (struct sockaddr*) &servAddr, sizeof(servAddr));
printf("Sent %d bytes\n", len);
// Receive echo string
len = recvfrom(s, buffer, len, 0, NULL, NULL);
printf("Received\n");
//Print and verify echoed string
buffer[len] = '\0';
printf("Echo string received: ");
fputs(buffer, stdout);
printf("\n");
// Close the socket
close(s);
// Stop the program
exit(0);
}

Answer 1

do
// Receive echo string
len = recvfrom(s, buffer, sizeof(buffer), 0, NULL, NULL);
printf("Received\n");
and make buffer bigger.

Answer 2

You're passing an uninitizlied clntAddrLen value to recvfrom, which is resulting in the Invalid argument error code. According to the documentation:
The argument addrlen is a value-result argument, which the caller should initialize before the call to the size of the buffer associated with src_addr, and modified on return to indicate the actual size of the source address.
So you need to initialize it with:
clntAddrLen = sizeof(clntAddr);

Answer 3

Make sure you initialize the client address length variable before using it in sendto or recvfrom.
The problem is that on the first calling to sendto from client.c, the servers sees the client's ip as 0.0.0.0, after that on the second, third,... calls the client.c get an ip and have a legal ip such as 127.0.0.3:3212.
You can see that second, third, ... clients work.
Initialize length variable to sizeof(struct sockaddr_in)