posix_fadvise

Standard C Library (libc, -lc)

#include <fcntl.h>

int
posix_fadvise(int fd, off_t offset, off_t len, int advice);

The posix_fadvise() system call allows a process to describe to the system its data access behavior for an open file descriptor fd. The advice covers the data starting at offset offset and continuing for len bytes. If len is zero, all data from offset to the end of the file is covered.

The behavior is specified by the advice parameter and may be one of:

POSIX_FADV_NORMAL

Tells the system to revert to the default data access behavior.

POSIX_FADV_RANDOM

Is a hint that file data will be accessed randomly, and prefetching is likely not advantageous.

POSIX_FADV_SEQUENTIAL

Tells the system that file data will be accessed sequentially. This currently does nothing as the default behavior uses heuristics to detect sequential behavior.

POSIX_FADV_WILLNEED

Tells the system that the specified data will be accessed in the near future. The system may initiate an asynchronous read of the data if it is not already present in memory.

POSIX_FADV_DONTNEED

Tells the system that the specified data will not be accessed in the near future. The system may decrease the in-memory priority of clean data within the specified range and future access to this data may require a read operation.

POSIX_FADV_NOREUSE

Tells the system that the specified data will only be accessed once and then not reused. The system may decrease the in-memory priority of data once it has been read or written. Future access to this data may require a read operation.

The posix_fadvise() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The posix_fadvise() system call will fail if:

[EBADF]

The fd argument is not a valid file descriptor.

[EINVAL]

The advice argument is not valid.

[EINVAL]

The offset or len arguments are negative, or offset + len is greater than the maximum file size.

[ENODEV]

The fd argument does not refer to a regular file.

[ESPIPE]

The fd argument is associated with a pipe or FIFO.

madvise(2)

The posix_fadvise() interface conforms to IEEE Std 1003.1-2001 (“POSIX.1”).

The posix_fadvise() system call first appeared in FreeBSD 9.1.