pipe (7) - Linux Man Pages
pipe: overview of pipes and FIFOs
pipe - overview of pipes and FIFOs
DESCRIPTIONPipes 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 can be used to create a communication channel between related processes; see pipe(2) for an example.
A FIFO (short for First In First Out) has a name within the filesystem (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(7) for further details. Note: although FIFOs have a pathname in the filesystem, I/O on FIFOs does not involve operations on the underlying device (if there is one).
I/O on pipes and FIFOsThe 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. Nonblocking 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.
It is not possible to apply lseek(2) to a pipe.
Pipe capacityA 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 i386). Since Linux 2.6.11, the pipe capacity is 16 pages (i.e., 65,536 bytes in a system with a page size of 4096 bytes). Since Linux 2.6.35, the default pipe capacity is 16 pages, but the capacity can be queried and set using the fcntl(2) F_GETPIPE_SZ and F_SETPIPE_SZ operations. See fcntl(2) for more information.
The following ioctl(2) operation, which can be applied to a file descriptor that refers to either end of a pipe, places a count of the number of unread bytes in the pipe in the int buffer pointed to by the final argument of the call:
flag for the read end of a pipe causes a signal
by default) to be generated when new input becomes available on the pipe.
The target for delivery of signals must be set using the
is supported for pipes and FIFOs only since kernel 2.6.
Before Linux 4.9, bugs similar to points (1) and (3) could also occur
when the kernel allocated memory for a new pipe buffer;
that is, when calling
and when opening a previously unopened FIFO.
On Linux, the following files control how much memory can be used for pipes:
POSIX.1 says that
of less than
bytes must be atomic: the output data is written to the pipe as a
Writes of more than
bytes may be nonatomic: the kernel may interleave the data
with data written by other processes.
to be at least 512 bytes.
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:
Open file status flags
The only open file status flags that can be meaningfully applied to
a pipe or FIFO are
On some systems (but not Linux), pipes are bidirectional:
data can be transmitted in both directions between the pipe ends.
POSIX.1 requires only unidirectional pipes.
Portable applications should avoid reliance on
bidirectional pipe semantics.
Before Linux 4.9, some bugs affected the handling of the
limits when using the
operation to change a pipe's capacity:
This page is part of release 5.05 of the Linux
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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. The target for delivery of signals must be set using the fcntl(2) F_SETOWN command. On Linux, O_ASYNC is supported for pipes and FIFOs only since kernel 2.6.
Before Linux 4.9, bugs similar to points (1) and (3) could also occur when the kernel allocated memory for a new pipe buffer; that is, when calling pipe(2) and when opening a previously unopened FIFO.