C++: How to check if a file/directory is readable? (PHP equivalent: is_readable)

Question

I am trying to validate a directory with C++.

bool is_readable ( string $filename )
Tells whether a file (or directroy) exists and is readable.

What would be the equivalent of the above in C++?

I am already using the boost/filesystem library to check that the directory exists. I have checked the documentation:
http://www.boost.org/doc/libs/1_44_0/libs/filesystem/v3/doc/index.htm
but I cannot find the equivalent of PHP's is_readable().

If it is not possible with the boost/filesystem library, what method would you use?

score 4 · Answer 1 · answered Aug 25 '10 at 05:59

4

Most operating systems provide stat().

answered Aug 25 '10 at 05:59

Tony Delroy

Alex B · Accepted Answer · 2010-08-25T06:11:35.710

4

Since you've tagged the question "Linux", there is a POSIX function to check if the file is readable/writable/executable by the user of the current process. See man 2 access.
```
int access(const char *pathname, int mode);
```
For example,
```
if (-1 == access("/file", R_OK))
{
    perror("/file is not readable");
}
```
Alternatively, if you need portability, try to actually open the file for reading (e.g. std::ifstream). If it succeeds, the file is readable. Likewise, for directories, use boost::filesystem::directory_iterator, if it succeeds, directory is readable.

edited Aug 25 '10 at 06:11

answered Aug 25 '10 at 05:59

Alex B

Thanks. A quick follow up question: I don't have the relevant man pages installed ("No manual entry for access in section 2"). Would you know the package name to install on Debian/Kubuntu? – augustin Aug 25 '10 at 06:36
imho the correct way is no. 2 anyway, you have no guarantee if access returns success that it is still readable when you rty to read from it – jk. Aug 25 '10 at 08:38
@jk yes, there is always a TOCTOU race condition possible if you try to open it later, but it works for one-off reporting purposes (i.e. when only a check is needed). – Alex B Aug 25 '10 at 10:43
Yes, in this case, I don't actually want to read anything, but check that the directory is readable before saving a setting. – augustin Aug 25 '10 at 13:15

2 Answers2