cat terminal , check usb removed (perror)? - c

Question

For an assignment we have to create C program that functions similar to the cat command. The first hand-in requires it to mimic very minimal operations of cat....i.e print to output, redirect. The issue I'm having is that one requirement is to print an error in the case that an output file residing on a usb drive is lost, i.e usb pulled out whilst redirecting stdout to it.
How do I catch such an error, also how can perform a test-case for that particular error ??
Many Thanks....really have no idea
UPDATE CODE TEMP
int main(){
char c;
while((c = getchar()) != EOF){
putchar(c);
// Ensure newly created file exists
}
return EXIT_SUCCESS;
}

Answer 1

Assuming you are using fprintf(), from the man pages:
On success, the total number of characters written is returned.
So:
store the size of the char array you will write into a variable x
if fprintf() is less than x, the writing was interrupted.
exit gracefully
EDIT:
There are 2 things I'm thinking of:
1: When putchar() fails, it indicates an error when writing to the file. Since writing one byte doesn't take very long, this should be unlikely since it will be in a safe state once the byte is written (or you assume).
You can do this like so
if(putchar(c) == EOF){
//write error
}
2: If you're asked to quit the instant you detect a file removal, then you need to monitor the directory. Luckily, you're only looking at one directory. However that while loop gets in the way of things because getchar() is a blocking function (cannot return until something happens). You should use inotify to monitor the directory, then probably poll to poll the file descriptor of inotify(). When I did this I used select because we were forced to.
Some kind of an idea how to monitor a directory with inotify()
int length, i = 0;
char buffer[EVENT_BUF_LEN];
memset(buffer, 0, EVENT_BUF_LEN*sizeof(char));
//init inotify
fd = inotify_init();
if(fd < 0){
perror("inotify init");
}
//add directory to watch list
wd = inotify_add_watch(fd, path , IN_DELETE |
IN_DELETE_SELF | IN_MODIFY | IN_MOVE_SELF | IN_MOVED_FROM | IN_MOVED_TO);
fd_set fds;
FD_ZERO(&fds);
FD_SET(fd, &fds);
//wait for event, since read() blocks
length = read( fd, buffer, EVENT_BUF_LEN );
if ( length < 0 ) {
perror("zero event length");
}
struct inotify_event *event;
while (i < length){
//cast the event to a char buffer
event = (struct inotify_event*) &buffer[i];
if (event->len){
//this was a custom function of mine
storeEvent(event);
}
i += EVENT_SIZE + event->len;
}
You'll have to check which attributes to use when adding a directory (like IN_DELETE or IN_MODIFY) since they will determine what triggers an inotify() event. Note this code will only detect one event, and blocks at the read() statement.