A. The client functions

void newdownload(char* filename)
{
// The client UI will call this function if user
// wants to download a file from the beginning,
// not restart an aborted file transfer procedure
//
long tbt = 0; // total bytes transferred
char buf[2048]; // buffer
long bt; // bytes transferred in
// a single block
int fd; // local file descriptor
fd = _open(filename, O_WRONLY); // Create a new local file
bt = svr_read(&buf); // returns buf size
while (bt > 0) {
if (isrestartmarker(buf)){ // if buf is a restart marker
updatelogservertimestamp(filename,buf); // then update Log file
} else {
tbt = tbt + bt; // increment total bytes transferred
updatelogbytesize(filename, tbt);
_write(fd, buf, sizeof(buf));
};
bt = svr_read(&buf);
};
_close(fd); // close local file
}
void restartdownload(char* filename)
{
// The client UI will call this function if user
// wants to restart aborted file transfer process
//
long tbt = 0; // total bytes transferred
char buf[2048]; // buffer
long bt; // bytes transferred in a single block
int fd; // local file descriptor
TIMESTAMP sts; // last recorded server timestamp in Log 
TIMESTAMP cts; // last recorded client timestamp in Log 
int ret; // return code for error recovery
// Check if the local file size matches that in Transfer Log
//
tbt = getlogbytes
ize(filename);
sts = getlogservertimestamp(filename);
cts = getlogclienttimestamp(filename);
fd = _open(filename, O_APPEND); // Create a new local file
if (tbt! = _filelength(fd) ||
cts! = getclientfiletimetstamp(filename)) {
// local file has changed 
// since download
_close(fd); // abort restart procedure
return();
};
ret = svr_restart(filename, // ask server to start 
tbt, // (RPC Call) 
sts); 
if (ret == -1) goto shutdown; // error recovery 
bt = svr_read(&buf); 
while (bt > 0) {
// if buf is a restart marker 
// then update Log file
if (isrestartmarker(buf)) {
updatelogtimestamp(filename, buf);
} else {
tbt = tbt + bt; // increment total bytes 
// transferred 
u
pdatelogbytesize(filename, tbt);
_write(fd, buf, sizeof(buf));
};
bt = svr_read(&buf);
};
shutdown:
close(fd); // close local file
}
void updatelogservertimestamp(char* filename,char* buf)
{
int TSLENGTH = 8; // Timestamp length
int fd; // file descriptor of log file
LOGSTRUCT ls = NULL;
char t[8];
fd = _open(filename, O_RDWR);
while (read(fd, ls, sizeof(LOGSTRUCT))) {
if (strcmp(ls.filename, filename) == 0) {
// I found the Log record!
strncpy(t, ls.filename, TSLENGTH); 
// get the timestamp out of buf
ls.rt = (TIMESTAMP)t; 
// cast into timestamp
_write(fd, ls);
exit;
};
};
_close(fd);
}
void updatelogclienttimestamp(char* filename, char* buf)
{
int TSLENGTH = 8; // Timestamp length
int fd; // file descriptor of l
og file
LOGSTRUCT ls = NULL;
char t[8];
fd = _open(filename, O_RDWR);
while (read(fd, ls, sizeof(LOGSTRUCT))) {
if (strcmp(ls.filename, filename) == 0) {
// I found the Log record!
strncpy(t, ls.filename, TSLENGTH); 
// get the timestamp out of buf
ls.ct = (TIMESTAMP)t; 
// cast into timestamp
_write(fd, ls);
exit;
};
};
_close(fd);
}
updatelogbytesize(char* filename, long tbt)
{
int fd; // file descriptor of log file
LOGSTRUCT ls = NULL;
fd = _open(filename, O_RDWR);
while (read(fd, ls, sizeof(LOGSTRUCT))) {
if (strcmp(ls.filename, filename) == 0) {
// I found the Log record!
ls.bytestransferred = tbt; 
_write(fd, ls);
exit;
};
};
_close(fd);
}
TIMESTAMP getlogservertimestamp(char* filename)
{
int fd; // file descri
ptor of log file
LOGSTRUCT ls = NULL;
fd = _open(filename, O_RDWR);
while (read(fd, ls, sizeof(LOGSTRUCT))) {
if (strcmp(ls.filename, filename) == 0) { // I found the Log record!
return ls.rt; 
// return timestamp
};
};
_close(fd);
return NULL;
}
TIMESTAMP getlogclienttimestamp(char* filename)
{
int fd; // file descriptor of log file
LOGSTRUCT ls = NULL;
fd = _open(filename, O_RDWR);
while (read(fd, ls, sizeof(LOGSTRUCT))) {
if (strcmp(ls.filename, filename) == 0) {
// I found the Log record!
return ls.ct; 
// return timestamp
};
};
_close(fd);
return NULL;
}
long getlogbytesize(char *filename)
{
int fd; // file descriptor of log file
LOGSTRUCT ls = NULL;
fd = _open(filename, O_RDONLY);
while (read(fd, ls, sizeof(LOGSTRUCT))) {
if (strcmp(ls.filename, filename) == 0) {
// I found the Log record!
return ls.bytestransferred;
};
};
_close(fd);
return 0;
}
TIMESTAMP getclientfiletimestamp(char *filename)
{
TIMESTAMP t = NULL;
int fd; // file descriptor of log file
char buf[2048];
fd = _open(filename, O_RDONLY);
if (fd > 0) {
t = _fstat(fd, &buffer);
_close(fd);
};
return t.st_time;
}

B. The server functions

static LOGSTRUCT ls;
static int fd;
static int tbt; // total bytes transferred 
// during this session
static int rc; // counter for restart marker intervals
#define RESTART_INTERVAL 100000
int svr_restart(char* filename, long tbt, TIMESTAMP ts)
{
int c = 2048;
char buf[2048];
struct _stat buffer;
fd = _open(filename, O_RDONLY);
_fstat(fd, &buffer);
if (buffer.st_time! = ts) return -1; 
// file has changed!
while (tbt 
> 0) { // Otherwise, move file pointer 
// by tbt bytes
if (tbt < c) c = tbt;
read(fd, buf, c);
tbt = tbt - c;
};
}
int svr_read(char* filename,char **buf)
{
int c = 2048, n;
struct _stat buffer;
strcpy(ls.filename, filename);
if (rc > RESTART_INTERVAL) { 
// Send Restart Marker
rc = 0; // every 100K bytes
ls.bytestransferred = tbt;
_fstat(fd, &buffer);
ls.ts = buffer.st_time;
buf = (char **)ls; 
} else { // else just move the
n = _read(fd, *buf, c); // data down the pipe
tbt = tbt + n;
rc = rc + n;
};
if (n == 0) _close(fd); // close the file when EOF
return(strlen(*buf));
};