stblksize

stblksize, commonly written as st_blksize, is a field in the POSIX struct stat that indicates the preferred block size for filesystem I/O. It represents a hint from the kernel about the optimal size of transfer units for this file, rather than a fixed limit. The value can help tune buffering and I/O strategies but should not be treated as a guarantee of performance.

In practice, programs use st_blksize to size their I/O buffers. A buffer sized to approximately st_blksize can improve throughput for sequential reads or writes, particularly on filesystems where block-based caching is effective. It is not the file's size; st_size stores the total size in bytes, and st_blocks reports the number of 512-byte blocks allocated.

Platform variations: st_blksize is defined by POSIX as part of struct stat, but its exact value depends on the filesystem, device, and operating system. Values vary widely and may be 4096 on common local filesystems, larger on certain networks, or zero if not applicable. Some environments may expose the field only under certain headers or types.

Notes: st_blksize is a hint for I/O efficiency, not a hard requirement. For direct I/O or memory-mapped I/O, alignment considerations may apply and boundaries related to st_blksize interact with system constraints.