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CS156 WhatIsAProgram: What is a Program?

What is a Program?

• A program is set of instructions for a computer, often called “code”.
• Examples: The Unix kernel, bash, cat
• A compiled program is translated into a format that the CPU can read directly, called instructions, “machine language”, or “machine code”.
• An interpreted program is not translated into machine code but executed by an interpreter.

What is a Programming Language?

• A programming language is a language for describing programs.
• Examples: C, Java, Perl, IBM Assembly Language
• Programming languages are defined by:
  • a set of rules defining valid programs: syntax
  • a set of meaning for valid programs: semantics

Syntax vs. Semantics

• Syntax: the structure of something.
• Semantics: what it means.
• Syntactically invalid: “Dog black my is.”
• Syntactically valid, but semantically invalid: “My dog is black.”
• Syntactically & semantically valid: “My dog is white.”
• If a program compiles, it has valid syntax.
• If your program works, it has valid semantics.

What is C?

• C is an imperative programming language.
• C was originally written for use in Unix, but is now used on many operating systems.
• It was designed for writing system software and much of Unix is written in C.
• C is a relatively low-level language that allows the programmer great control over how a program executes.

What is Compilation?

• Compilation is a process of translating a program into machine code.
• There are actually several steps, but we’ll ignore the distinction for a while:
  • Preprocessing
  • Compilation
  • Linking

An Example C Program

int main() {                       
    return 0;                       
}

All C programs have a main function.

• int main () is the function header.
  • The function main has a return type of int, an integer.
  • The () defines function parameters.
• The {} defines the beginning and end of the function.
  • The program execution begins with the first statement in {}.
  • return 0; returns an integer value of zero, meaning that all went well.

The type of main

• main() returns an int
• main() does not return void

The ANSI standard says that main() returns int. It doesn't matter if void works for another compiler. We teach ANSI C in this class, not Microsoft C or GNU C or any particular compiler.

An Example C Program

int main() {                       
    /*  There may
        be one or more lines
        in this type of comment.
    */              
    return 0;  // Comment to the end of line  
}

Programs may have comments.

• // starts a comment, the rest of the line is ignored by the compiler
  • You can nest this type of comment // something // else
• /* ... */ is a block comment that can span lines
  • You cannot nest this type of comment, /* is ignored within a comment. The first matching */ ends the comment.

An Example C Program

#include <stdio.h>                  // Preprocessor Directive
int main() {                        // Function
    // Print Hello, world           // Comment
    printf("Hello, world!\n");      // Function Call
    return 0;                       // Function return value
}

Declarations appear at the top of the program

• #include <stdio.h>
  • #include
    • Add a copy of the header file to this file during compilation
  • <stdio.h>
    • Standard input/output
    • .h = header file
    • Header files include declaration of functions, macros, templates and other programming elements

An Example C Program

#include <stdio.h>                  // Preprocessor Directive
int main() {                        // Function
    // Print Hello, world           // Comment
    printf("Hello, world!\n");      // Function Call
    return 0;                       // Function return value
}

Functions defined by the header file are used in the program.

• printf ("Hello, world!\n");
  • printf = name of function
  • Inside ( ) are parameters
  • printf prints (displays) the parameter values to the screen
  • \n = new line character

Creating a C Program in Unix

• For this class you will be doing all programming on the Linux operating system.
• You will create your programs using an editor: pico, vim, or whatever editor you like.
• I don’t care which editor you use.
• For that matter, I can’t even tell which editor you use.

gcc: The GNU C Compiler

• Using c11 is just like using gcc -std=c11.
• The gcc is a front end for a number of compilation tools.
• Preprocessor — handle lines that start with #.
• Compiler — Convert C code to machine code.
• Linker — combine machine code files into executable file
• The gcc is equipped with many options that change its behavior. The options affect such things as optimization, output format, warnings, etc.

Common c11 Options

• Many c11 options you won’t need to remember, but some are used often.
  • -o file — make an executable program called file instead of the default program a.out.
    

    % c11 -o hello hello_world.c
    
  • -v — give verbose output, show the commands being executed
  • --help — give some basic usage information about the compiler
  • -Wall — give tons of useful warnings

Example Continued

% cp ~cs156/pub/hello_world.c .
% ls -l
total 4
-r-------- 1 cs156 class 84 Nov 21 21:41 hello_world.c
% cat hello_world.c
#include <stdio.h> 
int main() { 
    printf("Hello, world!\n"); 
    return 0; 
} 
% c11 -Wall hello_world.c
% ls -l
total 24
-rwx------ 1 cs156 class 18104 Nov 21 21:41 a.out
-r-------- 1 cs156 class    84 Nov 21 21:41 hello_world.c
% ./a.out
Hello, world!

Example Continued, using -o

% cp ~cs156/pub/hello_world.c .
% ls -l
total 4
-r-------- 1 cs156 class 84 Nov 21 21:41 hello_world.c
% cat hello_world.c
#include <stdio.h> 
int main() { 
    printf("Hello, world!\n"); 
    return 0; 
} 
% c11 -Wall -o hello hello_world.c
% ls -l
total 24
-rwx------ 1 cs156 class 18104 Nov 21 21:41 hello
-r-------- 1 cs156 class    84 Nov 21 21:41 hello_world.c
% ./hello
Hello, world!

Get the order correct!

% cp ~cs156/pub/hello_world.c .
% c11 -Wall -o hello hello_world.c
% ls -l
total 24
-rwx------ 1 cs156 class 18104 Nov 21 21:41 hello
-r-------- 1 cs156 class    84 Nov 21 21:41 hello_world.c
% c11 -Wall -o hello_world.c hello
hello: In function `_fini':
(.fini+0x0): multiple definition of `_fini'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crti.o:(.fini+0x0): first defined here
hello: In function `data_start':
(.data+0x0): multiple definition of `__data_start'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.data+0x0): first defined here
hello:(.rodata+0x8): multiple definition of `__dso_handle'
/usr/lib/gcc/x86_64-redhat-linux/8/crtbegin.o:(.rodata+0x0): first defined here
hello:(.rodata+0x0): multiple definition of `_IO_stdin_used'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.rodata.cst4+0x0): first defined here
hello: In function `_dl_relocate_static_pie':
(.text+0x30): multiple definition of `_dl_relocate_static_pie'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.text[.text.group]+0x0): first defined here
hello: In function `_start':
(.text+0x0): multiple definition of `_start'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.text+0x0): first defined here
hello: In function `_init':
(.init+0x0): multiple definition of `_init'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crti.o:(.init+0x0): first defined here
/usr/lib/gcc/x86_64-redhat-linux/8/crtend.o:(.tm_clone_table+0x0): multiple definition of `__TMC_END__'
hello:(.data+0x8): first defined here
/bin/ld: error in hello(.eh_frame); no .eh_frame_hdr table will be created.
collect2: error: ld returned 1 exit status
% ls -l
total 20
-rwx------ 1 cs156 class 18104 Nov 21 21:41 hello

Disaster results from -o in the wrong order!

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