Does the function scandir handle relative paths?
In other words, are the following two pieces of code equivalent?
int NUM = scandir("/home/hello/wait/../pqr/../../hello", &LIST, 0, alphasort);
int NUM = scandir("/home/hello", &LIST, 0, alphasort);
If not, is there some simple buit-in function to convert /home/hello/wait/../pqr/../../hello to /home/hello?
Does the function scandir handle relative paths?
I believe the answer is yes. From posix scandir we know only that:
The scandir() function shall scan the directory dir, [...]
From posix definitions:
3.129 Directory
A file that contains directory entries. No two directory entries in the same directory have the same name
The argument to scandir is char *, so it's a string. And from posix definitions we know that:
3.271 Pathname
A string that is used to identify a file. [...]
Another section named 4.13 Pathname Resolution delves on how pathname is resolved, and tells us that:
If the pathname does not begin with a <slash>, the predecessor of the first filename of the pathname shall be taken to be either the current working directory of the process [...] (such pathnames are referred to as "relative pathnames").
So because scandir takes a pathname to a directory and because of how a pathname is resolved, yes, you can give relative paths to scandir.
In other words
It's not an "other word" - both pathnames you showed are absolute, not relative. Using dot-dot in a pathname doesn't make the pathname relative.
are the following two pieces of code equivalent?
Yes.
is there some simple buit-in function to convert /home/hello/wait/../pqr/../../hello to /home/hello?
There is no "builtin function" to canonicalize a pathname in any compiler I am aware of. There exists a completely normal function provided from POSIX for that purpose and is called realpath(). GNU library may allocated the returned string dynamically and it also has canonicalize_file_name.