Use fopen independently on the current directory - c

Question

Let's say I am in 2 differents directory:
On the one hand:
print_text_from_file.c
text_to_be_printed.txt
On the other hand:
src/
print_text_from_file.c
text_to_be_printed.txt
And the source code from the .c:
// can't use argv
int main(void){
FILE *fp = fopen ("text_to_be_printed.txt", "r");
if (fp != NULL) {
// working function which takes fp and prints it as expected
print_from_file(fp);
fclose (fp);
}
else {
printf("The file doesn't exist\n");
}
return 0;
}
And let's say, the file only contains Hello World !
If I compile it
After compiling it from being in the first directory with gcc print_text_from_file.c
and after executing it with ./a.out, the output is:
Hello world ! (exactly what I want)
However if I'm in the second case, I'll compile being on src : gcc print_text_from_file.c
and then execute it with being out of src: ./src/a.out, the output is:
The file doesn't exist.
So basically the challenge is that I can't use argv AND I can't know from where the user will execute the file (so I don't know if using absolute paths would be correct). From anywhere, the program will have to print Hello World !.

Answer 1

You really should read something like Advanced Linux Programming and something about operating systems.
And you might also look into syscalls(2) and credentials(7)
The important notions are working directory, process, glob, unix shell, and PATH
See also chdir(2), path_resolution(7), glob(7), getcwd(3), exec(3), proc(5)

Answer 2

There are several issues here:
gcc generates the a.out file at the working directory. It does not matter whether your build print_text_from_file.c or src/print_text_from_file.c, the a.out file is created at the same place, at the working directory (not at src directory). When you run src/a.out, I don't know what did you actually run, maybe an old artifact.
The file opened by the program calling fopen is relative to the working directory. If you want to open the file src/text_to_be_printed.txt you should give path which is relative to the working directory (i.e. src/text_to_be_printed.txt).
Alternative way is to give the full path of the file. e.g. something like /home/myuser/src/text_to_be_printed.txt; this way, the program will find the input file, not as relative to the working directory but as absolute path.
In order to enable the user to give you the path, you shall change the main function signature to get arguments from the user. It should be: int main(int ac, char **av). The user shall run the command like this ./a.out INPUT_FILE_PATH, where INPUT_FILE_PATH is either relative to the working directory or absolute path.
You can access INPUT_FILE_PATH within your code as av[1]. Code will look like this:
int main(int ac, char **av){
const char *input_path = av[1];
FILE *fp = fopen (input_path, "r");
...