Manpage of PIPE
PIPESection: Linux Programmer's Manual (7)
NAMEpipe - overview of pipes and FIFOs
DESCRIPTIONPipes and FIFOs (also known as named pipes) provide a unidirectional interprocess communication channel. A pipe has a read endand 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_RDONLYflag; the write end is opened using the O_WRONLYflag. 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_SETFLoperation to enable the O_NONBLOCKopen 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 SIGPIPEsignal 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/EPIPEare 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_NONBLOCKflag 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. Since Linux 2.6.35, the default pipe capacity is 65536 bytes, but the capacity can be queried and set using the fcntl(2) F_GETPIPE_SZand F_SETPIPE_SZoperations. See fcntl(2) for more information.
PIPE_BUFPOSIX.1 says that write(2)s of less than PIPE_BUFbytes must be atomic: the output data is written to the pipe as a contiguous sequence. Writes of more than PIPE_BUFbytes may be nonatomic: the kernel may interleave the data with data written by other processes. POSIX.1 requires PIPE_BUFto be at least 512 bytes. (On Linux, PIPE_BUFis 4096 bytes.) The precise semantics depend on whether the file descriptor is nonblocking (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 nbytes are written atomically; write(2) may block if there is not room for nbytes to be written immediately
- O_NONBLOCK enabled, n <= PIPE_BUF
- If there is room to write nbytes to the pipe, then write(2) succeeds immediately, writing all nbytes; otherwise write(2) fails, with errnoset to EAGAIN.
- O_NONBLOCK disabled, n > PIPE_BUF
- 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 nbytes have been written.
- O_NONBLOCK enabled, n > PIPE_BUF
- If the pipe is full, then write(2) fails, with errnoset to EAGAIN. Otherwise, from 1 to nbytes 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 flagsThe only open file status flags that can be meaningfully applied to a pipe or FIFO are O_NONBLOCKand O_ASYNC.
Setting the O_ASYNCflag for the read end of a pipe causes a signal (SIGIOby 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_SETOWNcommand. On Linux, O_ASYNCis supported for pipes and FIFOs only since kernel 2.6.
Portability notesOn 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.
SEE ALSOmkfifo(1), dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), socketpair(2), splice(2), stat(2), mkfifo(3), epoll(7), fifo(7)
- SEE ALSO
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Time: 22:28:03 GMT, June 20, 2016