write pipe

1. PIPE_BUF
   
2.        POSIX.1 says that write(2)s of less than PIPE_BUF bytes must be atomic:
   
3.        the  output  data  is  written  to  the  pipe as a contiguous sequence.
   
4.        Writes of more than PIPE_BUF bytes may be non-atomic:  the  kernel  may
   
5.        interleave  the  data  with  data  written by other processes.  POSIX.1
   
6.        requires PIPE_BUF to be at least 512 bytes.   (On  Linux,  PIPE_BUF  is
   
7.        4096 bytes.)  The precise semantics depend on whether the file descrip-
   
8.        tor is non-blocking (O_NONBLOCK), whether there are multiple writers to
   
9.        the pipe, and on n, the number of bytes to be written:
   
10. 
    
11.        O_NONBLOCK disabled, n <= PIPE_BUF
    
12.               All  n bytes are written atomically; write(2) may block if there
    
13.               is not room for n bytes to be written immediately
    
14. 
    
15.        O_NONBLOCK enabled, n <= PIPE_BUF
    
16.               If there is room to write n bytes to  the  pipe,  then  write(2)
    
17.               succeeds  immediately,  writing  all n bytes; otherwise write(2)
    
18.               fails, with errno set to EAGAIN.
    
19. 
    
20.        O_NONBLOCK disabled, n > PIPE_BUF
    
21.               The write is non-atomic: the  data  given  to  write(2)  may  be
    
22.               interleaved with write(2)s by other process; the write(2) blocks
    
23.               until n bytes have been written.
    
24. 
    
25.        O_NONBLOCK enabled, n > PIPE_BUF
    
26.               If the pipe is full, then write(2)  fails,  with  errno  set  to
    
27.               EAGAIN.   Otherwise,  from  1 to n bytes may be written (i.e., a
    
28.               "partial write" may occur; the caller should  check  the  return
    
29.               value  from  write(2)  to see how many bytes were actually writ-
    
30.               ten), and these bytes may be interleaved with  writes  by  other
    
31.               processes.