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Given an arbitrary file descriptor, can I make it blocking if it is non-blocking? If so, how?
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Note: The default mode is blocking. If data is not available to the socket, and the socket is in blocking and synchronous modes, the READ call blocks the caller until data arrives.
Normally, write() will block until it has written all of the data to the file. If you try to write data to a pipe with a full internal buffer, your write() will not return until someone has read enough out of the pipe to make room for all of the data that you're trying to add.
When a process makes a successful request to open a file, the kernel returns a file descriptor which points to an entry in the kernel's global file table. The file table entry contains information such as the inode of the file, byte offset, and the access restrictions for that data stream (read-only, write-only, etc.).
close() closes a file descriptor, so that it no longer refers to any file and may be reused. Any record locks (see fcntl(2)) held on the file it was associated with, and owned by the process, are removed (regardless of the file descriptor that was used to obtain the lock).
Its been a while since I played with C, but you could use the fcntl() function to change the flags of a file descriptor:
#include <unistd.h>
#include <fcntl.h>
// Save the existing flags
saved_flags = fcntl(fd, F_GETFL);
// Set the new flags with O_NONBLOCK masked out
fcntl(fd, F_SETFL, saved_flags & ~O_NONBLOCK);
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