Page 140 - ARM 64 Bit Assembly Language

126 Chapter 5

Listing 5.19 Calling scanf and printf in C.

1 #include <stdio.h>
2
3 static char str1[] = "%d";
4 static char str2[] = "You entered %d\n";
5 static int n = 0;
6
7 int main()
8 {
9 scanf(str1, &n);
10 printf(str2, n);
11 return 0;
12 }

5.4.3 Standard C library functions

Subroutines may be defined within a program, or a set of subroutines may be packaged to-
gether in a library. Libraries of subroutines may be used by multiple programs, and most
languages provide some built-in library functions. The C language has a very large set of
functions in the C standard library. All of the functions in the C standard library are avail-
able to any program that has been linked with the C standard library. Even assembly programs
can make use of this library. Linking is done automatically when gcc is used to assemble the
program source. All that the programmer needs to know is the name of the function and how
to pass parameters to it.

5.4.4 Passing parameters

Listing 5.19 shows a very simple C program which reads an integer from standard input using
scanf and prints the integer to standard output using printf. An equivalent program written
in AArch64 assembly is shown in Listing 5.20. These examples show how parameters can be
passed to subroutines in C and equivalently in assembly language.

All processor families have their own standard methods, or function calling conventions,
which specify how arguments are passed to subroutines and how function values are returned.
The function call standard allows programmers to write subroutines and libraries of subrou-
tines that can be called by other programmers. In most cases, the function calling standards
are not enforced by hardware, but assembly programmers and compiler writers conform to the
standards in order to make their code accessible to other programmers. The basic subroutine
calling rules for the AArch64 processor are simple: