ABS2REL(3)

💅 abs2rel(3) — bestie edition ✨

okay so real talk, this one lives in the Library Functions Manual and it's your go-to for turning a big scary absolute path into a cute lil relative one 🎀 it's <pink>abs2rel</pink>, from the <purple>libpathconv</purple> library, and honestly? she's iconic.

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✨ NAME ✨

abs2rel — make a relative path name from an absolute path name 🦋

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🌸 SYNOPSIS 🌸

library “libpathconv” char *
abs2rel(const char *path, const char *base, char *result, size_t size);

like yes, it's a library libpathconv, and yes, she returns a char * — we love a consistent queen 👑

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💖 DESCRIPTION 💖

so the abs2rel() function makes a relative path name out of an absolute path name path, based on a directory base, and then copies the resulting path name into the memory that result points at. mouthful, i know, but it slays 💅

⚠️ okay important: the result argument must refer to a buffer that can hold at least size characters. no cap, if you undersize it you're gonna have a bad time 😭

the resulting path name may include symbolic links — she's not out here resolving anything. the abs2rel() function does not check whether any path actually exists. it's giving "i don't do background checks", lowkey 🙈 so don't come for her if the path isn't real.

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🫶 RETURN VALUES 🫶

• on success? the abs2rel() function returns the relative path name. slay 💖
• if an error happens? it returns NULL. 😭

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🎀 EXAMPLES 🎀

like, picture this:

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

which yields:

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

and similarly, bestie:

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

which yields:

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

the way that "." just shows up at the end like she owns the place 😌 periodt.

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🔥 ERRORS 🔥

the abs2rel() function may fail and set the external variable errno to indicate the error. vibe check failed, basically 💀

• [EINVAL] — the base directory isn't an absolute path name, or the size argument is zero. like... one of those two, pick your struggle 🙈
• [ERANGE] — the size argument is greater than zero but smaller than the length of the pathname plus 1. so close, yet so far 😭

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🦋 SEE ALSO 🦋

• rel2abs(3) — the other half of the duo, her sibling, the reverse queen 👑

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👑 AUTHORS 👑

Shigio Yamaguchi (shigio@tamacom.com) — we stan a legend 🫶

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💀 BUGS 💀

okay this is the part where we get real, bestie.

if the base directory includes symbolic links, the abs2rel() function produces the wrong path. not "a little off" — wrong. like, wrong wrong 💀

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!! */

see that comment? /* It's wrong!! */ — the manpage itself is yelling at you 😭 iconic honesty honestly.

so you should convert the base directory into a real path in advance. like:

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

yields:

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

that is correct, but a little redundant. ngl, she's right — that's a lot of .. for one girl 🌈

so if you wish to 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. 🫶 that's not a vibe, that's a rule, so please respect it 💅

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anyway that's the whole page, mwah 💋 go forth and make your paths relative, you got this ✨

— with love, from the Debian girlies, dated December 13, 2025 🎀🦋💖 (and yes, this is ABS2REL(3), section 3, library functions, iykyk 👑)