Manpage of POSIX_FALLOCATE

POSIX_FALLOCATE

Section: Linux Programmer's Manual (3)
Updated: 2016-03-15
Index
 

NAME

posix_fallocate - allocate file space  

SYNOPSIS

#include <fcntl.h>int posix_fallocate(int fd, off_t offset, off_t len);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

posix_fallocate():

_POSIX_C_SOURCE >= 200112L
 

DESCRIPTION

The function posix_fallocate() ensures that disk space is allocated for the file referred to by the file descriptor fdfor the bytes in the range starting at offsetand continuing for lenbytes. 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.  

RETURN VALUE

posix_fallocate() returns zero on success, or an error number on failure. Note that errnois not set.  

ERRORS

EBADF
fdis not a valid file descriptor, or is not opened for writing.
EFBIG
offset+lenexceeds the maximum file size.
EINVAL
offsetwas less than 0, or lenwas less than or equal to 0, or the underlying filesystem does not support the operation.
ENODEV
fddoes not refer to a regular file.
ENOSPC
There is not enough space left on the device containing the file referred to by fd.
ESPIPE
fdrefers to a pipe.
EINTR
A signal was caught during execution.
 

VERSIONS

posix_fallocate() is available since glibc 2.1.94.  

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).
InterfaceAttributeValue
posix_fallocate() Thread safetyMT-Safe (but see NOTES)
 

CONFORMING TO

POSIX.1-2001.

POSIX.1-2008 says that an implementation shallgive the EINVALerror if lenwas 0, or offsetwas less than 0. POSIX.1-2001 says that an implementation shallgive the EINVALerror if lenis less than 0, or offsetwas less than 0, and maygive the error if lenequals zero.  

NOTES

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 fdhas been opened with the O_APPENDor O_WRONLYflags, the function will fail 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 EOPNOTSUPPis returned, otherwise the application itself will need to implement a fallback with all the same problems as the emulation provided by glibc.  

SEE ALSO

fallocate(1), fallocate(2), lseek(2), posix_fadvise(2)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
VERSIONS
ATTRIBUTES
CONFORMING TO
NOTES
SEE ALSO

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Time: 22:27:54 GMT, June 20, 2016