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posix Module¶

The posix module exposes low-level POSIX APIs. On POSIX platforms, os is implemented on top of posix, and most users should prefer os for portability.

Complexity Reference¶

Operation	Time	Space	Notes
`posix.open(path, flags, mode=0o777)`	O(1) + syscall	O(1)	Path lookup cost depends on filesystem
`posix.read(fd, n)`	O(n) + syscall	O(n)	n = bytes read
`posix.write(fd, data)`	O(n) + syscall	O(1)	n = bytes written
`posix.close(fd)`	O(1) + syscall	O(1)	Release descriptor
`posix.lseek(fd, pos, how)`	O(1) + syscall	O(1)	Adjust file offset
`posix.stat(path)`	O(1) + syscall	O(1)	Path lookup cost depends on filesystem
`posix.fstat(fd)`	O(1) + syscall	O(1)	Descriptor-based stat
`posix.listdir(path)`	O(m) + syscall	O(m)	m = directory entries returned
`posix.mkdir(path, mode=0o777)`	O(1) + syscall	O(1)	Path lookup cost depends on filesystem
`posix.rmdir(path)`	O(1) + syscall	O(1)	Directory must be empty
`posix.unlink(path)`	O(1) + syscall	O(1)	Remove a name (file or symlink)
`posix.chmod(path, mode)`	O(1) + syscall	O(1)	Change permissions
`posix.chown(path, uid, gid)`	O(1) + syscall	O(1)	Change owner/group

Low-Level File I/O¶

import posix

fd = posix.open("example.txt", posix.O_CREAT | posix.O_WRONLY | posix.O_TRUNC, 0o644)
try:
    posix.write(fd, b"hello\n")  # O(n) in bytes
finally:
    posix.close(fd)

fd = posix.open("example.txt", posix.O_RDONLY)
try:
    data = posix.read(fd, 1024)  # O(n) in bytes
    print(data.decode("utf-8"))
finally:
    posix.close(fd)

File Descriptors and Stat¶

import posix

fd = posix.open("example.txt", posix.O_RDONLY)
try:
    info = posix.fstat(fd)  # O(1)
    print(info.st_size)
finally:
    posix.close(fd)

Directory Listing¶

import posix

entries = posix.listdir(".")  # O(m) where m = directory entries
for name in entries:
    print(name)

Notes¶

Many functions raise OSError for permission or filesystem errors.
posix is not available on Windows; use os for portability.

os Module - Portable OS interface (recommended)
posixpath Module - POSIX path operations
pathlib Module - Object-oriented paths