/* server.c */#include <stdio.h>#include <string.h>#include <unistd.h>#include <ctype.h>#include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>#include <stdlib.h>#include <errno.h>#define MAXLINE 80int             port = 8000;intmain( void ){    struct sockaddr_in sin;    struct sockaddr_in pin;    int             listen_fd;    int             conn_fd;    int             address_size;    char            buf[MAXLINE];    char            str[INET_ADDRSTRLEN];    int             i;    int             n;    int             len;    bzero( &sin, sizeof( sin ) );    sin.sin_family = AF_INET;    sin.sin_addr.s_addr = INADDR_ANY;    sin.sin_port = htons( port );    listen_fd = socket( AF_INET, SOCK_STREAM, 0 );    if ( listen_fd == -1 )    {        perror( "call to socket" );        exit( 1 );    }    n = bind( listen_fd, ( struct sockaddr * ) &sin, sizeof( sin ) );    if ( n == -1 )    {        perror( "call to bind" );        exit( 1 );    }    n = listen( listen_fd, 20 );    if ( n == -1 )    {        perror( "call to listen" ), exit( 1 );    }    printf( "Accepting connections ...\n" );    while ( 1 )    {        conn_fd =            accept( listen_fd, ( struct sockaddr * ) &pin, &address_size );        if ( conn_fd == -1 )        {            perror( "call to accept" );            exit( 1 );        }      readagain:        n = read( conn_fd, buf, MAXLINE );        if ( n == -1 )        {            if ( errno == EINTR )            {                goto readagain;            }            else            {                perror( "call to read" );                exit( 1 );            }        }        else if ( n == 0 )        {            printf( "the other side has been closed.\n" );            close( conn_fd );            continue;        }        printf( "received from client %s at port %d: %s\n",                inet_ntop( AF_INET, &pin.sin_addr, str, sizeof( str ) ),                ntohs( pin.sin_port ), buf );        // just convert the characters to upper case        len = strlen( buf );        for ( i = 0; i < len; i++ )        {            buf[i] = toupper( buf[i] );        }      writeagain:        n = write( conn_fd, buf, len + 1 );        if ( n == -1 )        {            if ( errno == EINTR )            {                goto writeagain;            }            else            {                printf( "call to write" );                exit( 1 );            }        }        n = close( conn_fd );        if ( n == -1 )        {            perror( "call to close" );            exit( 1 );        }    }}