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


6       pipe - overview of pipes and FIFOs


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.
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.
19       A  FIFO (short for First In First Out) has a name within the filesystem
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 filesystem, I/O on FIFOs does not involve opera‐
25       tions on the underlying device (if there is one).
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.
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.   Nonblocking
36       I/O  is  possible by using the fcntl(2) F_SETFL operation to enable the
37       O_NONBLOCK open file status flag.
39       The communication channel provided by a pipe is a byte stream: there is
40       no concept of message boundaries.
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.
52       It is not possible to apply lseek(2) to a pipe.
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.
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 i386).  Since  Linux  2.6.11,
65       the  pipe  capacity  is 16 pages (i.e., 65,536 bytes in a system with a
66       page size of 4096 bytes).  Since Linux 2.6.35, the default pipe  capac‐
67       ity  is  16  pages,  but  the capacity can be queried and set using the
68       fcntl(2) F_GETPIPE_SZ and F_SETPIPE_SZ operations.   See  fcntl(2)  for
69       more information.
71       The  following  ioctl(2)  operation,  which  can  be  applied to a file
72       descriptor that refers to either end of a pipe, places a count  of  the
73       number  of unread bytes in the pipe in the int buffer pointed to by the
74       final argument of the call:
76           ioctl(fd, FIONREAD, &nbytes);
78       The FIONREAD operation is not specified in any standard,  but  is  pro‐
79       vided on many implementations.
81   /proc files
82       On  Linux,  the following files control how much memory can be used for
83       pipes:
85       /proc/sys/fs/pipe-max-pages (only in Linux 2.6.34)
86              An upper limit, in pages, on the capacity that  an  unprivileged
87              user (one without the CAP_SYS_RESOURCE capability) can set for a
88              pipe.
90              The default value for this limit is 16 times  the  default  pipe
91              capacity (see above); the lower limit is two pages.
93              This  interface  was  removed  in  Linux  2.6.35,  in  favor  of
94              /proc/sys/fs/pipe-max-size.
96       /proc/sys/fs/pipe-max-size (since Linux 2.6.35)
97              The maximum size (in bytes) of individual pipes that can be  set
98              by  users  without  the  CAP_SYS_RESOURCE capability.  The value
99              assigned to this file may be  rounded  upward,  to  reflect  the
100              value  actually  employed  for  a convenient implementation.  To
101              determine the rounded-up value, display  the  contents  of  this
102              file after assigning a value to it.
104              The default value for this file is 1048576 (1 MiB).  The minimum
105              value that can be assigned to this file is the system page size.
106              Attempts  to  set a limit less than the page size cause write(2)
107              to fail with the error EINVAL.
109              Since Linux 4.9, the value on this file also acts as  a  ceiling
110              on the default capacity of a new pipe or newly opened FIFO.
112       /proc/sys/fs/pipe-user-pages-hard (since Linux 4.5)
113              The hard limit on the total size (in pages) of all pipes created
114              or set by a single unprivileged user (i.e., one with neither the
115              CAP_SYS_RESOURCE  nor the CAP_SYS_ADMIN capability).  So long as
116              the total number of pages allocated to  pipe  buffers  for  this
117              user  is  at  this  limit,  attempts to create new pipes will be
118              denied, and attempts to  increase  a  pipe's  capacity  will  be
119              denied.
121              When  the value of this limit is zero (which is the default), no
122              hard limit is applied.
124       /proc/sys/fs/pipe-user-pages-soft (since Linux 4.5)
125              The soft limit on the total size (in pages) of all pipes created
126              or set by a single unprivileged user (i.e., one with neither the
127              CAP_SYS_RESOURCE nor the CAP_SYS_ADMIN capability).  So long  as
128              the  total  number  of  pages allocated to pipe buffers for this
129              user is at this limit, individual pipes created by a  user  will
130              be limited to one page, and attempts to increase a pipe's capac‐
131              ity will be denied.
133              When the value of this limit is zero, no soft limit is  applied.
134              The default value for this file is 16384, which permits creating
135              up to 1024 pipes with the default capacity.
137       Before Linux 4.9, some bugs affected the  handling  of  the  pipe-user-
138       pages-soft and pipe-user-pages-hard limits; see BUGS.
140   PIPE_BUF
141       POSIX.1 says that write(2)s of less than PIPE_BUF bytes must be atomic:
142       the output data is written  to  the  pipe  as  a  contiguous  sequence.
143       Writes  of  more  than  PIPE_BUF bytes may be nonatomic: the kernel may
144       interleave the data with data  written  by  other  processes.   POSIX.1
145       requires  PIPE_BUF  to  be  at least 512 bytes.  (On Linux, PIPE_BUF is
146       4096 bytes.)  The precise semantics depend on whether the file descrip‐
147       tor  is nonblocking (O_NONBLOCK), whether there are multiple writers to
148       the pipe, and on n, the number of bytes to be written:
150       O_NONBLOCK disabled, n <= PIPE_BUF
151              All n bytes are written atomically; write(2) may block if  there
152              is not room for n bytes to be written immediately
154       O_NONBLOCK enabled, n <= PIPE_BUF
155              If  there  is  room  to write n bytes to the pipe, then write(2)
156              succeeds immediately, writing all n  bytes;  otherwise  write(2)
157              fails, with errno set to EAGAIN.
159       O_NONBLOCK disabled, n > PIPE_BUF
160              The write is nonatomic: the data given to write(2) may be inter‐
161              leaved with write(2)s by  other  process;  the  write(2)  blocks
162              until n bytes have been written.
164       O_NONBLOCK enabled, n > PIPE_BUF
165              If  the  pipe  is  full,  then write(2) fails, with errno set to
166              EAGAIN.  Otherwise, from 1 to n bytes may be  written  (i.e.,  a
167              "partial  write"  may  occur; the caller should check the return
168              value from write(2) to see how many bytes  were  actually  writ‐
169              ten),  and  these  bytes may be interleaved with writes by other
170              processes.
172   Open file status flags
173       The only open file status flags that can be meaningfully applied  to  a
174       pipe or FIFO are O_NONBLOCK and O_ASYNC.
176       Setting  the  O_ASYNC  flag  for the read end of a pipe causes a signal
177       (SIGIO by default) to be generated when new input becomes available  on
178       the  pipe.   The  target  for delivery of signals must be set using the
179       fcntl(2) F_SETOWN command.  On Linux, O_ASYNC is  supported  for  pipes
180       and FIFOs only since kernel 2.6.
182   Portability notes
183       On  some  systems (but not Linux), pipes are bidirectional: data can be
184       transmitted in both directions between the pipe ends.  POSIX.1 requires
185       only unidirectional pipes.  Portable applications should avoid reliance
186       on bidirectional pipe semantics.
188   BUGS
189       Before Linux 4.9, some bugs affected the  handling  of  the  pipe-user-
190       pages-soft  and  pipe-user-pages-hard  limits  when  using the fcntl(2)
191       F_SETPIPE_SZ operation to change a pipe's capacity:
193       (1)  When increasing the pipe capacity, the checks against the soft and
194            hard  limits  were made against existing consumption, and excluded
195            the memory required for the  increased  pipe  capacity.   The  new
196            increase in pipe capacity could then push the total memory used by
197            the user for pipes (possibly far) over a limit.  (This could  also
198            trigger the problem described next.)
200            Starting  with  Linux  4.9, the limit checking includes the memory
201            required for the new pipe capacity.
203       (2)  The limit checks were performed even when the  new  pipe  capacity
204            was  less  than  the  existing  pipe capacity.  This could lead to
205            problems if a user set a large pipe capacity, and then the  limits
206            were  lowered,  with  the  result  that  the  user could no longer
207            decrease the pipe capacity.
209            Starting with Linux 4.9, checks against the limits  are  performed
210            only  when  increasing a pipe's capacity; an unprivileged user can
211            always decrease a pipe's capacity.
213       (3)  The accounting and checking against the limits were done  as  fol‐
214            lows:
216            (a) Test whether the user has exceeded the limit.
217            (b) Make the new pipe buffer allocation.
218            (c) Account new allocation against the limits.
220            This  was racey.  Multiple processes could pass point (a) simulta‐
221            neously, and then allocate pipe buffers that  were  accounted  for
222            only  in  step  (c),  with  the result that the user's pipe buffer
223            allocation could be pushed over the limit.
225            Starting with Linux 4.9, the accounting step is  performed  before
226            doing  the  allocation, and the operation fails if the limit would
227            be exceeded.
229       Before Linux 4.9, bugs similar to points (1) and (3) could  also  occur
230       when  the  kernel allocated memory for a new pipe buffer; that is, when
231       calling pipe(2) and when opening a previously unopened FIFO.


234       mkfifo(1), dup(2),  fcntl(2),  open(2),  pipe(2),  poll(2),  select(2),
235       socketpair(2),  splice(2),  stat(2),  tee(2),  vmsplice(2),  mkfifo(3),
236       epoll(7), fifo(7)


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246Linux                             2017-09-15                           PIPE(7)