C server socket's read function writing dirty buffer - c

Question

I have the following C server and client:
SERVER
int main(int argc, char **argv) {
int listenfd, connfd, n;
struct sockaddr_in servaddr;
char recvline[1025];
if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
perror("Socket Error");
exit(1);
}
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(1024);
if (bind(listenfd, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0) {
perror("Bind Error");
exit(1);
}
if (listen(listenfd, 1024) < 0) {
perror("Listen Error");
exit(1);
}
while(1) {
printf("Waiting connection...\n");
if ((connfd = accept(listenfd, (struct sockaddr *) NULL, NULL)) < 0) {
perror("Accept Error");
exit(1);
}
printf("Waiting for input.\n");
while ((n = read(listenfd, recvline, sizeof(recvline))) > 0) {
recvline[n] = '\0';
if (fputs(recvline, stdout) == EOF) {
fprintf(stderr,"fputs Error\n");
exit(1);
}
}
printf("Read string: %s", recvline);
close(connfd);
}
}
CLIENT
int main(int argc, char **argv) {
int sockfd, connfd, n;
struct sockaddr_in servaddr;
char buff[1025], matricola[15];
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
perror("Socket Error");
exit(1);
}
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(1024);
if (inet_pton(AF_INET, argv[1], &servaddr.sin_addr) < 0) {
fprintf(stderr,"inet_pton Error for %s\n", argv[1]);
exit (1);
}
if (connect(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0) {
fprintf(stderr,"Connect Error\n");
exit(1);
}
printf("Matricola: ");
fflush(stdin);
scanf("%s", matricola);
strcpy(buff, matricola);
if (write(sockfd, buff, strlen(buff)) != strlen(buff)) {
perror("Write Error");
exit(1);
}
close(connfd);
}
I'm under Linux, I can start the server correctly from terminal that starts listening, but when I open another terminal instance and start the client, the server automatically executes the rest of the code and prints Read string: ### (random characters like dirty buffer). Is this because the server is not waiting for the client to write? What am I doing wrong in this code?
What I tought was that the server didn't wait for the client to write something, so I tried to add a sleep(10) before the server's read command, but nothing changes.
EDIT: I edited the read function from
n = read(listenfd, recvline, strlen(recvline) + 1)) > 0
to
n = read(listenfd, recvline, sizeof(recvline))) > 0
But still it doesn't fix the issue.

Answer 1

Change your inner while cycle to read from the connected file descriptor, not the listening file descriptor, like this:
while ((n = read(connfd, recvline, strlen(recvline) + 1)) > 0) {
recvline[n] = '\0';
if (fputs(recvline, stdout) == EOF) {
fprintf(stderr,"fputs Error\n");
exit(1);
}
}

Answer 2

This line
while ((n = read(listenfd, recvline, sizeof(recvline))) > 0) {
tries to read from the listening socket. This does not make sense.
You need to read from the accepted socket. So it should be:
while ((n = read(connfd, recvline, sizeof(recvline))) > 0) {
Moreover it should be
while ((n = read(connfd, recvline, sizeof(recvline) - 1)) > 0) {
as after the read() you do
recvline[n] = '\0';
For the OP's source if you'd have read sizeof recvline bytes recvline[n] would address one behind the last element of recvline.