J.E.Jones
OSU CSE 2421
OSU CSE 2421
• C is procedural, not object-oriented
• C is fully compiled (to machine code), not to byte-code
• C allows direct manipulation of memory (via pointers)
• C does not have garbage collection; the software writer must do explicit memory management when it is required, and failure to do so can result in significant problems (e.g., memory leaks, where the memory used by the program may grow potentially without bound)
• Many of the basic language constructs in C act similarly to the way they work in Java; nonetheless, there are sometimes important differences which need to be understood
• C has many nuanced, yet important details
J.E.Jones
OSU CSE 2421
• • • •
•
C does not support the notion of Classes or Objects
We will talk about classes in C, but it has no relationship to classes in C++
•
ANSI C does not have a Boolean type
• •
C is not an object-oriented language, it is procedural.
This means development is usually top-down rather than bottom-up C breaks down to functions while Java breaks down to objects
C does not itself support Encapsulation, as all memory is technically accessible and modifiable by any instruction of an executable
C does not itself support polymorphism, virtual function, inheritance, operator overloading, namespace concepts
J.E.Jones
OSU CSE 2421
In this class, we will learn ANSI C, which was originally standardized in 1989 by the American National Standards Institute. This version of C is sometimes referred to as “C89” (also sometimes referred to as “Standard C”). The ISO, or International Standards Organization, also adopted an equivalent version of C in 1990, which is often referred to as C90. Therefore, C89 and C90 are, in effect, equivalent.
There are later versions of C, for example, C99, which differ from ANSI C; for example, they may support features that ANSI C does not support (such as just-in-time declaration, for example).
J.E.Jones
OSU CSE 2421
There are a few words that you will see in different contexts as we go through this semester
The words Dynamic and Static are two of the worst offenders.
To determine context, check out the word these two adjectives describe: ◦ Dynamic memory
◦ Dynamic memory allocation
◦ Dynamic error
◦ Static error
◦ Static memory
◦ Static memory allocation ◦ Static identifier class
I can’t change these things. I can’t tell you why it is this way. We all must live with it – even though it can get confusing. So be aware!
J.E.Jones
OSU CSE 2421
In computer science, we distinguish events that can occur when a program is being compiled or built, called compile time, from events which can occur when the program is being executed, or running, called run time.
Certain kinds of errors can be identified (or occur) only at compile time, and while others only at run time.
Errors which occur at compile/build time: syntax errors/static errors [these make a program invalid, i.e., in the case of C, not a valid C program], or possibly linkage errors – more on this in a few weeks. In this case, the compiler may or may not generate an executable depending upon the severity of the error.
Here’s the 1
Compile/build time events are often referred to as static and run time events as dynamic.
Errors which occur at run time are semantic errors/dynamic errors [these do not make a program invalid, because only a valid program can be built and then executed]. An example of such an error would be division by zero or some other logic error. The compiler can not discover such an error, but when the program runs, an exception will sometimes be generated (e.g., segmentation fault), and the operating system will terminate the program.
st
occurrence!
J.E.Jones
OSU CSE 2421
Four General Categories of Statements in Computer Languages
◦ Declarations (optional in some languages like Python)
◦ Data Movement
• Memorytofunctionvariables
• Functionvariablestofunctionvariables • Functionvariablestomemory
◦ Arithmetic/Logical Operations • Comparesomething
• Calculatesomething
◦ Control-Flow
• Procedure/functioncalls • Looping
• Conditionals
J.E.Jones
OSU CSE 2421
/* Version 1 */
#include
void main (int argc, char **argv) {
/* Version 2 */ #include
}
/* Version 3 */ #include
printf(“Hello, ”); printf(“World!\n”);
printf(“Hello, World!\n”); return (0);
}
printf(“Hello, World!\n”);
return (EXIT_SUCCESS); }
J.E.Jones
OSU CSE 2421
Notice the lines at the top which begin with the # character. These are known as preprocessor directives and are used by a translation program called the preprocessor, which is the first program called when you build C source code (in our case, with gcc).
Required Reading: Pointers on C, Chapter 14, The Preprocessor
◦ You can omit section 14.5
◦ The engineering library has a copy if you didn’t purchase the book.
◦ 20 pages, but the print is large. By far, the largest reading assignment all semester, but worth it.
◦ You must be able to answer questions on the content by next week.
J.E.Jones
OSU CSE 2421
Prepares a .c file for the compiler
Input is from sourcecode.c and output goes to sourcecode.i
Strips all comments from sourcecode.c
Using any #include file preprocessor directives in the source code file, copy the entire contents of the file in to sourcecode.i
Replace any MACROs.
J.E.Jones
OSU CSE 2421
#include
main(){
Contents of stdio.h Contents of local.h main(){
.
.
.
EXIT_SUCCESS
} } .c file
. 0
.i file
J.E.Jones
OSU CSE 2421
One of the first things the preprocessor does is find, in the area on disk where library files are kept, the file with the name stdio.h or stdlib.h and copy the contents of that file into the source file. The text in the preprocessor directive (for example “#include
The header file may contain various things, but one thing it usually contains is one or more function prototypes (function declarations – see below), which tell the compiler information about a function (return type and parameters) that it needs to be able to do error checking while compiling the source code.
The header file does not contain the code for any of the functions…only the prototype.
J.E.Jones
OSU CSE 2421
The file accessed when #include
The preprocessor also removes comments from the source file.
Comments must be enclosed between /* and */
◦ [Note: no single line comments with // are permitted in ANSI C].
◦ Also, the preprocessor for ANSI C is not written to find nested comments, so they are disallowed. /* comment1 */ /*comment2*/
When you write comments in code on exams/homework/etc, correct format is expected.
J.E.Jones
OSU CSE 2421
Another thing the preprocessor does is to replace macros, which are fragments of code, defined in the source file, or in a header file, with the code they are defined to represent
◦ Keep in mind that code in a source file is just text, so macros are just chunks of text.
These macros, or code fragments, are defined with the define preprocessor
directive, as follows:
#define string1 string2
◦ string1 cannot contain any white space characters (spaces, tabs, new lines), but string2 can.
An example of this is in Version 3 of the Hello program, where you see EXIT_SUCCESS in the return statement at the end of the function. This macro is defined in the stdlib.h file, and that is why it has been included, using a preprocessor directive, in this version of the program.
J.E.Jones
OSU CSE 2421
In C, as in Java, statements must generally end with a semi-colon.
Remember, though, that preprocessor directives are NOT statements,
and are not terminated with a semi-colon!
C is also case sensitive, so if we have two variables, named num and Num in a C program, the compiler will treat them as two distinct variables.
J.E.Jones
OSU CSE 2421
C programs consist of zero or more:
◦ preprocessor directives,
◦ declarations and
◦ definitions (see explanation on a future slide) of:
One or more functions (which may contain variable declarations or definitions), and
Zero or more variables declared or defined outside of any function
J.E.Jones
OSU CSE 2421
Notice that all three versions of the Hello program have a main() function. Every C program must have exactly one main function, and program execution always begins in this function. main() does not necessarily have to be the first function defined in the program file.
Also, notice that, although Java also has a main method, it is in a class, but C has no classes!
In C, technically, we have no methods, only functions. Please pay attention to this distinction. It makes you look bad to other computer science professionals to talk about a “method” in a C program (remember the correct programming model and terminology for the language being discussed)! One of the fastest ways to lose the respect of your work-life peers is to use the wrong terminology. It tells them you don’t know what you are doing.
J.E.Jones
OSU CSE 2421
A number of statements grouped into a single logical unit is called a function
REMEMBER: It is required to have a single function ‘main’ in every C
program
A function prototype is a function declaration or definition which includes: ◦ Information about the number of arguments
◦ Information about the types of arguments
◦ What type of value the function returns
◦ NO CODE!
◦ Example: float add_floats(float a, float b);
note that there is a ‘;’ at the end rather than {}
Although you are allowed not to specify any information about a function’s arguments in a declaration, it is purely because of backwards compatibility with Old C and should be avoided (poor coding style).
◦ A declaration without any information about the arguments is not a prototype.
J.E.Jones
OSU CSE 2421
In general, C passes arguments to functions by value ◦ For example, int mult_values (int a, int b);
◦ This ensures that the values passed to the function can not be changed in the calling function by the called function.
But what if that’s what we would like to have happen?
◦ That’s when pointers get involved and we use what is called “pass by
reference”.
◦ We’ll look at this when we look at pointers in a week or so.
J.E.Jones
OSU CSE 2421
Declaration (type information only, no value(s)): ◦ Variable
it tells the compiler the type of a variable, but not its value
can only be declared once in a given block in a C program; ◦ Function
it tells the compiler the return type, and the number and types of its parameters (parameter names are optional), this is called a prototype.
can be declared multiple times in a C program, as long as all of the declarations are consistent (that is, identical with respect to types/# parameters).
Definition (type information and value): ◦ Variable
it tells the compiler the type of the variable and the initial value. ◦ Function
it tells the compiler the return type, parameter types, parameter names and the code that should be executed (i.e., the statements) when the function is called.
◦ A given variable or function can only be defined once in a C program.
◦ Note that a definition is also a declaration, since it contains type information.
J.E.Jones
OSU CSE 2421
In C:
◦ A variable must be declared (but not necessarily defined) before it can be
referenced in a non-declarative statement.
◦ int a; int a=7;
◦ a=7;
◦ A function must be declared (but not necessarily defined) before it can be referenced in a non-declarative statement (that is, before it can be called or invoked) . We declare a function with a prototype statement.
J.E.Jones
OSU CSE 2421
int funcA(int a); main(){
int c=3;
c=funcA(c);
}
int funcA(int a){ return(a);
}
main(){
int funcA(int a){ return(a);
}
int c=3;
c=funcA(c); }
J.E.Jones
OSU CSE 2421
Functions consist of one or more blocks (blocks can legally be empty).
A block in ANSI C has this form: { /* left curly brace */
Zero or more variable declarations/definitions
Zero or more non-declarative statements (i.e., statements that DO something) } /* right curly brace */
IMPORTANT: In ANSI C, all variable declarations in the block must precede the first non-declarative statement (i.e., no just-in-time declaration in ANSI C).
Yes, I know just-in-time declarations are more efficient.
Nested blocks are valid in C, but nested functions are not valid (that is, the compiler will generate errors, and will not produce an executable, if your source code contains one or more nested functions).
◦ If you tried to “nest” functions, then you would be defining one function within a block within another function.
◦ Other languages (e.g., ALGOL, MATLAB, C# (after 7.0), Scala) do support nested functions
In C, all functions have file scope. This means that any function declared in a file can be called from anywhere in the same file, after the point at which the function is declared.
In C, variables can have either file scope or block scope depending upon where they are declared. Block scope means that the variable is only accessible by the variable name within the block where it was declared.
◦ The caveat “by the variable name” is important. We will revisit this when we cover pointers.
J.E.Jones
OSU CSE 2421
#include
int function_level=0;
int functionA(int a, float b); int functionB(int a, float b); main(){
The variable function_level can be used anywhere within this file from this point down. Its initial value is 0.
}
int functionB(int a, float b){
}
int functionC(int a, float b){
}
.
.
function_level = ‘M’;
variable2 = functionA(variable1, float_val1); variable3 = functionB(variable1, float_val1);
Function prototype for functionC(). It can be used anywhere within this file from this point down. (i.e., main() can’t call functionC(), but functionA() or functionB() or functionC() could call it.
}
float functionC(int a, float b); float new_float;
int functionA(int a, float b){
The variable new_float can be used anywhere within this file from this point down. (i.e., main() can’t reference new_float, but functionA() or functionB() or functionC() can.
/* code within not germane*/
.
.
function_level=‘B’; new_float=functionC(a,b);
/* code within not germane */
Function prototypes for functionA() and functionB(). They can be used anywhere within this file from this point down.
J.E.Jones
OSU CSE 2421
#include
int total = 0; /* variable declaration and definition */ int i; /* variable declaration */
int values[4] = {12, 14, 18, 20}; /* array variable
declaration and definition */ for (i = 0; i < 4; i++) { /* nested block */
total = total + values[i]; }
printf(“The total is: %i\n”, total);
return (EXIT_SUCCESS); }
J.E.Jones