Is it safe to read directory entries via readdir() or scandir() while files are being created/deleted in this directory? Should I prefer one over the other?
EDIT: When I say "safe" I mean entries returned by these functions are valid and can be operated without crushing the program.
Thanks.
It depends by what you mean as "safe". They are safe in the sense that they should not crash your program. However, if you are creating/deleting files as you are reading/scanning that directory, the set of files you get back might not be up-to-date.
When reading/scanning a directory for directory entries, the file pointer (a directory is just a special type of file), moves forward. However, depending upon the file system, there may be nothing to prevent new files from being created in an empty directory entry slot behind your file pointer. Consequently, newly added directory entries may not be immediately detected by readdir()/scandir(). Similar reasoning applies for file deletion / directory entry removal.
Hope this helps.
What's your definition of safety? You won't crash the system, and readdir/scandir won't crash your program. Although they might give you data that is immediately out of date.
The usual semantics for reading a directory are that if you read the directory from beginning to end, you will see all of the files that didn't change during that time exactly once, and you will see files that were created or deleted during that time at most once.
On UNIX-like systems readdir() and scandir() are library functions implemented on top of the same underlying system call (getdents() in Linux, getdirentries() in BSD). So there shouldn't be much difference in their behavior in this regard. I think readdir() is a bit more standard, and therefore will be more portable.