Question

I’ve got a strange problem which I need help with. I’ve made a C++ program that gets some data from a .txt file and writes some data to another .txt file. When I compile and run the program by Xcode, it makes the output file in the same directory as the program file – that’s exactly what I need. But when I close Xcode and run the program just by double–clicking on it, it creates an output file in my Users directory. Is there a way to fix this?

Answer 1

You can use argv[0] to retrieve the full path of your program.

Then, either chdir to that path, or open the target file with a full path built from the first one.

For example:

char path_buffer[512];

strncpy(path_buffer, argv[0], 512); // This might be /home/user/Desktop/binary.exe

chdir(dirname(path_buffer));    // This is now /home/user/Desktop

fp = fopen("myfile.txt", "w");

It is safe to “blindly” chdir into whatever dirname returns, for dirname will return “.” (the current directory) in case of error.

Or also:

char path_buffer[512];
char new_buffer[512];

strncpy(path_buffer, argv[0], 512); // This might be /home/user/Desktop/binary.exe

snprintf(new_buffer, 512, "%s/%s", dirname(path_buffer), "myfile.txt");
fp = fopen(new_buffer, "w");

The above will set new_buffer to the full pathname of a file called myfile.txt in the same directory of the executable. We need two buffers because dirname may return a pointer to static storage, and it would not be safe to append “myfile.txt” directly to the returned string pointer trusting it is a modified version of path_buffer.

It would be possible to edit path_buffer, without resorting to dirname. This causes portability problems, for the directory separator isn’t necessarily “/”; it and might be “\”.

char path_buffer[512];
char *p, *q;
strncpy(path_buffer, argv[0], 512);
for (p = q = path_buffer; *p; p++)
    if (('/' == *p) || ('\\' == *p))
        q = p+1;
strncpy(q, "myfile.txt", 512 - (q - path_buffer));
// You may now use path_buffer as file name:
fp = fopen(path_buffer, "w");