Last modified: 2023-04-08

C

Wikipedia article

• C89 / ANSI is the most important standard, a must to know in C world
  • C89 and C90 are practically identical
• C99 and C11 are nice to have
• It is recommended to use gcc program.c -Wall -Wextra -pedantic -std=c89 to give you additional feedback and code checks
• Many things from C++ were backported into C99
  • like const, so you can avoid preprocessor's #define
  • and stdint.h
• specifications of C standards (source; they are technically drafts, but are valid)
  • c89 draft

Stuff to cover

• *.c and *.cpp contain the implementation
• *.h contain the definitions or prototypes
• Preprocessor
• Header files
• Macros
• construction enclosed in ( and )
• C language is stream oriented
• Memory Layout of C Programs

Operators

Sources:

• Wikipedia / Operators in C and C++

See also:

• Wikipedia / Operators in C and C++ / Operator precedence

• Unary operators

  • Perform action with single variable
  • Examples: ++, ++, !, sizeof, etc

• Binary operators

  • Perform action with two variables
  • Examples: +, -, *, >=, etc

• Ternary operator

  • ?
  • shorthand for if statement

int larger;
if (a > b)
	larger = a;
else
	larger = b;

int larger;
larger = (a > b) ? a : b;

Comparison operators / relational operators

C++ also has this:

Logic operators

• Non-zero values are TRUE
• Zero is always FALSE just like \0

Bitwise logic operators

Assignment operators

Member and pointer operators

Other operators

Variables

extern

static

Sources:

• stackoverflow.com / What does "static" mean in C?
• geeksforgeeks.org / Static Variables in C

static:

• A static variable inside a function keeps its value between invocations.
  • When you use static int x = 0;, the variable is initialized to 0 only on creation and is not reset in later iterations.
  • Can be used instead of global variable.
• A static global variable or a function is "seen" only in the file it's declared in.
• Static variables are allocated memory in data segment, not stack segment (see Memory Layout of C Programs)
• Static variables should not be declared inside structure. The reason is C compiler requires the entire structure elements to be placed together (i.e.) memory allocation for structure members should be contiguous.

Arrays

Size of array

Sources:

• stackoverflow.com / How can I find the number of elements in an array?

sizeof with first element

int a[20];
size_t length = sizeof(a) / sizeof(a[0]);

sizeof with type

int a[20];
size_t length = sizeof(a) / sizeof(int);

Macro

#define NUM(a) (sizeof(a) / sizeof(*a))
for (i = 0; i < NUM(a); i++)

Compiler should be smart enough to not calculate the macro at compile time.

Libraries

math vs cmath vs math.h vs cmath.h

• those with .h will pollute your global namespace with a lot of junk, those without may or may not (no guarantees)
• c vs c++: prefix c is for C++ and is missing the .h at the end

stdio.h

• int printf ( const char * format, ... ); - print C string
• int putchar ( int character ); - print single character
• int puts ( const char * str); - print C string with newline
• int scanf ( cost chat * format, ... ); - read formatted data from stdin
  • for example scanf ("format", &var);, do not use & for string or array
  • stops at first white-space character
  • discouraged to input strings because of possible buffer overflow
    • rather use fgets
• char * fgets ( char * str, int num, FILE * stream );

char input[64];
fgets(input,64,stdin);

Look into format specifiers to format data.

math.h

• usually use double
• Order of precedence
  • equations are evaluated left to right
  • multiplication and division are done first
  • addition and subtraction
  • parentheses used to prioritize some calculations

string.h

• C language does not have string, but rather arrays of characters
  • final character of string is null character (\0)
• char string[] where brackets can be empty if immediate assignment, otherwise length of desired string + 1 for null character
  • char string[] = "hello";
  • char string[6]; strcpy(string,"hello");
• string manipulation function are in string.h

stdlib.h

• void* malloc(size_t size);
  • successful allocation returns address
  • failed allocation returns NULL pointer
    • NULL is a contant pointer, use it to check if allocation was succesful (not the same thing as \0)

char *sto;
sto = malloc( sizeof(char) * 1024 ); // want to store 1024 chars

if (sto == NULL){
	printf("Unable to allocate memory\n");
	return(1);
}

Pointers

• pointer is declared with * operator
• pointer type must match variable type it is pointing at
• when used without * it represents an address
• when used with * it represents the value at the address

int pokey = 987;
int *p = &pokey;
printf("The address of 'pokey' is %p\n", &pokey);
printf("The address in 'p' is %p\n", p);

printf("The value of `pokey` is %d\n", pokey);
printf("The value of `*p` is %d\n", *p);

The address of 'pokey' is 0x7ffd437e4a8c
The address in 'p' is 0x7ffd437e4a8c

The value of `pokey` is 987
The value of `*p` is 987

Pointer arithmetic

int array[] = {11, 13, 17, 19};
int *aptr, *aptr2;
aptr = array;
aptr2 = &array[1];

printf("Elements is %d\n", *aptr);
printf("Elements is %d\n", *aptr2);

printf("Elements is %d\n", array[3]);
printf("Elements is %d\n", *(aptr+3));

Elements is 11
Elements is 13

Elements is 19
Elements is 19

Misc

• Required main() function (this is the entry point)
• operator sizeof returns number of bytes used by variable
  • can be even used of struct
• uninitialized variables contain garbage, this includes pointers
• modulo operator % is for remainder after integer division

sizeof

struct stuff {
	int a;
	float b;
	char c[32];
}
printf("Sizeof struct is %lu bytes", sizeof(struct stuff) );

returns:

Sizeof struct is 40 bytes