Manual Pages  — ABS2REL

abs2rel – make a relative path name from an absolute path name

SYNOPSIS DESCRIPTION RETURN VALUES EXAMPLES ERRORS SEE ALSO AUTHORS BUGS

char *
abs2rel(const char *path, const char *base, char *result, size_t size);

The abs2rel() function makes a relative path name from an absolute path name path based on a directory base and copies the resulting path name into the memory referenced by result. The result argument must refer to a buffer capable of storing at least size characters.

The resulting path name may include symbolic links. The abs2rel() function doesn't check whether or not any path exists.

The abs2rel() function returns relative path name on success. If an error occurs, it returns NULL.

char result[MAXPATHLEN];
char *path = abs2rel("/usr/src/sys", "/usr/local/lib", result, MAXPATHLEN);

yields:

path == "../../src/sys"

Similarly,

path1 = abs2rel("/usr/src/sys", "/usr", result, MAXPATHLEN);
path2 = abs2rel("/usr/src/sys", "/usr/src/sys", result, MAXPATHLEN);

yields:

path1 == "src/sys"
path2 == "."

The abs2rel() function may fail and set the external variable errno to indicate the error.

[EINVAL]
	The base directory isn't an absolute path name or the size argument is zero.
[ERANGE]
	The size argument is greater than zero but smaller than the length of the pathname plus 1.

Shigio Yamaguchi (shigio@tamacom.com)

If the base directory includes symbolic links, the abs2rel() function produces the wrong path. For example, if '/sys' is a symbolic link to '/usr/src/sys',

char *path = abs2rel("/usr/local/lib", "/sys", result, MAXPATHLEN);

yields:

path == "../usr/local/lib" /* It's wrong!! */

You should convert the base directory into a real path in advance.

path = abs2rel("/sys/kern", realpath("/sys", resolvedname), result, MAXPATHLEN);

yields:

path == "../../../sys/kern" /* It's correct but ... */

That is correct, but a little redundant. If you wish get the simple answer 'kern', do the following.

path = abs2rel(realpath("/sys/kern", r1), realpath("/sys", r2),
                                                                result, MAXPATHLEN);

The realpath() function assures correct result, but don't forget that realpath() requires that all but the last component of the path exist.