/* rlsd.c - a remote ls server - with paranoia */#include  <stdio.h>#include  <unistd.h>#include  <sys/types.h>#include  <sys/socket.h>#include  <netinet/in.h>#include  <netdb.h>#include  <time.h>#include  <strings.h>#define   PORTNUM  15000   /* our remote ls server port */#define   HOSTLEN  256#define   oops(msg)      { perror(msg) ; exit(1) ; }int main(int ac, char *av[]){	struct  sockaddr_in   saddr;   /* build our address here */	struct	hostent		*hp;   /* this is part of our    */	char	hostname[HOSTLEN];     /* address 	         */	int	sock_id,sock_fd;       /* line id, file desc     */	FILE	*sock_fpi, *sock_fpo;  /* streams for in and out */	FILE	*pipe_fp;	       /* use popen to run ls    */	char    dirname[BUFSIZ];       /* from client            */	char    command[BUFSIZ];       /* for popen()            */	int	dirlen, c;      /** Step 1: ask kernel for a socket **/	sock_id = socket( PF_INET, SOCK_STREAM, 0 );    /* get a socket */	if ( sock_id == -1 ) 		oops( "socket" );      /** Step 2: bind address to socket.  Address is host,port **/	bzero( (void *)&saddr, sizeof(saddr) ); /* clear out struct     */	gethostname( hostname, HOSTLEN );       /* where am I ?         */	hp = gethostbyname( hostname );         /* get info about host  */	bcopy( (void *)hp->h_addr, (void *)&saddr.sin_addr, hp->h_length);	saddr.sin_port = htons(PORTNUM);        /* fill in socket port  */	saddr.sin_family = AF_INET ;            /* fill in addr family  */	if ( bind(sock_id, (struct sockaddr *)&saddr, sizeof(saddr)) != 0 )	       oops( "bind" );      /** Step 3: allow incoming calls with Qsize=1 on socket **/	if ( listen(sock_id, 1) != 0 ) 		oops( "listen" );      /*       * main loop: accept(), write(), close()       */	while ( 1 ){	       sock_fd = accept(sock_id, NULL, NULL); /* wait for call */	       if ( sock_fd == -1 )		       oops("accept");       	       /* open reading direction as buffered stream */	       if( (sock_fpi = fdopen(sock_fd,"r")) == NULL )		       oops("fdopen reading");	       if ( fgets(dirname, BUFSIZ-5, sock_fpi) == NULL )		       oops("reading dirname");	       sanitize(dirname);	       /* open writing direction as buffered stream */	       if ( (sock_fpo = fdopen(sock_fd,"w")) == NULL )		       oops("fdopen writing");	       sprintf(command,"ls %s", dirname);	       if ( (pipe_fp = popen(command, "r")) == NULL )		       oops("popen");	      	       /* transfer data from ls to socket */	       while( (c = getc(pipe_fp)) != EOF )		       putc(c, sock_fpo);	       pclose(pipe_fp);	       fclose(sock_fpo);	       fclose(sock_fpi);       }}sanitize(char *str)/* * it would be very bad if someone passed us an dirname like * "; rm *"  and we naively created a command  "ls ; rm *" * * so..we remove everything but slashes and alphanumerics * There are nicer solutions, see exercises */{	char *src, *dest;	for ( src = dest = str ; *src ; src++ )		if ( *src == '/' || isalnum(*src) )			*dest++ = *src;	*dest = '\0';}