man pipe (Conventions) - overview of pipes and FIFOs
NAME
pipe - overview of pipes and FIFOs
DESCRIPTION
Pipes and FIFOs (also known as named pipes) provide a unidirectional interprocess communication channel. A pipe has a read end and a write end. Data written to the write end of a pipe can be read from the read end of the pipe.
A pipe is created using pipe(2), which creates a new pipe and returns two file descriptors, one referring to the read end of the pipe, the other referring to the write end. Pipes only allow communication between related processes: one process creates the pipe, and then allows another process to inherit duplicate file descriptors referring to the pipe as a result of calling fork(2).
A FIFO (short for First In First Out) has a name within the file system (created using mkfifo(3)), and is opened using open(2). Any process may open a FIFO, assuming the file permissions allow it. The read end is opened using the O_RDONLY flag; the write end is opened using the O_WRONLY flag. See fifo(4) for further details. Note: although FIFOs have a pathname in the file system, I/O on FIFOs does not involve operations on the underlying device (if there is one).
I/O on Pipes and FIFOs
The only difference between pipes and FIFOs is the manner in which they are created and opened. Once these tasks have been accomplished, I/O on pipes and FIFOs has exactly the same semantics.
If a process attempts to read from an empty pipe, then read(2) will block until data is available. If a process attempts to write to a full pipe (see below), then write(2) blocks until sufficient data has been read from the pipe to allow the write to complete. Non-blocking I/O is possible by using the fcntl(2) F_SETFL operation to enable the O_NONBLOCK open file status flag.
The communication channel provided by a pipe is a byte stream: there is no concept of message boundaries.
If all file descriptors referring to the write end of a pipe have been closed, then an attempt to read(2) from the pipe will see end-of-file (read(2) will return 0). If all file descriptors referring to the read end of a pipe have been closed, then a write(2) will cause a SIGPIPE signal to be generated for the calling process. If the calling process is ignoring this signal, then write(2) fails with the error EPIPE. An application that uses pipe(2) and fork(2) should use suitable close(2) calls to close unnecessary duplicate file descriptors; this ensures that end-of-file and SIGPIPE/EPIPE are delivered when appropriate.
Pipe Capacity
A pipe has a limited capacity. If the pipe is full, then a write(2) will block or fail, depending on whether the O_NONBLOCK flag is set (see below). Different implementations have different limits for the pipe capacity. Applications should not rely on a particular capacity: an application should be designed so that a reading process consumes data as soon as it is available, so that a writing process does not remain blocked.
In Linux versions before 2.6.11, the capacity of a pipe was the same as the system page size (e.g., 4096 bytes on x86). Since Linux 2.6.11, the pipe capacity is 65536 bytes.
PIPE_BUF
POSIX.1 says that write(2)s of less than PIPE_BUF bytes must be atomic: the output data is written to the pipe as a contiguous sequence. Writes of more than PIPE_BUF bytes may be non-atomic: the kernel may interleave the data with data written by other processes. POSIX.1 requires PIPE_BUF to be at least 512 bytes. (On Linux, PIPE_BUF is 4096 bytes.) The precise semantics depend on whether the file descriptor is non-blocking (O_NONBLOCK), whether there are multiple writers to the pipe, and on n, the number of bytes to be written:
- O_NONBLOCK disabled, n <= PIPE_BUF
- All n bytes are written atomically; write(2) may block if there is not room for n bytes to be written immediately
- O_NONBLOCK enabled, n <= PIPE_BUF
- If there is room to write n bytes to the pipe, then write(2) succeeds immediately, writing all n bytes; otherwise write(2) fails, with errno set to EAGAIN.
- O_NONBLOCK disabled, n > PIPE_BUF
- The write is non-atomic: the data given to write(2) may be interleaved with write(2)s by other process; the write(2) blocks until n bytes have been written.
- O_NONBLOCK enabled, n > PIPE_BUF
- If the pipe is full, then write(2) fails, with errno set to EAGAIN. Otherwise, from 1 to n bytes may be written (i.e., a "partial write" may occur; the caller should check the return value from write(2) to see how many bytes were actually written), and these bytes may be interleaved with writes by other processes.
Open File Status Flags
The only open file status flags that can be meaningfully applied to a pipe or FIFO are O_NONBLOCK and O_ASYNC.
Setting the O_ASYNC flag for the read end of a pipe causes a signal (SIGIO by default) to be generated when new input becomes available on the pipe (see fcntl(2) for details). On Linux, O_ASYNC is supported for pipes and FIFOs only since kernel 2.6.
Portability notes
On some systems (but not Linux), pipes are bidirectional: data can be transmitted in both directions between the pipe ends. According to POSIX.1, pipes only need to be unidirectional. Portable applications should avoid reliance on bidirectional pipe semantics.