read()/recv() successfully but buffer is unchanged and empty - c

Question

I am writing a client to read the data sent back from a server over socket under TCP with c script on LINUX.
The server is running forever and I validate if I can get the reply with netcat localhost [PORT_NUMBER] already. The server is in LISTEN state when checking with netstat -nap
The recv() function returns expected number of bytes, but buffer becomes empty and strlen(buffer) is 0. I also try changing to read() which I did not expect a different result, and the same problem was shown.
This is the code.
#include <stdio.h>
#include <string.h> //strlen
#include <sys/socket.h>
#include <arpa/inet.h> //inet_addr
#include <unistd.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
int socket_desc, msg_rep_size = 1200, msg_size = 100;
struct sockaddr_in server;
char message[msg_size], server_reply[msg_rep_size];
//Create socket
socket_desc = socket(AF_INET, SOCK_STREAM, 0);
if (socket_desc == -1)
{
printf("Could not create socket");
}
server.sin_addr.s_addr = inet_addr("0.0.0.0");
server.sin_family = AF_INET;
server.sin_port = htons(31114); // PORT_NUMBER
//Connect to remote server
if (connect(socket_desc, (struct sockaddr *)&server, sizeof(server)) < 0)
{
printf("connect error");
return 1;
}
printf("Connected");
while (1)
{
// send some data
bzero(message, msg_size);
strncpy(message, "REQUEST\n", msg_size);
if (send(socket_desc, message, msg_size, 0) < 0)
{
printf("Send failed");
return 1;
}
printf("Data Send\n");
// Receive a reply from the server
bzero(server_reply, msg_rep_size);
int read_result = recv(socket_desc, server_reply, msg_rep_size, 0);
if (read_result < 0)
{
printf("Receive failed\n");
return 1;
}
else
{
printf("Reply received\n");
printf("read_result: %d\n", read_result);
printf("strlen(server_reply): %d\n", (int)strlen(server_reply));
printf("Reply: %s\n", server_reply);
}
usleep(8);
}
return 0;
}
The printed result in bash is
Data send
Reply received
read_result: 1108
strlen(server_reply): 0
Reply:
The same result is shown if I change from
int read_result = recv(socket_desc, server_reply, msg_rep_size, 0);
to
int read_result = read(socket_desc, server_reply, msg_rep_size);
Thank you for your help. I am new to socket programming and could not track what is going on.
PS. The expected size of bytes from reply is 1108, which is correct. I intended to put maximum size as 1200 to confirm that correct number of byte is received.
PS2. Please feel free to also comment on the coding style of low-level c as well.

Answer 1

There is no evidence here of a problem, only bad code. If recv() returned 1088 it certainly transferred 1088 bytes into the buffer. Clearly the data received starts with a null byte. To print it correctly, use
printf("%.*s\n", read_result, server_reply);
NB:
'The expected size of bytes from reply is 1108, which is correct'. No it isn't. There is no 'expected size'. TCP is a streaming protocol. If recv() returns a positive number, the byte count transferred can be anything from 1 upwards to the buffer length supplied. You have to code a loop to be sure of getting exactly N bytes.
Before assuming it is positive, you must also check read_result for zero, which indicates that the peer has disconnected. Do not omit this step.
strlen(server_reply) is irrelevant. You don't know whether there is a trailing null at all. There could be no nulls at all, or several, including, as in this case, one right at the beginning.