make command & CLI

How do we get from source code to machine code? How does a command like make work?

make

• make is not actually a compiler, but a useful program that automates the process of running an actual compiler for you (clang for us in this case)
• when we’re using make hello
  • make runs clang, and the output by default is a.out (assembly output)
  • so if you use clang directly, you don’t get a program called hello but you get a.out

make hello
ls # hello
./hello
 
clang hello.c
ls # a.out
./a.out

Command line arguments

Inputs provided to commands at the command line

code hello.c
make hello 
./hello

• make hello
  • source to machine code manually
  • triggers the compilation
  • specifies what program to make
• ./hello
  • run the machine code

make to clang

clang -o hello hello.c -lcs40 
./hello

• arguments
  • -o hello hello.c
    • -o - output
    • outputs the default a.out name to hello
  • -lcs50
    • using 3rd party libraries:l stand for link against a library
    • When using clang or gcc directly, you must specify all needed libraries manually — which can be tedious.
    • this is why make is more convenient, it does all these under the hood

argc and argv

• If we want to use CLI at runtime instead of while the program is running, we can change our main method to look like this

#include <cs50.h>
#include <stdio.h>
 
int main(int argc, string argv[])
{
    if (argc == 2)
    {
        printf("hello, %s\n", argv[1]);
    }
    else
    {
        printf("hello, world\n");
    }
}

• argc - argument count
  • stores the number of command line arguments the user typed when the program was executed
  • ./greedy - argc == 1
  • ./greedy 100 - argc == 2
• argv[] - argument vector
  • array of the characters passed as arguments at the command line
  • stores strings
    • so even tho you typed ints in the CLI it is a string
  • first element is argv[0] and last is argv[argc-1