'\" t .\" Copyright (c) 2006, Michael Kerrisk .\" .\" 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 .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)