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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.Dd November 2, 2025 29.Dt POSIX_FALLOCATE 2 30.Os 31.Sh NAME 32.Nm posix_fallocate 33.Nd pre-allocate storage for a range in a file 34.Sh LIBRARY 35.Lb libc 36.Sh SYNOPSIS 37.In fcntl.h 38.Ft int 39.Fn posix_fallocate "int fd" "off_t offset" "off_t len" 40.Sh DESCRIPTION 41Required storage for the range 42.Fa offset 43to 44.Fa offset + 45.Fa len 46in the file referenced by 47.Fa fd 48is guaranteed to be allocated upon successful return. 49That is, if 50.Fn posix_fallocate 51returns successfully, subsequent writes to the specified file data 52will not fail due to lack of free space on the file system storage 53media. 54Any existing file data in the specified range is unmodified. 55If 56.Fa offset + 57.Fa len 58is beyond the current file size, then 59.Fn posix_fallocate 60will adjust the file size to 61.Fa offset + 62.Fa len . 63Otherwise, the file size will not be changed. 64.Pp 65Space allocated by 66.Fn posix_fallocate 67will be freed by a successful call to 68.Xr creat 2 69or 70.Xr open 2 71that truncates the size of the file. 72Space allocated via 73.Fn posix_fallocate 74may be freed by a successful call to 75.Xr ftruncate 2 76that reduces the file size to a size smaller than 77.Fa offset + 78.Fa len . 79.Sh RETURN VALUES 80If successful, 81.Fn posix_fallocate 82returns zero. 83It returns an error on failure, without setting 84.Va errno . 85.Sh ERRORS 86Possible failure conditions: 87.Bl -tag -width Er 88.It Bq Er EBADF 89The 90.Fa fd 91argument is not a valid file descriptor. 92.It Bq Er EBADF 93The 94.Fa fd 95argument references a file that was opened without write permission. 96.It Bq Er EFBIG 97The value of 98.Fa offset + 99.Fa len 100is greater than the maximum file size. 101.It Bq Er EINTR 102A signal was caught during execution. 103.It Bq Er EINVAL 104The 105.Fa len 106argument was less than or equal to zero, the 107.Fa offset 108argument was less than zero. 109.It Bq Er EIO 110An I/O error occurred while reading from or writing to a file system. 111.It Bq Er EINTEGRITY 112Corrupted data was detected while reading from the file system. 113.It Bq Er ENODEV 114The 115.Fa fd 116argument does not refer to a file that supports 117.Nm . 118.It Bq Er ENOSPC 119There is insufficient free space remaining on the file system storage 120media. 121.It Bq Er ENOTCAPABLE 122The file descriptor 123.Fa fd 124has insufficient rights. 125.It Bq Er EOPNOTSUPP 126The operation is not supported by the file system. 127.It Bq Er ESPIPE 128The 129.Fa fd 130argument is associated with a pipe or FIFO. 131.El 132.Sh SEE ALSO 133.Xr creat 2 , 134.Xr ftruncate 2 , 135.Xr open 2 , 136.Xr unlink 2 137.Sh STANDARDS 138The 139.Fn posix_fallocate 140system call conforms to 141.St -p1003.1-2024 . 142.Sh HISTORY 143The 144.Fn posix_fallocate 145function appeared in 146.Fx 9.0 . 147.Pp 148Previous versions of 149.Nm 150used 151.Er EINVAL 152to indicate that the operation is not supported by the file system, as specified 153in 154.St -p1003.1 155Base Specifications, Issue 7. 156.St -p1003.1 157Base Specifications, Issue 8 switched to requiring 158.Er EOPNOTSUPP 159for this error case. 160ZFS adopted the latter convention in 161.Fx 15.0 , 162and the remaining filesystems in base adopted it in 163.Fx 15.1 . 164.Sh AUTHORS 165.Fn posix_fallocate 166and this manual page were initially written by 167.An Matthew Fleming Aq Mt mdf@FreeBSD.org . 168