1PIPE(7)                    Linux Programmer's Manual                   PIPE(7)
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NAME

6       pipe - overview of pipes and FIFOs
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DESCRIPTION

9       Pipes  and  FIFOs  (also known as named pipes) provide a unidirectional
10       interprocess communication channel.  A pipe has a read end and a  write
11       end.  Data written to the write end of a pipe can be read from the read
12       end of the pipe.
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14       A pipe is created using pipe(2), which creates a new pipe  and  returns
15       two  file  descriptors,  one referring to the read end of the pipe, the
16       other referring to the write end.  Pipes can be used to create a commu‐
17       nication channel between related processes; see pipe(2) for an example.
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19       A FIFO (short for First In First Out) has a name within the file system
20       (created using mkfifo(3)), and is opened using  open(2).   Any  process
21       may  open a FIFO, assuming the file permissions allow it.  The read end
22       is opened using the O_RDONLY flag; the write end is  opened  using  the
23       O_WRONLY  flag.  See fifo(7) for further details.  Note: although FIFOs
24       have a pathname in the file system, I/O on FIFOs does not involve oper‐
25       ations on the underlying device (if there is one).
26
27   I/O on Pipes and FIFOs
28       The only difference between pipes and FIFOs is the manner in which they
29       are created and opened.  Once these tasks have been  accomplished,  I/O
30       on pipes and FIFOs has exactly the same semantics.
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32       If  a  process  attempts  to read from an empty pipe, then read(2) will
33       block until data is available.  If a process attempts  to  write  to  a
34       full  pipe  (see below), then write(2) blocks until sufficient data has
35       been read from the pipe to allow the write to  complete.   Non-blocking
36       I/O  is  possible by using the fcntl(2) F_SETFL operation to enable the
37       O_NONBLOCK open file status flag.
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39       The communication channel provided by a pipe is a byte stream: there is
40       no concept of message boundaries.
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42       If  all file descriptors referring to the write end of a pipe have been
43       closed, then an attempt to read(2) from the pipe will  see  end-of-file
44       (read(2) will return 0).  If all file descriptors referring to the read
45       end of a pipe have been closed, then a write(2) will  cause  a  SIGPIPE
46       signal to be generated for the calling process.  If the calling process
47       is ignoring this signal, then write(2) fails with the error EPIPE.   An
48       application  that uses pipe(2) and fork(2) should use suitable close(2)
49       calls to close unnecessary duplicate  file  descriptors;  this  ensures
50       that end-of-file and SIGPIPE/EPIPE are delivered when appropriate.
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52       It is not possible to apply lseek(2) to a pipe.
53
54   Pipe Capacity
55       A  pipe  has  a limited capacity.  If the pipe is full, then a write(2)
56       will block or fail, depending on whether the  O_NONBLOCK  flag  is  set
57       (see  below).   Different implementations have different limits for the
58       pipe capacity.  Applications should not rely on a particular  capacity:
59       an  application  should  be designed so that a reading process consumes
60       data as soon as it is available, so that a  writing  process  does  not
61       remain blocked.
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63       In Linux versions before 2.6.11, the capacity of a pipe was the same as
64       the system page size (e.g., 4096 bytes on x86).   Since  Linux  2.6.11,
65       the pipe capacity is 65536 bytes.
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67   PIPE_BUF
68       POSIX.1-2001  says  that  write(2)s of less than PIPE_BUF bytes must be
69       atomic: the output  data  is  written  to  the  pipe  as  a  contiguous
70       sequence.   Writes  of  more than PIPE_BUF bytes may be non-atomic: the
71       kernel may interleave the data with data written  by  other  processes.
72       POSIX.1-2001  requires  PIPE_BUF  to be at least 512 bytes.  (On Linux,
73       PIPE_BUF is 4096 bytes.)  The precise semantics depend on  whether  the
74       file  descriptor is non-blocking (O_NONBLOCK), whether there are multi‐
75       ple writers to the pipe, and on n, the number of bytes to be written:
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77       O_NONBLOCK disabled, n <= PIPE_BUF
78              All n bytes are written atomically; write(2) may block if  there
79              is not room for n bytes to be written immediately
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81       O_NONBLOCK enabled, n <= PIPE_BUF
82              If  there  is  room  to write n bytes to the pipe, then write(2)
83              succeeds immediately, writing all n  bytes;  otherwise  write(2)
84              fails, with errno set to EAGAIN.
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86       O_NONBLOCK disabled, n > PIPE_BUF
87              The  write  is  non-atomic:  the  data  given to write(2) may be
88              interleaved with write(2)s by other process; the write(2) blocks
89              until n bytes have been written.
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91       O_NONBLOCK enabled, n > PIPE_BUF
92              If  the  pipe  is  full,  then write(2) fails, with errno set to
93              EAGAIN.  Otherwise, from 1 to n bytes may be  written  (i.e.,  a
94              "partial  write"  may  occur; the caller should check the return
95              value from write(2) to see how many bytes  were  actually  writ‐
96              ten),  and  these  bytes may be interleaved with writes by other
97              processes.
98
99   Open File Status Flags
100       The only open file status flags that can be meaningfully applied  to  a
101       pipe or FIFO are O_NONBLOCK and O_ASYNC.
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103       Setting  the  O_ASYNC  flag  for the read end of a pipe causes a signal
104       (SIGIO by default) to be generated when new input becomes available  on
105       the  pipe  (see  fcntl(2) for details).  On Linux, O_ASYNC is supported
106       for pipes and FIFOs only since kernel 2.6.
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108   Portability notes
109       On some systems (but not Linux), pipes are bidirectional: data  can  be
110       transmitted  in  both  directions  between the pipe ends.  According to
111       POSIX.1-2001, pipes only need to be unidirectional.  Portable  applica‐
112       tions should avoid reliance on bidirectional pipe semantics.
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SEE ALSO

115       dup(2),  fcntl(2), open(2), pipe(2), poll(2), select(2), socketpair(2),
116       stat(2), mkfifo(3), epoll(7), fifo(7)
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120Linux 2.6.15                      2005-12-08                           PIPE(7)
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