Section (3) posix_fallocate
posix_fallocate — allocate file space
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
len, then the file is increased
to this size; otherwise the file size is left unchanged.
zero on success, or an error number on failure. Note that
errno is not set.
fdis not a valid file descriptor, or is not opened for writing.
offset+lenexceeds the maximum file size.
A signal was caught during execution.
offsetwas less than 0, or
lenwas less than or equal to 0, or the underlying filesystem does not support the operation.
fddoes not refer to a regular file.
There is not enough space left on the device containing the file referred to by
fdrefers to a pipe.
For an explanation of the terms used in this section, see attributes(7).
||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
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.
fdhas been opened with the
O_WRONLYflags, 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.
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Copyright (c) 2006, Michael Kerrisk <mtk.manpagesgmail.com>
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