.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
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.TH F_GETPIPE_SZ 2const 2025-07-20 "Linux man-pages 6.15"
.SH NAME
F_GETPIPE_SZ,
F_SETPIPE_SZ
\-
get/set the capacity of a pipe
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #define _GNU_SOURCE
.B #include <fcntl.h>
.P
.BI "int fcntl(int " fd ", F_SETPIPE_SZ, int " arg );
.BI "int fcntl(int " fd ", F_GETPIPE_SZ);"
.fi
.SH DESCRIPTION
.TP
.B F_SETPIPE_SZ
Change the capacity of the pipe referred to by
.I fd
to be at least
.I arg
bytes.
An unprivileged process can adjust the pipe capacity to any value
between the system page size and the limit defined in
.I /proc/sys/fs/pipe\-max\-size
(see
.BR proc_sys_fs (5)).
Attempts to set the pipe capacity below the page size are silently
rounded up to the page size.
Attempts by an unprivileged process to set the pipe capacity above the limit in
.I /proc/sys/fs/pipe\-max\-size
yield the error
.BR EPERM ;
a privileged process
.RB ( CAP_SYS_RESOURCE )
can override the limit.
.IP
When allocating the buffer for the pipe,
the kernel may use a capacity larger than
.IR arg ,
if that is convenient for the implementation.
(In the current implementation,
the allocation is the next higher power-of-two page-size multiple
of the requested size.)
The actual capacity (in bytes) that is set is returned as the function result.
.IP
Attempting to set the pipe capacity smaller than the amount
of buffer space currently used to store data produces the error
.BR EBUSY .
.IP
Note that because of the way the pages of the pipe buffer
are employed when data is written to the pipe,
the number of bytes that can be written may be less than the nominal size,
depending on the size of the writes.
.TP
.B F_GETPIPE_SZ
Return (as the function result) the capacity of the pipe referred to by
.IR fd .
.SH RETURN VALUE
The pipe capacity.
.P
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
See
.BR fcntl (2).
.TP
.B EBUSY
.I op
is
.B F_SETPIPE_SZ
and the new pipe capacity specified in
.I arg
is smaller than the amount of buffer space currently
used to store data in the pipe.
.TP
.B EPERM
.I op
is
.B F_SETPIPE_SZ
and the soft or hard user pipe limit has been reached; see
.BR pipe (7).
.SH STANDARDS
Linux.
.SH HISTORY
Linux 2.6.35.
.SH SEE ALSO
.BR fcntl (2)