fallocate(2) [linux man page]
FALLOCATE(2) Linux Programmer's Manual FALLOCATE(2) NAME
fallocate - manipulate file space SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */ #include <fcntl.h> int fallocate(int fd, int mode, off_t offset, off_t len); DESCRIPTION
This is a nonportable, Linux-specific system call. For the portable, POSIX.1-specified method of ensuring that space is allocated for a file, see posix_fallocate(). fallocate() allows the caller to directly manipulate the allocated disk space for the file referred to by fd for the byte range starting at offset and continuing for len bytes. The mode argument determines the operation to be performed on the given range. Currently only one flag is supported for mode: FALLOC_FL_KEEP_SIZE This flag allocates and initializes to zero the disk space within the range specified by offset and len. After a successful call, subsequent writes into this range are guaranteed not to fail because of lack of disk space. Preallocating zeroed blocks beyond the end of the file is useful for optimizing append workloads. Preallocating blocks does not change the file size (as reported by stat(2)) even if it is less than offset+len. If FALLOC_FL_KEEP_SIZE flag is not specified in mode, the default behavior is almost same as when this flag is specified. The only differ- ence is that on success, the file size will be changed if offset + len is greater than the file size. This default behavior closely resem- bles the behavior of the posix_fallocate(3) library function, and is intended as a method of optimally implementing that function. Because allocation is done in block size chunks, fallocate() may allocate a larger range than that which was specified. RETURN VALUE
fallocate() returns zero on success, and -1 on failure. ERRORS
EBADF fd is not a valid file descriptor, or is not opened for writing. EFBIG offset+len exceeds the maximum file size. EINTR A signal was caught during execution. EINVAL offset was less than 0, or len was less than or equal to 0. EIO An I/O error occurred while reading from or writing to a file system. ENODEV fd does not refer to a regular file or a directory. (If fd is a pipe or FIFO, a different error results.) ENOSPC There is not enough space left on the device containing the file referred to by fd. ENOSYS The file system containing the file referred to by fd does not support this operation. EOPNOTSUPP The mode is not supported by the file system containing the file referred to by fd. VERSIONS
fallocate() is available on Linux since kernel 2.6.23. Support is provided by glibc since version 2.10. CONFORMING TO
fallocate() is Linux-specific. SEE ALSO
ftruncate(2), posix_fadvise(3), posix_fallocate(3) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2010-09-10 FALLOCATE(2)
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POSIX_FALLOCATE(3) Linux Programmer's Manual POSIX_FALLOCATE(3) 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 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. RETURN VALUE
posix_fallocate() returns zero on success, or an error number on failure. Note that errno is not set. ERRORS
EBADF fd is not a valid file descriptor, or is not opened for writing. EFBIG offset+len exceeds the maximum file size. EINTR A signal was caught during execution. EINVAL offset was less than 0, or len was less than or equal to 0, or the underlying filesystem does not support the operation. ENODEV fd does not refer to a regular file. ENOSPC There is not enough space left on the device containing the file referred to by fd. ESPIPE fd refers to a pipe. VERSIONS
posix_fallocate() is available since glibc 2.1.94. ATTRIBUTES
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) | +------------------+---------------+-------------------------+ CONFORMING TO
POSIX.1-2001. 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. 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 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. SEE ALSO
fallocate(1), fallocate(2), lseek(2), posix_fadvise(2) COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. GNU
2017-09-15 POSIX_FALLOCATE(3)