NAME

dirname — report the parent directory name of a file pathname

SYNOPSIS

[XSI] #include <libgen.h>

char *dirname(char *path);

DESCRIPTION

The dirname() function shall take a pointer to a character string that contains a pathname, and return a pointer to a string that is a pathname of the directory containing the entry of the final pathname component. The dirname() function shall not perform pathname resolution; the result shall not be affected by whether or not path exists or by its file type. Trailing '/' characters in the pathname that are not also leading '/' characters shall not be counted as part of the pathname.

If the pathname does not contain a '/', then dirname() shall return a pointer to the string ".". If path is a null pointer or points to an empty string, dirname() shall return a pointer to the string ".".

It is unspecified whether redundant '/' characters and '.' pathname components in path are removed after determining the pathname to output. However, ".." pathname components occurring prior to the final component shall not be removed.

The dirname() function may modify the string pointed to by path, and may return a pointer into the input string. The returned pointer might be invalidated if the input string is subsequently modified or freed. If path does not contain a '/', is a null pointer, or points to an empty string the returned pointer may point to constant data that cannot be modified.

RETURN VALUE

The dirname() function shall return a pointer to a string as described above.

The dirname() function shall always be successful and no return value is reserved to indicate an error.

ERRORS

No errors are defined.

EXAMPLES

The following code fragment reads a pathname, changes the current working directory to the parent directory, and opens the file.

char *path = NULL, *pathcopy;
size_t buflen = 0;
ssize_t linelen = 0;
int fd;


linelen = getline(&path, &buflen, stdin);


path[linelen-1] = 0;
pathcopy = strdup(path);
if (chdir(dirname(pathcopy)) < 0) {
    ...
}
if ((fd = open(basename(path), O_RDONLY)) >= 0) {
    ...
    close (fd);
}
...
free (pathcopy);
free (path);

The EXAMPLES section of the basename() function (see basename) includes a table showing examples of the results of processing several sample pathnames by the basename() and dirname() functions and by the basename and dirname utilities.

APPLICATION USAGE

The dirname() and basename() functions together yield a complete pathname. The expression dirname(path) obtains the pathname of the directory where basename(path) is found.

Since the meaning of the leading "//" is implementation-defined, dirname("//foo") may return either "//" or "/" (but nothing else).

Note that in some circumstances, the returned pointer might point into constant data. Therefore, if the application needs to modify the returned data, it should be copied first.

RATIONALE

An implementation should prefer the shortest output possible; however, this is not required, in part because earlier versions of the standard did not mention whether elision of redundant <slash> characters or dot (".") components was permitted. Removal of the dot-dot ("..") pathname component is not permitted, because eliding it correctly would require performing pathname resolution to ensure the resulting string would still point to the correct pathname if the original string resolved as a pathname. On implementations where pathname "//" has an implementation-defined meaning distinct from the pathname "/", the dirname of "//" will be "//".

Earlier versions of this standard seemed to allow thread-safe and non-thread-safe implementations of basename() and dirname(), but did not allow implementations to return a null pointer and require that errno be set when that happened. The standard now requires thread-safe behavior for both of these functions and clearly states that they are always successful.

FUTURE DIRECTIONS

None.

SEE ALSO

basename

XBD <libgen.h>

XCU basename, dirname

CHANGE HISTORY

First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Normative text previously in the APPLICATION USAGE section is moved to the DESCRIPTION.

A note indicating that this function need not be reentrant is added to the DESCRIPTION.

Issue 7

Austin Group Interpretation 1003.1-2001 #156 is applied.

The EXAMPLES section is revised.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0068 [75] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0077 [830], XSH/TC2-2008/0078 [612], XSH/TC2-2008/0079 [830], XSH/TC2-2008/0080 [656], and XSH/TC2-2008/0081 [612] are applied.

Issue 8

Austin Group Defect 1064 is applied, requiring dirname() to be thread-safe and allowing it to return a pointer to constant data under certain conditions.

Austin Group Defect 1073 is applied, changing "parent directory of that file" to "directory containing the entry of the final pathname component" and clarifying that redundant '/' characters and '.' pathname components may be removed after determining the pathname to output.