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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |ATTRIBUTES |STANDARDS |HISTORY |CAVEATS |SEE ALSO |COLOPHON | |
posix_fallocate(3) Library Functions Manualposix_fallocate(3)posix_fallocate - allocate file space
Standard C library (libc,-lc)
#include <fcntl.h>int posix_fallocate(intfd, off_toffset, off_tsize); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):posix_fallocate(): _POSIX_C_SOURCE >= 200112L
The functionposix_fallocate() ensures that disk space is allocated for the file referred to by the file descriptorfd for the bytes in the range starting atoffset and continuing forsize bytes. After a successful call toposix_fallocate(), subsequent writes to bytes in the specified range are guaranteed not to fail because of lack of disk space. If the size of the file is less thanoffset+size, then the file is increased to this size; otherwise the file size is left unchanged.
posix_fallocate() returns zero on success, or an error number on failure. Note thaterrno is not set.
EBADFfd is not a valid file descriptor, or is not opened for writing.EFBIGoffset+size exceeds the maximum file size.EINTRA signal was caught during execution.EINVALoffset was less than 0, orsize was less than or equal to 0, or the underlying filesystem does not support the operation.ENODEVfd does not refer to a regular file.ENOSPCThere is not enough space left on the device containing the file referred to byfd.EOPNOTSUPP The filesystem containing the file referred to byfd does not support this operation. This error code can be returned by C libraries that don't perform the emulation shown in CAVEATS, such as musl libc.ESPIPEfd refers to a pipe.
For an explanation of the terms used in this section, seeattributes(7). ┌───────────────────┬───────────────┬────────────────────────────┐ │Interface│Attribute│Value│ ├───────────────────┼───────────────┼────────────────────────────┤ │posix_fallocate() │ Thread safety │ MT-Safe (but see CAVEATS) │ └───────────────────┴───────────────┴────────────────────────────┘
POSIX.1-2008.
glibc 2.1.94. POSIX.1-2001 POSIX.1-2008 says that an implementationshall give theEINVAL error ifsize was 0, oroffset was less than 0. POSIX.1-2001 says that an implementationshall give theEINVALerror ifsize is less than 0, oroffset was less than 0, andmay give the error ifsize equals zero.
In the glibc implementation,posix_fallocate() is implemented using thefallocate(2) system call, which is MT-safe. If the underlying filesystem does not supportfallocate(2), then the operation is emulated with the following caveats: • The emulation is inefficient. • There is a race condition where concurrent writes from another thread or process could be overwritten with null bytes. • 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. • Iffd has been opened with theO_APPENDorO_WRONLYflags, the function fails with the errorEBADF. In general, the emulation is not MT-safe. On Linux, applications may usefallocate(2) if they cannot tolerate the emulation caveats. In general, this is only recommended if the application plans to terminate the operation ifEOPNOTSUPPis returned, otherwise the application itself will need to implement a fallback with all the same problems as the emulation provided by glibc.
fallocate(1),fallocate(2),lseek(2),posix_fadvise(2)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17posix_fallocate(3)Pages that refer to this page:fallocate(1), rsync(1), fallocate(2), lseek(2), posix_fadvise(2), off_t(3type)
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