Programming Sockets in C, error to send and receive files - c

Question

I made an app in C for Linux environment. This app works, but after
some time (5 hours more or less) its performance is erratic.
This is the background. I have some remote computers (400 more or
less) that send its data through sockets (my app) every 5 minutes. All
process is a loop and I need that works all day. The app first send
data (if exists) and then receive data (also, if exists). My app works
like a Server side with some arguments or it works like a Client side
with other arguments. The app working as Server mode, after the listening state,
only waits for client connections. Once that a client it's connected,
the App in Client mode sends a message to indicates that will be sent
data, then sends a new message with the size and name concatenated to
server side (always sends 64 bytes, so my app writes 64 bytes and in
the other side reads 64 bytes), then sends data (the file). The Server
reads the message of size and name of file to receive (of 64 bytes),
splits the message, storing the size in a variable and file name in
other, and then after received the file data. compares the size stored
in the variable with the size of data readed, if all is ok, then the
server sends a message and stores the data in a new file with the name
that was it before received. So on untill the last file will be sent
it.
When my app was in state of development, I noticed that after some
hours the messages they become corrupted, I mean that the messages are
incomplete or have more data, probably data of the next ones. So the
file data too. So, how can i send the file data through the same
socket?? Note: I have a buffer to send and receive messages, and
other buffer for send and receive file data.
And other question:
Original code:
int copydata(int readfd, int writefd, int offset, int bytes) {
char buffer[4096];
int crbytes = 0, cwbytes = 0, trbytes = 0, twbytes = 0;
if (offset) {
if (lseek(readfd, offset, SEEK_CUR) < 0) {
return -1;
}
}
while ((crbytes = read(readfd, buffer, bytes)) > 0) {
if (crbytes < 0) {
return -1;
}
trbytes += crbytes;
//printf("dbgmsg::Readed data <%dB> | Total readed data <%dB>\n", crbytes, trbytes);
while (crbytes > 0) {
cwbytes = write(writefd, buffer, crbytes);
if (cwbytes < 0) {
return -1;
}
twbytes += cwbytes;
crbytes -= cwbytes;
//printf("dbgmsg::Written data <%dB> | Total written data <%dB>\n", cwbytes, twbytes);
}
}
return twbytes;
}
This code is used to send and receive file data. The side that sends
data uses the file descriptor of the file that we want to send (to
read) <readfd> and writes on the file descriptor of the socket (to
write) <writefd>. The side that receives data, use <readfd> to read
from the socket file descriptor and <writefd> to write in the file
descriptor from the file where we want to write the data.
If are sent other messages before use this function, both, client and
server will be stuck in the end of the inner "while loop", in other
words, the client sends all file data, and the server receives all
file data (in this point, the received data is complete, how i know?
Because i can open the the received file). No errors, only one, "no message of
desired type". If i not sent messages before this function all works
fine.
To skip this little problem, i modify the code, passing a file size as
argument too and write between two whiles one if structure.
int copydata(int readfd, int filesz, int writefd, int offset, int bytes) {
char buffer[4096];
int crbytes = 0, cwbytes = 0, trbytes = 0, twbytes = 0;
if (offset) {
if (lseek(readfd, offset, SEEK_CUR) < 0) {
return -1;
}
}
while ((crbytes = read(readfd, buffer, bytes)) > 0) {
if (crbytes < 0) {
return -1;
}
trbytes += crbytes;
//printf("dbgmsg::Readed data <%dB> | Total readed data <%dB>\n", crbytes, trbytes);
while (crbytes > 0) {
cwbytes = write(writefd, buffer, crbytes);
if (cwbytes < 0) {
return -1;
}
twbytes += cwbytes;
crbytes -= cwbytes;
//printf("dbgmsg::Written data <%dB> | Total written data <%dB>\n", cwbytes, twbytes);
}
if (twbytes == filesz) { break; }
}
return twbytes;
}
Thanks for advance, and sorry for my english!!

Answer 1

If the program encounters an "incomplete" write(), your inner loop resends the beginning of the buffer.
while (crbytes > 0) {
cwbytes = write(writefd, buffer, crbytes);
if (cwbytes < 0) {
return -1;
}
twbytes += cwbytes;
crbytes -= cwbytes;
//printf("dbgmsg::Written data <%dB> | Total written data <%dB>\n", cwbytes, twbytes);
}
You could change it to something like:
for ( sentbytes=0; sentbytes < crbytes; sentbytes += cwbytes ) {
cwbytes = write(writefd, buffer+sentbytes, crbytes - sentbytes);
if (cwbytes < 0) { return -1; }
twbytes += cwbytes;
}