C Sock-Programming Problems with accept - c

Question

Im programming a server/client program and have a weird problem here....
The server binds a socket and listening to a socket. But it is stucked at accepting the client. It is a simulation of a bank program. The server stores a balance of one account and do what the client wants. The weird situation is that I've got no error messages at accepting and the server prints my message on the console "Accepted", but it did not do the next print message. But the writing of the balance to the client did well because the client gets the right value.....the next print message is also ignored by the server.....whats wrong here??? The program is not ready yet and i'm only testing a few functionalities here.
#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <netdb.h>
#include <arpa/inet.h>
#define MAX 20
int main(int argc, char **argv){
int sockfd,csocket;
struct sockaddr_in addr,client_addr;
socklen_t clilen;
clilen = sizeof(client_addr);
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) {
fprintf(stderr, "Error in opening socket - %s\n", strerror(errno));
return EXIT_FAILURE;
}
addr.sin_family = AF_INET;
addr.sin_port = htons(atoi(argv[1]));
addr.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(sockfd, (struct sockaddr *) &addr, sizeof(struct sockaddr_in)) < 0){
fprintf(stderr, "Error in binding - %s\n", strerror(errno));
return EXIT_FAILURE;
}
printf("Bind\n");
int list = listen(sockfd, SOMAXCONN);
if (list < 0) {
fprintf(stderr, "Error in listening - %s\n", strerror(errno));
return EXIT_FAILURE;
}
printf("Listening\n");
char sel;
int tmp,bal=666;
while(1) {
//newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
csocket = accept(sockfd,(struct sockaddr *) &client_addr,&clilen);
if (csocket < 0) {
fprintf(stderr, "Error in accepting - %s\n", strerror(errno));
return EXIT_FAILURE;
}
printf("Accepted\n");
printf("Writing balance");
if (write(csocket,&bal,sizeof(int))<0){
fprintf(stderr, "Error in Writing - %s", strerror(errno));
return EXIT_FAILURE;
}
printf("Writing balance");
if(read(csocket, &sel, sizeof(char))<0){
fprintf(stderr, "Error in reading - %s\n", strerror(errno));
return EXIT_FAILURE;
}
printf("User wählt:%c\n",sel);
/********Einzahlen**********/
if (sel=='e'){
printf("Neuer Kontostand: %d", bal);
if(read(csocket, &tmp, sizeof(int))<0){
fprintf(stderr, "Error in accepting - %s\n", strerror(errno));
return EXIT_FAILURE;
}
bal+=tmp;
printf("Neuer Kontostand: %d", bal);
}
if(sel=='q'){
close(csocket);
printf("User beendet\n");
}
}
}
The Client
{add include part here}
int main(){
int sockfd;
socklen_t addrlen;
addrlen = sizeof(struct sockaddr_in);
struct sockaddr_in addr;
sockfd = socket(AF_INET, SOCK_STREAM, 0);
char ip[16];
printf("IP (Press 0 for 127.0.0.1):");
scanf("%15s", ip);
//printf("\n");
if (ip[0]=='0'){
strcpy(ip,DEFAULT_IP);
printf("Setting IP to 127.0.0.1\n");
}
printf("%s",ip);
int port;
printf("Port:");
scanf("%d",&port);
printf("\n");
addr.sin_family = AF_INET;
addr.sin_port = htons(port);
addr.sin_addr.s_addr=inet_addr(ip);
int tmp;
if (connect(sockfd, (struct sockaddr *) &addr, sizeof(struct sockaddr_in))<0){
fprintf(stderr, "Error in connecting - %s\n", strerror(errno));
}
printf("Connected\n");
int bal,w;
if(read(sockfd, &bal, sizeof(int))<0){
fprintf(stderr, "Error in accepting - %s\n", strerror(errno));
return EXIT_FAILURE;
}
printf("Balance:%d\n",bal);
printf("Geld [e]inzahlen\n"); //add money to account
printf("Geld [a]uszahlen\n"); //Getting money from account
char msg[MAX];
printf("Enter your choce:\n");
scanf("%s",msg);
if (msg[0]=='e'){
printf("How much?:\n");
scanf("%i",&w);
if (write(sockfd,&w,sizeof(int))<0 ){
fprintf(stderr, "Error in Writing - %s", strerror(errno));
return EXIT_FAILURE;
}
bal+=w;
}
printf("sended");
if (close(sockfd) < 0) {
fprintf(stderr, "Error in closing socket - %s\n", strerror(errno));
}
}

Answer 1

The immediate problem is combination of two things.
First, you print "Accepted" without newline. Standard output is line buffered, so it will not be printed until you print a newline (or read input from stdin, or explicit fflush(stdout);).
Second, you then enter read, which apparently never returns. Theoretically, also the write could block, but you say client receives the write, so server must be stuck at the read.
End result, your server program is indefinitely stuck at the read. And the output you thought you printed is stuck at the stdout buffers, waiting to be flushed.
The reason for this is simple: your client writes only 4 bytes, but your server tries to read 4+1 bytes. So actually it is the 2nd read at server, which blocks indefinitely.
Some general hints:
At Stack Overflow, please look at the preview and make sure your code is properly indented, or fewer people will bother to even look at it (hint: avoid TABs, use only spaces to indent, to avoid extra hassle).
Always check return value of any scanf functions, and doubly so if you are reading user input from keyboard. Even if it is just experimental code, you don't want to waste time solving bugs which are not bugs but normal behaviour of scanf.
When copy-pasting, try to fix the strings you copy paste, such as function names in your error messages, or you will be confused by "wrong" error message texts of your own.

Answer 2

You need a call to the select() function to test if the file descriptor is ready prior to the accept(). Also look up fd_set and the related FD_ZERO, FD_SET, and FD_ISSET macros. They are needed to monitor whether the socket is ready to accept the incoming connection. This is a great resource for socket programming.