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Diffstat (limited to 'man3/posix_fallocate.3')
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diff --git a/man3/posix_fallocate.3 b/man3/posix_fallocate.3 new file mode 100644 index 0000000..1c5c189 --- /dev/null +++ b/man3/posix_fallocate.3 @@ -0,0 +1,182 @@ +'\" t +.\" Copyright (c) 2006, Michael Kerrisk <mtk.manpages@gmail.com> +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.TH posix_fallocate 3 2023-07-20 "Linux man-pages 6.05.01" +.SH NAME +posix_fallocate \- allocate file space +.SH LIBRARY +Standard C library +.RI ( libc ", " \-lc ) +.SH SYNOPSIS +.nf +.B #include <fcntl.h> +.PP +.BI "int posix_fallocate(int " fd ", off_t " offset ", off_t " len ); +.fi +.PP +.ad l +.RS -4 +Feature Test Macro Requirements for glibc (see +.BR feature_test_macros (7)): +.RE +.PP +.BR posix_fallocate (): +.nf + _POSIX_C_SOURCE >= 200112L +.fi +.SH DESCRIPTION +The function +.BR posix_fallocate () +ensures that disk space is allocated for the file referred to by the +file descriptor +.I fd +for the bytes in the range starting at +.I offset +and continuing for +.I len +bytes. +After a successful call to +.BR posix_fallocate (), +subsequent writes to bytes in the specified range are +guaranteed not to fail because of lack of disk space. +.PP +If the size of the file is less than +.IR offset + len , +then the file is increased to this size; +otherwise the file size is left unchanged. +.SH RETURN VALUE +.BR posix_fallocate () +returns zero on success, or an error number on failure. +Note that +.I errno +is not set. +.SH ERRORS +.TP +.B EBADF +.I fd +is not a valid file descriptor, or is not opened for writing. +.TP +.B EFBIG +.I offset+len +exceeds the maximum file size. +.TP +.B EINTR +A signal was caught during execution. +.TP +.B EINVAL +.I offset +was less than 0, or +.I len +was less than or equal to 0, or the underlying filesystem does not +support the operation. +.TP +.B ENODEV +.I fd +does not refer to a regular file. +.TP +.B ENOSPC +There is not enough space left on the device containing the file +referred to by +.IR fd . +.TP +.B EOPNOTSUPP +The filesystem containing the file referred to by +.I fd +does not support this operation. +This error code can be returned by C libraries that don't perform the +emulation shown in NOTES, such as musl libc. +.TP +.B ESPIPE +.I fd +refers to a pipe. +.SH ATTRIBUTES +For an explanation of the terms used in this section, see +.BR attributes (7). +.TS +allbox; +lb lb lbx +l l l. +Interface Attribute Value +T{ +.na +.nh +.BR posix_fallocate () +T} Thread safety T{ +.na +.nh +MT-Safe (but see NOTES) +T} +.TE +.sp 1 +.SH STANDARDS +POSIX.1-2008. +.SH HISTORY +glibc 2.1.94. +POSIX.1-2001 +.PP +POSIX.1-2008 says that an implementation +.I shall +give the +.B EINVAL +error if +.I len +was 0, or +.I offset +was less than 0. +POSIX.1-2001 says that an implementation +.I shall +give the +.B EINVAL +error if +.I len +is less than 0, or +.I offset +was less than 0, and +.I may +give the error if +.I len +equals zero. +.SH CAVEATS +In the glibc implementation, +.BR posix_fallocate () +is implemented using the +.BR fallocate (2) +system call, which is MT-safe. +If the underlying filesystem does not support +.BR fallocate (2), +then the operation is emulated with the following caveats: +.IP \[bu] 3 +The emulation is inefficient. +.IP \[bu] +There is a race condition where concurrent writes from another thread or +process could be overwritten with null bytes. +.IP \[bu] +There is a race condition where concurrent file size increases by +another thread or process could result in a file whose size is smaller +than expected. +.IP \[bu] +If +.I fd +has been opened with the +.B O_APPEND +or +.B O_WRONLY +flags, the function fails with the error +.BR EBADF . +.PP +In general, the emulation is not MT-safe. +On Linux, applications may use +.BR fallocate (2) +if they cannot tolerate the emulation caveats. +In general, this is +only recommended if the application plans to terminate the operation if +.B EOPNOTSUPP +is returned, otherwise the application itself will need to implement a +fallback with all the same problems as the emulation provided by glibc. +.SH SEE ALSO +.BR fallocate (1), +.BR fallocate (2), +.BR lseek (2), +.BR posix_fadvise (2) |