Realpath

Realpath is a term used in computing to describe a function or process that resolves a pathname to its canonical absolute form. This involves resolving symbolic links, removing redundant separators, and eliminating references such as "." and ".." to produce a direct, unambiguous path.

In POSIX systems, the realpath() function performs this resolution. It takes a path and returns a canonicalized absolute pathname. The input path may be absolute or relative (in the latter case, it is interpreted relative to the current working directory). The function fills a provided buffer with the result and returns that buffer on success, or returns NULL on error and sets errno. If a NULL pointer is passed for the output, some implementations allocate memory with malloc for the caller to free. Common error conditions include ENOENT (path does not exist), EACCES (permission denied), and ENOTDIR (a component of the path is not a directory). Realpath typically requires that the path exist, since it resolves the actual filesystem structure.

Applications and languages store and use realpath in slightly different forms. In programming languages, realpath-like functions are available to obtain absolute paths while resolving symlinks, such as Python’s os.path.realpath and similar facilities in other ecosystems. Some systems also provide a related function, canonicalize_file_name (for path canonicalization without requiring existence), and Windows offers equivalent functionality via APIs like GetFullPathName.

Security considerations include the fact that resolving symlinks can reveal the true location of files used in include or load operations, potentially exposing sensitive directories. Realpath can also be involved in race conditions when used for security checks, so it is often supplemented with additional validation.