Is there a POSIX syscall to resolve file system paths?

Question

Is there a POSIX syscall to resolve filesystem paths? I have the CWD for a path, as well as the path to a file from that CWD. I can't use chdir to switch to the directory because I need to resolve paths from multiple threads simultaneously. I considered appending a / in between the CWD and the path, but for some reason it feels like that's hacky. Is that the proper way to resolve relative paths?

Answer 1

I think appending the / should be sufficient in pretty much all situations - even with .,.., extra /s, or symlinks it should just do the right thing. If you really want to do this with the standard library, you can use realpath(3) to do path normalization but I don't know of a way to do exactly what you want.

Answer 2

Depending on what you want to do with the file, and if relatively recent additions in POSIX.1-2008 are acceptable for you, openat and friends may be of interest:

int dirfd = open(desired_cwd_path, O_RDONLY);
int fd = openat(dirfd, file_relpath, O_RDONLY);
close(dirfd);
// ...use fd

(These system calls have existed on e.g. Solaris and Linux for quite some time.)