Add a header to a string and sending it with TCP socket - c

Question

I have a client/server that exchanges messages, and i am trying to add the size of the string i am sending, in the beginning of the string, in order for the server to know how many bytes to read.
I added the message starting from the +4 pos of the char* and used memcpy to copy the strlen of the string. It doesnt seem to work and something tells me its the wrong way to do it. This is my code.
//*CLIENT*//
send_message = malloc(1024 * sizeof(char));
strcpy(send_message + 4,"GETFILES ");
strcat(send_message,"/");
strcat(send_message,directory_name);
size = strlen(send_message) + 1;
csize = malloc(4 * sizeof(char));
csize = (char*)&size;
memcpy(&send_message,csize,4);
if((sent = send(sock, send_message, strlen(send_message) + 1, 0)) < 0)
perror("write");
//*SERVER*//
while(1){
count = recv(events[i].data.fd, buf, sizeof(buf),0);
if(count == -1){
//if errno = EAGAIN we have read all data. going back to main loop
if(errno != EAGAIN){
perror("read");
done = 1;
}
break;
}
else if(count == 0){
//End of file. The remote has closed the connections
done = 1;
break;
}
printf("received message %s and count %d\n", buf, count);
}
if i comment these lines
csize = malloc(4 * sizeof(char));
csize = (char*)&size;
memcpy(send_message,csize,4);
I get this output:
received message ▒�w�GETFILES /test and count 19
otherwise i get no output..Any ideas how to fix it and add the header so my server knows in advance how many bytes to read?

Answer 1

As commented already, the main problem is the use of strlen(), but there is some more error.
First the strlen() and other str functions could be used in this way.
strcpy(send_message + 4,"GETFILES ");
strcat(send_message + 4,"/");
strcat(send_message + 4,directory_name);
size = strlen(send_message + 4) + 1;
It is not a good way to fix it, but it is easier to understand why your code is not working.
This is unnecessary
csize = malloc(4 * sizeof(char));
csize = (char*)&size;
memcpy(&send_message,csize,4);
You could simply do this
memcpy(send_message,&size,4);
BUT, for good practice and portability, replace all your magic 4 for sizeof(int32_t).
send_message is an array, so you do not need to get it address (&send_message), it may work this way, but if it was a pointer instead of an array it would break your code.
And last, you are printing the entire buff, but you forgot that you have a 4 bytes header, and that is the main reason it do not print anything if you properly initialize the csize.
If you try to do this
printf("received message %s and count %d\n", buf+4, count);
It will probably works, but it do not mean it is correct.
Edit: I will not update my answer to not make it bigger and harder to you see your mistake, but consider #thuovila comments below and look for more informations about htonl ntohl and how to use them.