How to open a file no matter where I run my program from?

fopen always tries to load files from the working path. To reproduce the bug, I wrote a demo code below:

#include<stdio.h>
int main(void) {
    FILE* my_file = NULL;
    const char* file_name = "some_file.dat";
    errno_t errcode = fopen_s(&my_file, file_name, "rb");
    if (errcode != 0) {
        printf("Cannot open the file `%s`. Error code = %d", file_name, errcode);
    }
    else {
        puts("Success!!");
    }
    return errcode;
}

I have a file d:\path1\some_file.dat and a directory d:\path2. Then I compile the code above to the program named D:\path1\myprogram.exe.
Input command
```
cd d:\path1
myprogram
```
and the program will print "Success!!".
Input command
```
cd d:\path2
..\path1\myprogram
```
and the program will print "Cannot open the file some_file.dat. Error code = 2".

My question is how to successfully open the file no matter where I run my program from. If fopen can't do this, is there a library that can do it?

Reply to the comments:

I know that fopen can load files from absolute paths but I want to make my program portable. myprogram.exe and some_file.dat are always in the same path.

Solution

Assuming that the file is in the same folder as the executable.
You can use the argv[0] passed to the program – which is the executable name.
I have shown it run three ways: two from the console, and one from GUI file manager.

#include <stdio.h>
#include <string.h>

#define SLASH_CHAR  '\\'    // Windows
//#define SLASH_CHAR  '/'    // Linux

int main(int argc, char *argv[]) {

    const char* file_name = "some_file.dat";    // desired filename
    printf("argv[0] = %s\n", argv[0]);          // the executable
    
    // allocate enough memory to create a file name
    char *fname = malloc(strlen(argv[0]) + strlen(file_name) + 1);
    if(fname == NULL)
        return 1;

    // find the last path separator in the executable (if any)
    char *slash_ptr = strrchr(argv[0], SLASH_CHAR);
    if (slash_ptr == NULL) {
        // just use the file name
        strcpy(fname, file_name);
    }
    else {
        // create a new file name
        strcpy(fname, argv[0]);
        size_t slash_ind = slash_ptr - argv[0];
        strcpy(fname + slash_ind + 1, file_name);
    }
    
    printf("fname = %s\n", fname);
    free(fname);
    getchar();
    return 0;
}

Run from the console current directory

argv[0] = test
fname = some_file.dat

Run in the console from a parallel folder

argv[0] = ..\temp\test
fname = ..\temp\some_file.dat

Run from Windows file manager

argv[0] = F:\TEMP\test.exe
fname = F:\TEMP\some_file.dat