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Pointers ​

Memory and addresses ​

  • In upper-level languages give the programmer little or no control over the assignment of memory addresses

  • You do get to pick what type of memory, Control What is stored in memory, But not where it’s stored

  • You control what is in your PO box based on what type of mail you get

  • You decide how big it is

  • The & used with scanf refers to the address of the variable

  • Dont use it with arrays because the name of the array is the address of the array

  • %p Conversion specifier for printing addresses Forms vary from computer systems
  • %x Hex
  • %o Octal

0x7fff Stack address

Pointer ​

A variable that can store the address of another variable Just holds addresses of other variables, they have no idea whats in the variable

Pointers indirectly references values Pointers also have their own addresses

Int intvarA = 8765;
Int *intvarAptr = &intvarA;
  • Pointers are separate data types
  • Every datatype has a pointer type
  • Cant mix pointer types and data types
Int dog
Char *frog = &dog doesnt work

We can have pointers to pointers, nested pointers

  • makes pointers

Pointer initialization and the null pointer ​

  • Null should be used to indicated that a pointer is not pointing yet

Int *A=NULL;

  • famous words of donna french
  • "Like putting safety on a gun"

  • Its good to put NULL, because 0 could not work on some systems, this is a good habit

  • Pointers are variables

Dereferneing a Pointer variable ​

Print the addresses of variables Could be useful for debugging

Not often a permanent part of program More interested in value pointed to by pointer Value can be changed through pointers

Dereferencing operator

  • is used to get to the contents of something printf("%d", *ptrcat)

Prints contents of the address of the ptrcat Dereferencing can be used on either side of an assignment

  • is like the key of the PO box

Unary operators & and * when used with porters have equal precedence with each other

  • Expresiosions combining them are evaluated from left ot right

Unary operators have higher precedence than binary operators IntVar2 *intvarprtr1 + *&intvarptr2 *& cancel each other out

Size of pointers ​

  • Holds 8 bytes
  • Pointers hold addresses
  • Same size as long

Why do pointers need to have a type? ​

For the same reason that languages have a type system in the first place, it helps to detect invalid usage of pointers when compiler time comes rather than runtime

If you pass a ptr to char that points to an int, it wont work!

  • Compiler dont like different types

Pointer Arithmetic ​

Makes it easier why pointers have types You can move pointers around by integers for arrays Take ptr add 2 to an int array

Do while ​

Executes contents of loop at least once Expression evaluated after statement While (expression); <- need this for DO while

Switch ​

Expression needs be int Switch (expression) Cases below

Continue and Break ​

While For Dowhile Use inside a loop or switch then fall out of loop Used to alter flow of control

Return ​

Used in main to terminate the progam Also terminates functions to Lets function end too Functions in c arent required to return it

Exit() ​

Takes single parameter of type int Terminates the program Needs <stdlib.h>

  • Not allowed to use exit unless assignments say to
  • Use return 0; in main, not exit(0)

\0 at the end of initialization of char arr “ABC” -> char A[4] the real size is ABC\0 just dont put size

  • The name of the array is the address of the first element of the array
Char *PtrArray[] ={"The","quick","fox","jumps",""};

Array doesnt hold strings ^

  • Pointer array is an array of pointers
  • T is the memory location of the string “the\0” same with q, f and j
  • The “” is just empty space, it is just a \0

So an arrays use pointers, a pointer array uses an array of pointers

  • Char **PtrPtr = PtrArray;

An address to get to another address to get out contents

Char *ptrarray[9];

Construct is used in c to represent an array of strings

strlen(cat), counts until theres a \0

  • C does not put a limit on the number of levels of indirection. “Fun”

Makefile ​

Adding in steps so we can link multiple object files

Gcc -c (command to ONLY compile) You get to see object file Gcc -g code_name.o -o code_name.e (names executable We can run the .e file

What is make ​

Unix utility that is designed to start execution of a makefile

A Makefile Is a special file, containing shell commands that you create and name.

  • People dont do this by hand

Make looks for the directory you’re in make (looks for file named makefile)

  • Coding assignments need separate folders now
  • Make keeps track of the last time files were updated, itl only compile new updated code
  • Without makefile this sucks ass

Makefile works well in one shell but may not in another

Makefile contains list of rule, tells make what to do The rules, have to begin in column 1 No indentions

    • Dependcency line
    • Recipe

Must be indented with a tab not spaces After its been created a make file or a text file Make will stop when it runs into an error

  • Rule : dependencies
    • System commands (recipe)

All is default rule where it starts Dependency Recipe

Makefiles works its way down then back up Executes backwards *** missing separator means that you didnt use a tab

Make cant find makefile.txt, make cant also find makefile.mak

  • We neeed to have it as makefile (no extensions)
SRC = code.c
OBJ = $(SRC: .c=.o)
EXE = $(SRC:.c=.e)

CFLAGS = -g

All : $(EXE)

$(EXE) : $(OBJ)

$() -> signals to do substitutions

We dont need to memorize makefile on a test, but instead understand what it does.

  • To use multiple c files SRC1 = code_main.c SRC2 = libarray_code.c OBJ1 = $(SRC1:.c=.o) OBJ2 = $(SRC2:.c=.o) EXE = $(SRC1:.c=.e)

With multiple file = multiple sources and objects but only 1 exe file with src1 being main.

Will ask what section is compiler section

Compiler creates an object file, or a .o Linker links files to make 1 .e or executable

Libraries ​

  • Libraries are not executable
  • Do not contain a main() function
  • Only contain functions and declarations
  • Cannot gcc a library then ./a.out it
  • No more gcc or a.out after we start using make
  • Coding assignment 4 your library has 2 files
  • Mylibrary.c
  • And mylibrary.h

Stdio.h ​

Stdio.h is a standard library file, there is no executable code in stdio.h the prototype is in the .h the real code is in stdio.c which is linked with the system library

We need something for our header files to prevent stupid from happening

Include guard ​

  • Guards our original file, our personal bouncer
  • Will make our header file idempotent
  • A file being idempotent means it tells the preprocessor to grab whatever header only 1 if its even there 100 times
#ifndef (if not defined)
#define (define it)
Then put prototype
Somecode();
morefunccode();
#endif (if its included again it goes back to top)
  • Include guard is for our safety
  • Include guard is the bread to our grilled cheese sandwich

Take name of file #ifndef _FUNCPRO_H (always unique this way)

Compiling and Linking ​

Main program Filelib.c FTP.c

  • We compile and link with make individual files, because we dont open files that we dont need to open
  • Allow multiple people in industry to work on separate files to work at multiple places at once and then compare if it works

Comments are made by # sign

  • make -B (causes make to recompile everything)

In C

  • All parameters are passed by value (pass a copy of them) The ability to pass by reference does not exist

  • We can simulate it (pass by reference) We pass the address of the variable

  • Addresses cannot be modified Contents of address can be modified

  • This can be done in functions If you pass an address to the function We can modify the contents of the variable because we passed its address

Calling it: we print it like cat(&dog); In func: void cat(int *Bird)

Strings ​

No string data type, in other languages they are objects

  • C doesn’t have objects

string.h what is it? ​

  • A string is a sequence of characters from the underlying character set
  • A string in C is terminated by a null character. ‘\0’
  • A string is accessed via a pointer in its first character
  • A string is like an array of characters - both are stored in contiguous memory
  • Arrays do not require a null character
  • Strings must have a null character at the end
  • Strings are a null terminated character array

String library functions expect strings or Null terminated char arrays

Char Cat[]"Put things in memory with a \0"
  • The address is the first element

If you put a \0 in the middle of string it just cuts off your string

" please dont cut me \0 off"
Prints: please dont cut me

Char cat[10] = {};
Char *Cat;

These things are not interchangeable directly

  • If assignment says make an array make a gosh dang array
%s ​

Basically just telling printf(%s) will print until it finds the \0

scanf(“%s”, array) ​
  • Dont need & because name of the array is the address of the first element of the array

If you want to print a % sign in string you do %%

  • Scanf doesnt read through whitespace
  • When it sees whitespace it stops

Scanf adds a null terminator to the array What happens if array is not large enough to hold input?

  • Overflowwwwwwwwwwwwwwwwwwwwwwwwwwwwww

A string can be stored in an array at the type the array is declared

Variable Strings ​

To input a variable length string

Fgets (inbuff, n, fp) fp = stdin ​

  • Will not stop at whitespaces
  • Everything it reads is a string
  • Only returns character

For strings that have spaces in them Puts null terminator at the end So always do n-1

Returns a value a char* value(the address of inbuff) or NULL We dont even need it For now

fgets must get an array to write into

  • Cant use a pointer for thisss

gets() ​

Takes array, and assume you’re reading from stdin gets(inputarray); It doesnt, have the second parameter to stop it from reading too much Thats no good Doesnt have overflow controlThe compiler tells you that gets is dangerous and should not be used lmao

String library Functions ​

strlen(string) ​

  • Calculates the length of string
  • Parameters, null terminated array
  • Return value: the length of a string not including a null

size_t is an unsigned int, or like one (8 bits?) 0 and up, ints positive Gets rid of the that pesky \n UserString[strlen (UserString) -1] = '\0';

  • Implicit declaration of built in function warning means u need to #include

strcpy(buffer, string) ​

Copies string into buffer

Writes over what was there before, not wiping it clean, just writes over the parts that were there Result always ends up in first parameter

strcat(buffer, string) ​

Glues two things together Glues two strings togetheerr

Hellow there
How are you
Hellow therehow are you

Second string is unharmed First one is glued together with second

strncat (buffer, string, n) ​

n lets us tell it how many character to glue together strncat (string, string1, 2) This string was This string was gl

strcmp(string1, string2) ​

Just compares strings no altering It return a 0 -string identical Positive - first string is > second string negative - first string is < second string Compares first char in string, then moves on if they’re diff

Shorter string will always be less than

strncmp(string1, string2, n) ​

Like strcmp, but the n is just how many characters its comparing

Requiring use of pointers ​

strchr(string, ch) ​

Looks in a string for a given character ch If it cant find it gives you NULL If it does find it, it gives you the address of it back If you have an odd string like qbcabqc It will only find the first letter of the pointer, like q will only be the first q

To get around this you use a while loop with a pointer not = null You would dereference with first occurrence and slam the first occurrence with a - Then ask it to get another strchr after u do that

strstr(string1, string2) ​

Like strchr but with pointers Points back to the first letter of the string)

Technique for coding assignment 5 ​

Get pointer to first occurrence Same while loop as strchr Find the distance between the 1st string and first occurrence Then for loop counting up to strlen of userstring 2 Inside for loop, userstring1[distance + i] = ‘X’ First occur = strstr(userstring1, userstring2);

while (FirstOccur != NULL)
{
    distance=abs(UserString1)-FirstOccur;
    for(i=0;i<strlen(UserString2);i++)
    {
        UserString1[distance+i]='X';
        
    }
    FirstOccur=strstr(UserString1,UserString2);
}

strpbrk(string, char_set) ​

Like string char, but uh whole set of char’s Same loop with string char A more efficient string char

While (firstoccur != NULL)
{
	
}

strtok(buffer, delimiters) ​

How you take a string, and put them into separate variables or tokenize them Delimiters divide up tokens

We break them up because we want to handle things separately

Stringtok changes our strings when it sees delimiters, it will replace it with a NULL

Creates internal static pointer that points to the point after the delimiter it just replaced with NULL Strtok can be told to not start over, pick up where it left off

Using NULL instead of the array name invokes the pointer to the spot after NULL If you dont use NULL It just startsss over at the beginning

You can use char *Delmiters = “{}”; Char *Token = NULL;

Is functions ​

Only works for single character

  • islower(ch) is lowercase
  • isupper(ch)
  • Isalpha(ch)
  • isalnum(ch)

Checks for things on single char Can use with if statements to return 1 or 0

To func. ​

  • tolower(ch) returns the lowercase version of ch
  • toupper(ch) returns the uppercase version of ch

Needs to be passed by reference or be used by passed by value to store a variable to the return of toupper

Ex. x = toupper(chrX);

Atoi and Atof ​

Takes string that has a number value and gives back the int or float vers. Of the char number

Char num[5] = {“540”}; printf(“%d”, atoi(num));

Does not work if char array has letters before numbers Works with letters after