server sends back unwanted characters - c

Question

#include <io.h>
#include <stdio.h>
#include <winsock2.h>
#include <string.h>
#define MY_PORT 8989
#define MAXBUF 256
int main(int argc, char *argv[]){
WSADATA wsa;
SOCKET sockfd ,clientfd; //define 2 sockets
struct sockaddr_in self;
char buffer[MAXBUF];
printf("\nInitialising Winsock...");
if (WSAStartup(MAKEWORD(2,2),&wsa) != 0){
printf("Failed. Error Code : %d",WSAGetLastError());
return 1;
}
printf("Initialised.\n");
/*---create streaming socket---*/
if ( (sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0 ){
perror("Socket");
exit(errno);
}
printf("Socket created.\n");
/*---initialize address/port structure---*/
/* bzero(&self, sizeof(self));*/
self.sin_family = AF_INET;
self.sin_port = htons(MY_PORT);
self.sin_addr.s_addr = INADDR_ANY;
/*---assign a port number to the socket---*/
if ((bind(sockfd, (struct sockaddr*)&self, sizeof(self)))!=0){
perror("socket--bind");
exit(errno);
}
puts("Bind done");
/*---make it a "listening socket"---*/
if ((listen(sockfd, 20))!=0){
perror("socket--listen");
exit(errno);
}
puts("Waiting for incoming connections...");
/*---forever... ---*/
while (1){
struct sockaddr_in client_addr;
int addrlen=sizeof(client_addr);
/*---accept a connection (creating a data pipe)---*/
clientfd = accept(sockfd, (struct sockaddr*)&client_addr, &addrlen);
//create variables
int length=recv(clientfd, buffer, MAXBUF, 0);
//output in reverse order & lowercase
for(int i=0; buffer[i]!='\0'; i++){
if(buffer[i]>=65 && buffer[i]<=90){ // check ascii value
buffer[i]= buffer[i]+32; // if uppercase thn change
}
}
int loop=0, len=strlen(buffer);
char temp; //temporary holds character to swap
len--; // remove null and start from 0
while (loop<len){
temp=buffer[loop];
buffer[loop]=buffer[len];
buffer[len]=temp;
loop++;
len--;
}
//send back
send(clientfd, buffer, sizeof(buffer), 0);
/*---close connection---*/
close(clientfd);
}
/*---clean up (should never get here!)---*/
close(sockfd);
WSACleanup();
return 0;
}
The reversing of string from client works well but it also gives out many other unwanted characters behind it and two 'PuTTy's, I'm very new to socket programming so any help is much appreciated. I can't use any string functions other than strlen. Currentlly using puTTy to open server on 8989. Telnet connection is passive.

Answer 1

You code assumes that buffer is NUL terminated, however there is no guaranty that the NUL character will be received by recv (which may be a partial recv and this is streaming socket).
It would make more sense to reverse the data using the return value from recv (if a positive value). This way you do not have to worry about message boundaries.
int length=recv(clientfd, buffer, MAXBUF, 0);
//output in reverse order & lowercase
for(int i=0; i < length; i++){
if(buffer[i]>=65 && buffer[i]<=90){ // check ascii value
buffer[i]= buffer[i]+32; // if uppercase thn change
}
}

Answer 2

You are ALWAYS sending MAXBUF (ie 256) bytes because in
send(clientfd, buffer, sizeof(buffer), 0);
sizeof(buffer) is the size of the char array declared by
#define MAXBUF 256
char buffer[MAXBUF];
Fix this by using strlen(buffer) for example, or keeping the len you got previously.