Print name of the binary - c

Question

I am sorry if this is a stupid question, but I have been searching but unable to find the answer anywhere else. Its most likely pretty simple, but here goes:
I am asked to write a C program which takes one argument. If I try to give it more than one, it should print a usage message to stderr, with the format Usage: %s argument\n, where %s is the name of the binary.
It is this last part which I do not understand.
I know it should look like this if it fails:
$./hello
Usage: ./hello argument
Can someone please explain this to me?
EDIT:
Got it working like this:
#include<stdlib.h>
#include<stdio.h>
#include<errno.h>
#include<string.h>
int args_test(int args, char argument[]){
if(args < 1){
fprintf(stderr, "Usage: %s argument\n", argument);
exit(EXIT_SUCCESS);
}
if(args < 2){
fprintf(stderr, "Usage: %s argument\n", argument);
exit(EXIT_SUCCESS);
}
if(args > 2){
fprintf(stderr, "Usage: %s argument\n", argument);
exit(EXIT_SUCCESS);
}
return 0;
}
int args_usage(char argument[]){
printf("Hello, %s!\n", argument);
exit(EXIT_SUCCESS);
}
int main(int args, char* argv[]){
if(args < 2 || args > 2){
args_test(args, argv[0]);
}
args_test(args, argv[1]);
args_usage(argv[1]);
exit(EXIT_SUCCESS);
}

Answer 1

Hell first of all you can try and hardcode it :) Just kidding.
If you are on linux, you can get binary name withing /proc/pid/exe.
For example:
auto ret = readlink("/proc/self/exe", buf, sizeof(buf));
if (ret == -1) //error
std::string fpath(buf, ret);
Now in fpath you have absolute path to your program. You can parse your program out with function like dirname.
So first you need to get absolute path to char*. This can be done with strcpy
char *fullpath = new char[fpath.length()+1];
strcpy(fullpath, fpath.c_str());
std::string dir = dirname(fullpath);
std::string program_name = fpath.substr(dir.length());
printf("Usage: %s argument\n", program_name);
And that is it. I will just leave it for you to decide size of the buffer ;)
If you are on Windows, you can use function GetModuleFileName
https://msdn.microsoft.com/en-us/library/windows/desktop/ms683197(v=vs.85).aspx
Enjoy