I have a program that runs correctly if I start it manually. However, if I try to add a registry key to start it automatically during startup, I get this error:

Debug assertion failed (str!=null) fprintf.c line:55

I tried to add Sleep(20000) before anything hapens, but same error.

here`s the code:

main()
{
    FILE* filetowrite;
    filetowrite = fopen("textfile.txt", "a+");

    writefunction(filetowrite);
}


int writefunction(FILE* filetowrite) {

    fprintf(filetowrite, "%s", "\n\n");
    ...
}

I also tried with passing filename as char* and opening it in wrtiefunction(), but same error.