C++ fopen relative Path

Question

I read through some topic about relative path, but i still got i wrong. I hope sb can help me :). I am using Visual studio 2013, windows 7

I got the following directories:

Here is my .exe file D:\uni\c++\ex5\msvc2013\ex5\Debug

Here is the file i want to read D:\uni\c++\ex5\res\thehead.raw

The code for opening the file:

FILE* f;
f = fopen("..\\..\\..\\res\\thehead.raw", "rb");
if (f == NULL)
printf("FAIL!!");

As i need to use relative paths i figured it out as following: ..\ gets to parent directory.

so "..\..\..\" should get me into the folder "D:\uni\c++\ex5\".

\res should open the res foulder.

Needless to say it fails and i have no idea why. Any help would be appreciated.

Where your program launches and what your starting working directory are are not necessarily the same things. — Joe, Jul 06 '14 at 14:45
Look up the term "current working directory". Relative paths are relative to this directory. — n. m. could be an AI, Jul 06 '14 at 14:49
thx, im looking that up. it does not crash. f is just NULL so it prints "FAIL". — Käptn Freiversuch, Jul 06 '14 at 14:50
[**@Käptn Freiversuch**](http://stackoverflow.com/users/1700016/k%c3%a4ptn-freiversuch) Note, you're sailing tough seas here ;) ... — πάντα ῥεῖ, Jul 06 '14 at 14:50
It may help to print the value of [`errno`](http://en.cppreference.com/w/c/error/errno), using [`strerror`](http://en.cppreference.com/w/c/string/byte/strerror). Although `fopen` is not required to set `errno`, many implementations will do so. — NicholasM, Jul 06 '14 at 14:52
Thx, i did that. the output is: Can't open "..\..\..\res\thehead.raw": No error — Käptn Freiversuch, Jul 06 '14 at 14:58
@NicholasM Care to provide a reference for `fopen` not being required to set `errno`? How is a correct program to report the error? — user4815162342, Jul 06 '14 at 15:00
@KäptnFreiversuch Try printing the current working directory: http://stackoverflow.com/questions/143174/how-do-i-get-the-directory-that-a-program-is-running-from — NicholasM, Jul 06 '14 at 15:03
@user4815162342 For the C function fopen, see section 7.21.5.3 of the [C11 Draft Standard (large PDF)](http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf): "If the open operation fails, fopen returns a null pointer." — NicholasM, Jul 06 '14 at 15:06
Yes i knew, that if it fails fopen returns a nullpointer. using _fullpath(NULL,path,40) i found my mistake, as i was going one directory too high — Käptn Freiversuch, Jul 06 '14 at 15:08
@NicholasM Technically C++ (at least C++11) references the C99 standard. — Alan Stokes, Jul 06 '14 at 15:10
@AlanStokes Good point. I should have referred to the [C99 Standard](http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf), which contains the same statement in 7.19.5.3. — NicholasM, Jul 06 '14 at 15:14

user4815162342 · Accepted Answer · 2021-04-12T08:47:27.530

7

Relative paths are relative to the current working directory, not the path of the executable. The current working directory is the directory from which you started the program.

To treat a path as relative to the position of the executable, the simplest portable option is to access the executable as argv[0], extract the directory, and chdir() into it. Note that this will work only as long as the program was itself started with the full path name.

edited Apr 12 '21 at 08:47

answered Jul 06 '14 at 15:03

user4815162342

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1

Under Windows it's common to use `GetModuleFilename`, passing 0 for `hModule`, since `argv[0]` may not be right under various circumstances. – Alan Stokes Jul 06 '14 at 15:07
Thanks for the help. i figured it out using printf(" %s", _fullpath(NULL, path, 40)); apparently i was going one directory to high. "..\\..\\res\\thehead.raw"; instead of "..\\..\\..\\res\\thehead.raw" works fine now. thanks for the help – Käptn Freiversuch Jul 06 '14 at 15:07
Hello, thanks for the helpful answer. *"Relative paths are relative to the current working directory, not the path of the executable*" - this indeed seems to be true on my (Windows) system. But is this guaranteed by the standard? Or is this prone to behave differently on different machines? – Aviv Cohn Mar 29 '20 at 15:58
@AvivCohn It's definitely the case on Unix-like systems as well, although the syntax of relative and absolute paths differs. (E.g. `c:\xyz` is an absolute path on Windows, whereas the same path would be relative on Unix.) Neither C nor C++ have the concept of a working directory in the language standard, so they provide no guarantees either way. – user4815162342 Mar 29 '20 at 18:04
`the simplest option is to access the executable as argv[0], extract the directory, and chdir() into it.` This is not always true. argv[0] is what gets typed in the shell, not necessarily be the program's real path. See [link](https://stackoverflow.com/a/42291142/14233885) – user239216 Apr 12 '21 at 08:35
@user239216 You're right, I've now amended the answer to warn of this. – user4815162342 Apr 12 '21 at 08:47

score 3 · Answer 2 · answered Jul 06 '14 at 18:24

@Käptn With some modifications to the code, due to some warnings encountered, I found the following worked, although I used the C drive and not D drive as my system does not have a D drive. Effectively, the following code works same and my system will have the "file opened" message that I have added. I find this works the same whether it is run through the debugger, or executed directly from the executable in the Debug folder.

Paths

Here is my .exe file C:\uni\c++\ex5\msvc2013\ex5\Debug    
Here is the file i want to read C:\uni\c++\ex5\res\thehead.raw

Source Code

#include <iostream>

int main (int argc, char ** argv)
{
    FILE* f;
    fopen_s(&f, "..\\..\\..\\res\\thehead.raw", "rb");
    if (f == NULL)
    {
        printf("FAIL!!");
    }
    else
    {
        printf("File opened.");
    }
    return 0;
}

C++ fopen relative Path

2 Answers2