Sub-functions to send and receive string over socket - c++

Question

I assume that for messages that are of only 1 byte (a char), I will use read() and write() directly.
For those messages having size > 1 bytes, I use two subfunctions to read and write them over sockets.
For example, I have the server construct a string called strcities (list of city) and print it out --> nothing strange. Then send the number of bytes of this string to the client, and then the actual string.
The client will first read the number of bytes, then the actual city list.
For some reason my code sometimes work and sometimes doesn't. If it works, it also prints out some extra characters that I have no idea where they come from. If it doesn't, it hangs and forever waits in the client, while the server goes back to the top of the loop and wait for next command from the client. Could you please take a look at my codes below and let me know where I did wrong?
Attempt_read
string attempt_read(int rbytes) { // rbytes = number of bytes of message to be read
int count1, bytes_read;
char buffer[rbytes+1];
bool notdone = true;
count1 = read(sd, buffer, rbytes);
while (notdone) {
if (count1 == -1){
perror("Error on write call");
exit(1);
}
else if (count1 < rbytes) {
rbytes = rbytes - count1; // update remaining bytes to be read
count1 = read(sd, buffer, rbytes);
}
else {notdone = false;}
} // end while
string returnme;
returnme = string(buffer);
return returnme;
}
Attempt_write
void attempt_write(string input1, int wbytes) { // wbytes = number of bytes of message
int count1;
bool notdone = true;
count1 = write(sd, input1.c_str(), wbytes);
while (notdone) {
if (count1 == -1){
perror("Error on write call");
exit(1);
}
else if (count1 < wbytes) {
wbytes = wbytes - count1;
count1 = write(sd, input1.c_str(), wbytes);
}
else {notdone = false;}
} // end while
return;
}

Answer 1

1) string class has a method size() that will return the length of the string, so you do not actually need a second attempt_write parameter.
2) You can transfer length of message before message or you can transfer a terminating 0 after, if you only will sent an ASCII strings. Because your connection could terminate at any time, it is better to send exact length before sending the string, so your client could know, what to expect.
3) What compilator do you use, that would allow char buffer[rbytes+1]; ? A standard c++ would require char buffer = new char[rbytes+1]; and corresponding delete to avoid a memory leaks.
4) In your code, the second read function call use same buffer with no adjustment to length, so you, practically, overwrite the already received data and the function will only work, if all data will be received in first function call. Same goes for write function
I would suggest something like this:
void data_read(unsigned char * buffer, int size) {
int readed, total = 0;
do {
readed = read(sd, buffer + total, size - total);
if (-1 == writted) {
perror("Error on read call");
exit(1);
}
total += readed;
} while (total < size);
}
string attempt_read() {
int size = 0;
data_read((unsigned char *) &size, sizeof(int));
string output(size, (char) 0x0);
data_read((unsigned char *) output.c_str(), size);
return output;
}
void data_write(unsigned char * buffer, int size) {
int writted, total = 0;
do {
writted = write(sd, buffer + total, size - total);
if (-1 == writted) {
perror("Error on write call");
exit(1);
}
total += writted;
} while (total < size);
}
void attempt_write(string input) {
int size = input.size();
data_write((unsigned char *) &size, sizeof(int));
data_write((unsigned char *) input.c_str(), size);
}