Problem - Kevin Tafuro

if (spc_environ) free(environ);
environ = new_environ;
spc_environ = 1;
return 1;
}
See Also
Recipe 1.1
3.7 Validating Filenames and Paths
Problem
You need to resolve the path of a file provided by a user to determine the actual file
that it refers to on the filesystem.
Solution
On Unix systems, use the function realpath( ) to resolve the canonical name of a file
or path. On Windows, use the function GetFullPathName( ) to resolve the canonical
name of a file or path.
Discussion
You must be careful when making access decisions for a file. Taking relative pathnames
and links into account, it is possible for multiple filenames to refer to the
same file. Failure to take this into account when attempting to perform access checks
based on filename can have severe consequences.
On the surface, resolving the canonical name of a file or path may appear to be a reasonably
simple task to undertake. However, many programmers fail to consider symbolic
and hard links. On Windows, links are possible, but they are not as serious an
issue as they are on Unix because they are much less frequently used.
Fortunately, most modern Unix systems provide, as part of the standard C runtime,
a function called realpath( ) that will properly resolve the canonical name of a file or
path, taking relative paths and links into account. Be careful when using realpath( )
because the function is not thread-safe, and the resolved path is stored in a fixed-size
buffer that must be at least MAXPATHLEN bytes in size.
The function realpath( ) is not thread-safe because it changes the current
directory as it resolves the path. On Unix, a process has a single
current directory, regardless of how many threads it has, so changing
the current directory in one thread will affect all other threads within
the process.
Validating Filenames and Paths | 97
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