data deletion with concurrent reading/writing in file system

Question

Most file systems use locking to handle concurrent read/write. But what if after a read call, a write call is executed which deletes the data preceding the previous read call.

Is the pointer for a file open for reading updated to reflect the new start of the now smaller file?

Answer 1

The question isn't really valid, because you can't delete data using the write system call. You can overwrite data using the write(2) system call, but you can't delete data. Now, you can truncate the file using the truncate(2) system call. This changes the size of the file (reported via the st_size field by the stat(2) system call), and any bytes after the end of the file as reported by changed st_size will be zero. You can increase the size of the file using the truncate system call by requesting a new size which is larger than the current size. It is undefined (per the POSIX specification) whether this is allowed, or what the system will do when it recieves a truncate larger than the current size of the file. On many file systems it will simply set the size of the file to requested size.

OK, a few more concepts. Associated with each open file structure is a file offset pointer. Attempts to read or write a file using the read(2) or write(2) system call will advance the offset pointer by the number of bytes read or written. If you open a file twice using the open(2) system call, you will get two file descriptors, which each refer to a different open file structure, and in that case, a read(2) or write(2) using one file descriptor will not change the file offset for the other file descriptor. (If you clone a file descriptor using the dup(2) system call, then then you will get a second file descriptor which points to the same file structure, and then changes made to the file structure via one file descriptor, using the read(2), write(2), or lseek(2) system calls will be reflected via the cloned file descriptor. But that's a side issue, so that's all I will say on this topic for now.)

Now, if you truncate the file, this doesn't change the file offset in the file descriptor. However, any bytes after the truncated size will be zero if read. So the answer is that file offset pointer won't be updated after the truncate, but an attempt to read beyond the truncated size of the file will return all zeros.