Question

I’d like to perform a quick check whether or not a file can be opened. It should be written in portable C, or at least to work on Win32 and POSIX systems. #ifdefs are acceptable.

I’m trying to avoid this:

int openable(const char*filename) {     FILE *f = fopen(filename,'r');     if (!f)         return 0; /* openable */     fclose(f);     return 1; /* not openable */ } 

From what I can tell stat(), in its simplest form, can be used to check if file exists, but not to check if it’s actually openable.

Answer 1

The POSIX standard solution is access(), which is also in the Windows runtime as _access().

I guess it is slighly better than fopen()+fclose() because:

• It is a well-known, standard solution to the problem of testing a file for access rights
• It is very probably faster, with less memory overhead

Of course, it is just as susceptible to race conditions as any other way of doing such a test. In a way, the only safe way to know if a file is availble for reading is to open it and try to read, without closing it in between. Even then, you still need to watch for ‘unexpected’ EOF, of course. I/O is hard.