.\"
.\" Copyright (c) 1997 Shigio Yamaguchi. All rights reserved.
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.Dd August 7, 2022
.Dt ABS2REL 3
.Os
.Sh NAME
.Nm abs2rel
.Nd make a relative path name from an absolute path name
.Sh SYNOPSIS
.Ft "char *"
.Fn abs2rel "const char *path" "const char *base" "char *result" "size_t size"
.Sh DESCRIPTION
The
.Fn abs2rel
function makes a relative path name from an absolute path name
.Fa path
based on a directory
.Fa base
and copies the resulting path name into the memory referenced by
.Fa result .
The
.Fa result
argument must refer to a buffer capable of storing at least
.Fa size
characters.

The resulting path name may include symbolic links.
The
.Fn abs2rel
function doesn't check whether or not any path exists.
.Sh "RETURN VALUES"
The
.Fn abs2rel
function returns relative path name on success.
If an error occurs,
it returns
.Dv NULL .
.Sh EXAMPLES
    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 == "."
.Sh ERRORS
The
.Fn abs2rel
function may fail and set the external variable
.Va errno
to indicate the error.
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa base
directory isn't an absolute path name or the
.Fa size
argument is zero.
.It Bq Er ERANGE
The
.Fa size
argument is greater than zero but smaller than the length of the pathname plus 1.

.El
.Sh SEE ALSO
.Xr rel2abs 3
.Sh AUTHORS
.An Shigio Yamaguchi (shigio@tamacom.com)
.Sh BUGS
If the
.Fa base
directory includes symbolic links,
the
.Fn 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.
.Pp

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