run.md

Many tasks that are trivial in high level languages can be quite complicated in Assembly. So far we have printed a string to the screen and read the exit code of a program via echo $?. Sometimes, however, we just want to know what is the content of a register.

We can do this by writing a program that "returns" the value of that register by putting it in RAX and returning control to the caller, and writing a C program to call this assembly program and write its return to the screen.

Running

These are ret_int64.asm and caller.c. First, assemble the .asm program as usual:

$ nasm -felf64 tasks/call_from_c/ret_int64.asm -o bin/ret_int64.o

Then, use gcc to compile caller.c, linking it to our asm library:

$ gcc tasks/call_from_c/caller.c bin/ret_int64.o -o bin/caller_c

Notice that our ret_int64.asm does not have an entry point, denoted in assembly by the label _start. Here, the entry point will be provided by the C runtime, which will call the main function from caller.c, which in turn will call our program (source).

Now, just invoke caller_c:

$ bin/caller_c
10000000000
2540be400

That's the content of RAX at the moment before the ret call.

We will use this same C program with other assembly programs, all they need to have is a "function" asm_func declared globally with a ret instruction at the end. The return value will be read from RAX (if the assembly program does not set it, it will be random garbage).

A Shortcut

To quickly build caller.c against any assembly program in this project, you can use the caller_c script like this:

$ ./caller_c call_from_c ret_int64
10000000000
2540be400

Just as with the run script, this script takes the name of the directory under tasks and the program name without the .asm extension.

Negative Return

Since we defined our function in caller.c as returning an unsigned int, this is what we get from ret_neg.asm, which returns -1:

$ ./caller_c call_from_c ret_neg
DEC: 18446744073709551615
HEX: ffffffffffffffff
BIN: 1111111111111111111111111111111111111111111111111111111111111111

This is the binary representation of -1, but we interpreted it in C as a unsigned int, which in this case is the largest 64-bit number: 18,446,744,073,709,551,615.