Bad File Descriptor on a serial port - c

Question

I'm implementing a simple protocol to do file transfer between 2 PC's over serial port and I'm getting a weird error.
On the main I call a function "llopen":
int
llopen(int port, int type) {
int fd = 0;
char* PORT;
PORT = malloc( sizeof(char) * (strlen(COM) + 1) );
sprintf(PORT,"%s%d",COM,port);
fd = initialization(PORT); // Open(...): returns a file descriptor!
switch(type) {
case SENDER:
return connectSender(fd);
break;
case RECEIVER:
return connectReceiver(fd);
break;
}
return fd; // The fd value here is 5
}
After that, I call a function llwrite(int fd, ...) to write a string to the file descriptor, but I'm getting an error: "Bad file descriptor" on llwrite(int fd, ...). If I call again the initialization(port) function, before that, it works and it writes the N bytes on the file descriptor, but if I don't it gives to me the "Bad file descriptor" error again.
Here it is the llwrite(int fd, ...) function:
int
llwrite(int fileDescriptor, unsigned char* buffer, unsigned int length) {
// The fd value here is 5
return writeBuffer(fileDescriptor,buffer,length);
}
Even before the return statement if I call, for instance, the tcflush(...) function I'm getting the "Bad file descriptor" error.
Any clue? Thanks in advance!
EDIT:
The problem is solved.
llopen(...) was wrong. I was returning the number of bytes wrote on the ConnectReceiver(...) / ConnectSender(...) and not the file descriptor
Now it's right:
int
llopen(int port, int type) {
int fd = 0;
char* PORT;
PORT = malloc( sizeof(char) * (strlen(COM) + 1) );
sprintf(PORT,"%s%d",COM,port);
fd = initialization(PORT); // Open(...): returns a file descriptor!
switch(type) {
case SENDER:
if( connectSender(fd) > 0 ) return fd;
case RECEIVER:
if( connectReceiver(fd) > 0 ) return fd;
}
return -1;
}

Answer 1

There's not really enough information here, but it's worth a shot noting that you do
return connectSender(fd);
break;
The break there is dead code, since the return stops execution of the function. Perhaps you didn't mean to return?
If that's not the case try using strace to get more details about what's going on. If you're not on linux other OSes should have similar tools, such as dtruss or ktrace.