Pipes and FIFOs (also known as named pipes) provide a unidirectional interprocess communication channel. A pipe has a
. Data written to the write end of a pipe can be read from the read end of the pipe.
A pipe is created using
(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 can be used to create a communication channel between related processes; see
(2) for an example.
A FIFO (short for First In First Out) has a name within the file system (created using
(3)), and is opened using
(2). Any process may open a FIFO, assuming the file permissions allow it. The read end is opened using the
flag; the write end is opened using the
(7) for further details.
: 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
(2) will block until data is available. If a process attempts to write to a full pipe (see below), then
(2) blocks until sufficient data has been read from the pipe to allow the write to complete. Nonblocking I/O is possible by using the
operation to enable the
open file status flag.
The communication channel provided by a pipe is a
: 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
(2) from the pipe will see end-of-file (
(2) will return 0). If all file descriptors referring to the read end of a pipe have been closed, then a
(2) will cause a
signal to be generated for the calling process. If the calling process is ignoring this signal, then
(2) fails with the error
. An application that uses
(2) should use suitable
(2) calls to close unnecessary duplicate file descriptors; this ensures that end-of-file and
are delivered when appropriate.
It is not possible to apply
(2) to a pipe.
A pipe has a limited capacity. If the pipe is full, then a
(2) will block or fail, depending on whether the
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 i386). Since Linux 2.6.11, the pipe capacity is 65536 bytes.
POSIX.1-2001 says that
(2)s of less than
bytes must be atomic: the output data is written to the pipe as a contiguous sequence. Writes of more than
bytes may be nonatomic: the kernel may interleave the data with data written by other processes. POSIX.1-2001 requires
to be at least 512 bytes. (On Linux,
is 4096 bytes.) The precise semantics depend on whether the file descriptor is nonblocking (
), whether there are multiple writers to the pipe, and on
, the number of bytes to be written:
n bytes are written atomically;
write(2) may block if there is not room for
n bytes to be written immediately
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
The write is nonatomic: 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.
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
flag for the read end of a pipe causes a signal (
by default) to be generated when new input becomes available on the pipe (see
(2) for details). On Linux,
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.1-2001, pipes only need to be unidirectional. Portable applications should avoid reliance on bidirectional pipe semantics.