c++ - Why does realpath() return error EEXIST? -

my program running in linux environment, compiled gcc version 4.4.7.

i using realpath() "canonicalize" file paths. path of every directory , file feed realpath() exists, of course essential realpath() work properly.

however, realpath() fail error code 17, name eexist, string description "file exists".

that baffles me. of course exists, scream @ realpath(). realpath() unmoved ranting.

documentation realpath() @ http://pubs.opengroup.org/onlinepubs/009695399/functions/realpath.html lists errors cause fail, eexist not 1 of them.

why realpath() fail in way?

examples of directory , file paths cause eexist error:

• an absolute path directory: /alpha/bravo/charlie/delta
• an absolute path file: /alpha/bravo/charlie/foo.txt
• a relative path file: ../../charlie/foo.txt
• a path file has dot in it: /alpha/bravo/charlie/./foo.txt

but examples not definitive, because other files exact same patterns, , in same directories, succeeed.

there not seem rhyme or reason directory or file cause eexist error. error typically happens first file path try canonicalize, , not subsequent ones. however, cannot kludge around merely trying canonicalize first file again; error keep happening it.

program snippet:

#include <string> #include <stdio.h> #include <stdlib.h> #include <errno.h> #include <limits.h>    // path_max  using std;  string pathcanonicalize( string const & path ) {   string result;    char szresult[ path_max ];   ::realpath( path.c_str(), szresult );   if ( errno == eexist )   {     // why?     cerr << "realpath: error code " << errno << ", " << ::strerror( errno ) << ": '" << path << "'.  of course file exists!" << endl;      result = path;   }   else if ( errno )   {     cerr << "realpath: error code " << errno << ", " << ::strerror( errno ) << ": '" << path << "'" << endl;      result = path;   }   else   {     result = szresult;   }    return result;       } 

you should never, ever check errno without specific reason.

perhaps whatever internal operation realpath happened last failed eexist. or maybe errno happened eexist previous operation failed , realpath didn't change it.

if didn't cause realpath fail, why care?

from own link:

upon successful completion, realpath() shall return pointer resolved name. otherwise, realpath() shall return null pointer , set errno indicate error, , contents of buffer pointed resolved_name undefined.

notice doesn't errno set in particular if realpath succeeds. why checking errno before checking if realpath succeeded?