Manpage of POSIX_FALLOCATE
POSIX_FALLOCATESection: Linux Programmer's Manual (3)
NAMEposix_fallocate - allocate file space
#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_C_SOURCE >= 200112L
DESCRIPTIONThe 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.
RETURN VALUEposix_fallocate() returns zero on success, or an error number on failure. Note that errnois not set.
- fdis not a valid file descriptor, or is not opened for writing.
- offset+lenexceeds the maximum file size.
- 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 fd.
- fdrefers to a pipe.
- A signal was caught during execution.
VERSIONSposix_fallocate() is available since glibc 2.1.94.
ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7).
|posix_fallocate()||Thread safety||MT-Safe (but see NOTES)|
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.
NOTESIn 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 ALSOfallocate(1), fallocate(2), lseek(2), posix_fadvise(2)
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Time: 22:27:54 GMT, June 20, 2016