Socket recv() function get null message - c

Question

I have a problem about the recv() function.
When I sent the first message(ex: hi) to server first, the server would send back the same message to client.
And I sent again(ex: hi2), I would get a null message.
But I kept sending message,maybe I sent two times again. I got the second message I sent.
I want the result is client send to server and client receive the right message immediately.
How can I solve this problem?
my client:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int main(int argc , char *argv[])
{
int sockFD = 0;
sockFD = socket(AF_INET , SOCK_STREAM , 0);
if (sockFD == -1){
printf("Fail to create a socket.");
}
struct sockaddr_in info;
bzero(&info,sizeof(info));
info.sin_family = PF_INET;
info.sin_addr.s_addr = inet_addr("127.0.0.1");
info.sin_port = htons(8080);
if(connect(sockFD,(struct sockaddr *)&info,sizeof(info)) == -1){
perror("connect()");
}
char receiveMessage[100] = {};
memset(receiveMessage, 0, sizeof(receiveMessage));
char* input;
while(scanf("%s", input) == 1){
if(strcmp("exit", input) == 0){
break;
}
if(send(sockFD, input, sizeof(input), 0) < 0){
perror("send()");
}
if(recv(sockFD, receiveMessage, sizeof(receiveMessage), 0) < 0){
perror("recv()");
}
printf("Server: %s\n",receiveMessage);
}
printf("close Socket\n");
close(sockFD);
return 0;
}
my server:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int main()
{
int sockFD = 0, clientSockFD= 0;
char inputBuffer[256] = {};
//char outputBuffer[256] = {};
fd_set masterFD;
fd_set readFD;
int fdMax;
// create socket
sockFD = socket(AF_INET, SOCK_STREAM, 0);
if(sockFD == -1){
perror("socket()");
exit(0);
}
struct sockaddr_in serverInfo, clientInfo;
int addrLen = sizeof(clientInfo);
bzero(&serverInfo, sizeof(serverInfo));
serverInfo.sin_family = PF_INET;
serverInfo.sin_addr.s_addr = INADDR_ANY;
serverInfo.sin_port = htons(8080);
// bind port
if(bind(sockFD, (struct sockaddr *)&serverInfo, sizeof(serverInfo))){
perror("bind()");
}
// listen
if(listen(sockFD, 10)){
perror("listen()");
}
FD_SET(sockFD, &masterFD);
fdMax = sockFD;
while(1){
readFD = masterFD;
if(select(fdMax + 1, &readFD, NULL, NULL, NULL) == -1){
perror("select()");
exit(0);
}
for(int i = 0; i <= fdMax; i++){
if(FD_ISSET(i, &readFD)){
if(i == sockFD){
if((clientSockFD = accept(sockFD, (struct sockaddr *)&clientInfo, &addrLen)) < 0){
perror("accept()");
}
FD_SET(clientSockFD, &masterFD);
if(clientSockFD > fdMax){
fdMax = clientSockFD;
}
printf("selectserver: new connection\n");
}
else{
int recvStatus = recv(i, inputBuffer, sizeof(inputBuffer), 0);
if(recvStatus <= 0){
if(recvStatus < 0){perror("recv()");}
else if(recvStatus = 0){printf("colse connected");}
close(i);
FD_CLR(i, &masterFD);
}
else{
printf("client: %s\n", inputBuffer);
send(i, inputBuffer, sizeof(inputBuffer), 0);
}
}
}
}
}
return 0;
}

Answer 1

Change
send(i, inputBuffer, sizeof(inputBuffer), 0);
Into:
send(i, inputBuffer,recvStatus , 0);
The cause: you were sending a complete buffer, containing only a few intended characters, but padded (upto size=256) with NULs, (or whatever garbage is the buffer from previous calls))
recv() and read() return the number of characters received. (or 0 on EOF,or -1 on errors (some of which could be handled, for instance: EAGAIN, EINTR))