posix_fallocate — allocate file space


#include <fcntl.h>
int posix_fallocate( int fd,
  off_t offset,
  off_t len);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_POSIX_C_SOURCE >= 200112L


The function posix_fallocate() ensures that disk space is allocated for the file referred to by the file descriptor fd for the bytes in the range starting at offset and continuing for len bytes. After a successful call to posix_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 than offset+len, 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 that errno is not set.



fd is not a valid file descriptor, or is not opened for writing.


offset+len exceeds the maximum file size.


A signal was caught during execution.


offset was less than 0, or len was less than or equal to 0, or the underlying filesystem does not support the operation.


fd does not refer to a regular file.


There is not enough space left on the device containing the file referred to by fd.


The filesystem containing the file referred to by 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.


fd refers to a pipe.


posix_fallocate() is available since glibc 2.1.94.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
posix_fallocate() Thread safety MT-Safe (but see NOTES)



POSIX.1-2008 says that an implementation shall give the EINVAL error if len was 0, or offset was less than 0. POSIX.1-2001 says that an implementation shall give the EINVAL error if len is less than 0, or offset was less than 0, and may give the error if len equals zero.


In the glibc implementation, posix_fallocate() is implemented using the fallocate(2) system call, which is MT-safe. If the underlying filesystem does not support fallocate(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.

  • If fd has been opened with the O_APPEND or O_WRONLY flags, the function fails with the error EBADF.

In general, the emulation is not MT-safe. On Linux, applications may use fallocate(2) if they cannot tolerate the emulation caveats. In general, this is only recommended if the application plans to terminate the operation if EOPNOTSUPP is 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)


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  Copyright (c) 2006, Michael Kerrisk <>

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