openat

Openat is a system call and C library wrapper that opens a file relative to a directory file descriptor. It provides a way to perform file access operations using an existing directory descriptor rather than the process’s working directory. The typical signature is int openat(int dirfd, const char *pathname, int flags, ...); where dirfd is a directory file descriptor obtained from open or dup, or the special value AT_FDCWD to use the process’s current working directory. If pathname is absolute, dirfd is ignored.

When pathname is relative, its resolution is anchored to the directory referred to by dirfd. If O_CREAT is specified in flags, a mode argument must be supplied to set the file’s permissions when a new file is created. If the path name is not found, openat returns an error as with open, setting errno accordingly. The call supports the same access modes (read, write, or both) and the same file-opening flags as open, allowing a consistent way to create or modify files from an already-open directory.

Openat is part of a family of at functions that includes fstatat, mkdirat, unlinkat, renameat, and others, enabling system calls that operate relative to a directory file descriptor. This design helps write safer and more atomic code by avoiding reliance on the process’s current working directory, reducing certain race conditions when paths may be altered by other processes.

Openat has been complemented by openat2 in newer Linux kernels, which offers more flexible and extended pathname resolution options. Openat remains widely supported in POSIX-compatible C libraries and is commonly used in system utilities and language runtimes.