CODEWITHSUNDEEP
posixreaddir
John Bartholomew
John Bartholomew

Reputation: 6596

What guarantees does pathconf(..., _PC_NAME_MAX) provide?

Context:

The readdir_r function is used to read the next entry from a DIR* (there's also readdir, but that's not thread-safe). readdir_r takes a pointer to a user-allocated buffer to hold the output dirent. The manpage indicates that the size required for this buffer may be different on different systems, and provides an example of how to find a safe length at runtime:

len = offsetof(struct dirent, d_name) + pathconf(dirpath, _PC_NAME_MAX) + 1;

(warning: there is a race condition in the above, which can be avoided by using dirfd to get the file descriptor for the opened DIR* and using fpathconf instead of pathconf)

Question:

Looking at the manpage for pathconf, it states:

_PC_NAME_MAX returns the maximum length of a filename in the directory path or fd that the process is allowed to create. The corresponding macro is _POSIX_NAME_MAX.

However, in the notes section, it states:

Files with name lengths longer than the value returned for name equal to _PC_NAME_MAX may exist in the given directory.

Is this note true? If so, is the example code in the readdir_r man-page incorrect?

Upvotes: 4

Views: 1441

Answers (1)

jilles
jilles

Reputation: 11252

That interpretation of {NAME_MAX} does not agree with POSIX. POSIX says that implementations must treat names longer than {NAME_MAX} as an error and that a d_name buffer of {NAME_MAX}+1 bytes is sufficient.

An alternative (POSIX.1-2008) is to use scandir() which is thread-safe and abstracts this issue from the caller. Unfortunately, there is no scandirat() or fscandir() in any version of POSIX.

On many systems, it is also safe to use readdir() as long as the last access to the returned struct dirent happens-before the next call to readdir() (which follows from the existing requirements on the validity of the structure). I think there is little reason for POSIX not to permit this. readdir_r() requires quite a lot of extra code which makes things slower and more complex.

Upvotes: 3

Related Questions