unix tcp client/server weird behavior

Question

I've been trying to learn unix network programming so I tried to write a client/server program in which the client sends a message and the server returns the message converted to uppercase letters.

When I run the server and connect with a client to my own machine and send some test strings, it works fine until a point where the things I previously entered get written to the screen.

I suspect it has something to do with the buffer. Here's the sample run after I start the server:

can@ubuntu:~$ cd Desktop

can@ubuntu:~/Desktop$ ./tcpuppcli 127.0.0.1

Enter the string to echo: test

Echo response: TEST

Enter the string to echo: string2

Echo response: STRING2

Enter the string to echo: string3

Echo response: STRING3

Enter the string to echo: aaaaaaaaafsfagd

Echo response: AAAAAAAAAFSFAGD

Enter the string to echo: gdsgsg

Echo response: GDSGSG

AAFSFAGD ----------->!this is the weird line that has chars from the previous input!

Enter the string to echo: ^C

can@ubuntu:~/Desktop$

The code for the server and client is as follows:

// methods with prefix w_ are just error checking wrappers from UNP by Stevens    
// server
#include "socketwrap.h" // headers necessary and constants like MAXLINE
#include <ctype.h>

void sigchld_handler( int sig);
void str_upper( int connfd);
void toUpperCase( char buffer[], int length);

int main( int argc, char** argv)
{
  int listenfd;
  int connfd;
  pid_t childpid;
  struct sockaddr_in serverAddress;
  struct sockaddr_in clientAddress;
  socklen_t length;
  struct sigaction sa;

  // Create the socket
  listenfd = w_socket( AF_INET, SOCK_STREAM, 0);

  // Clear the serverAddress structure
  bzero( &serverAddress, sizeof( serverAddress));
  // Set up the serverAddress structure
  serverAddress.sin_family = AF_INET;
  serverAddress.sin_addr.s_addr = htonl( INADDR_ANY);
  serverAddress.sin_port = htons( 11979);

  // Bind the socket to a well-defined port
  w_bind( listenfd, (struct sockaddr*) &serverAddress, sizeof( serverAddress));

  // Start listening for connections
  w_listen( listenfd, BACKLOG);

  // Handle any zombie children by using a signal handler
  sa.sa_handler = sigchld_handler;
  sigemptyset( &sa.sa_mask);
  sa.sa_flags = SA_RESTART;
  if( sigaction( SIGCHLD, &sa, NULL) == -1)
  {
    perror( "signal error");
    exit( 1);
  }

  printf( "Waiting for connections...\n");

  while( 1)
  {
      length = sizeof( clientAddress);
      connfd = w_accept( listenfd, ( struct sockaddr*) &clientAddress, &length);
      if( connfd < 0)
      {
          if( errno == EINTR)
          {
            continue; // back to while
          }
          else
          {
            perror( "accept error");
            exit( 1);
          }
      }

      printf( "Obtained connection...\n");
      childpid = fork();
      if ( childpid == 0) /* child process */
      {
        w_close( listenfd); /* close listening socket */
        str_upper( connfd); // process the request
        exit( 0);
      }

    w_close( connfd); /* parent closes connected socket */
  }

}

void sigchld_handler( int sig)
{
  while( waitpid( -1, NULL, WNOHANG) > 0);
}

void str_upper( int connfd)
{
  char buffer[MAXLINE];
  while( read( connfd, buffer, MAXLINE - 1) > 0)
  {
    toUpperCase( buffer, strlen( buffer));
    write( connfd, buffer, strlen( buffer));
  }
}

void toUpperCase( char buffer[], int length)
{
  int i;
  for( i = 0; i < length - 1; i++)
  {
      buffer[i] = toupper( buffer[i]);
  }
}

// client
#include "socketwrap.h"

void str_cli( int connfd);

int main( int argc, char** argv)
{
  int sockfd;
  struct sockaddr_in serverAddress;

  if( argc != 2)
  {
      printf( "Invalid argument count\n");
      printf( "Correct usage: tcpcli4 <IPaddress>\n");
      exit( 1);
  }


  sockfd = w_socket( AF_INET, SOCK_STREAM, 0);
  bzero( &serverAddress, sizeof( serverAddress));
  serverAddress.sin_family = AF_INET;
  serverAddress.sin_port = htons( 11979);
  if( inet_pton( AF_INET, argv[1], &serverAddress.sin_addr) <= 0)
  {
      perror( "inet_pton error");
      exit( 1);
  }

  w_connect( sockfd, ( struct sockaddr*) &serverAddress, sizeof( serverAddress));

  str_cli( sockfd);
  exit( 0);
}

void str_cli( int connfd)
{
  char buffer[MAXLINE];

  printf( "Enter the string to echo: ");
  while( fgets( buffer, MAXLINE, stdin) > 0)
  {

    // Send string to echo server, and retrieve response
    write( connfd, buffer, strlen( buffer));
    read( connfd, buffer, MAXLINE - 1);

    // Output echoed string
    printf( "Echo response: %s\n", buffer);
    printf( "Enter the string to echo: ");
  }
}

If you need additional information or there's anything unclear please inform me

Thanks for all replies

Hmm. Does it happen any time you give a longer input followed by a shorter input? — aschepler
tried it and yes it does, no problems if the input gets longer and longer but long followed by shorter does this. — john27

Alan Curry Alan Curry · Accepted Answer · 2012-08-26T23:11:30

read() doesn't add a '\0' terminator to the string, so the result your strlen() call is not well-defined. You need to use the return value of read() (store it in a variable, don't just test it for > 0 then throw it away) to know how long the string is.

It seemed to be working correctly for a while, but that's just dumb luck with '\0's that were in the buffer when it was uninitialized. Try it with a memset(buffer, 'X', sizeof buffer) before the read loop and you'll get an earlier failure.

unix tcp client/server weird behavior

2 Answers