Copyright ©: Nahrstedt, Angrave, Abdelzaher1 C Basics Tarek Abdelzaher and Vikram Adve

Copyright ©: Nahrstedt, Angrave, Abdelzaher 1

C Basics

Tarek Abdelzaher and Vikram Adve

Copyright ©: Nahrstedt, Angrave, Abdelzaher

2

The C Language Spirit

Made by professional programmers for professional programmers

Very flexible, very efficient, very liberal Does not protect the programmers from

themselves. Rationale: programmers know what they are doing even if looks bad enough to deserve a “Darwin award” (see http://www.darwinawards.com/)

UNIX and most “serious” system software (servers, compilers, etc) are written in C.

Can do everything Java and C++ can. It’ll just look uglier in C


3

Compiler gcc See manual “man” for options man gcc

Preprocessor Compiler Linker

C89 versus C99 C99: inline functions, variable length arrays,

… make – a compilation utility

Google for make files (or GNU Make)


4

Programming in C

C = Variables + Instructions


5

Programming in C


intchar

float

…

pointerarray

struct { …}


6

Programming in C


intchar

float

struct { …}…

pointerarray

function callassignment

if

switch…

forwhile


7

10,000

10,004

10,012

10,016

10,020

…

Variables

Value1

Value2

Value3

Value4

Value5

x

y

z

p

d

Name (in program text)

Value

Memory

int x;double y;float z;double* p;int d;

Type of each variable(also determines size)


8

10,000

10,004

10,012

10,016

10,020

…

Variables

Value1

Value2

Value3

Value4

Value5

x

y

z

p

d

MemoryAddress(“name”at run-time)

Name (in program text)

Value

Memory

int x;double y;float z;double* p;int d;

Type of each variable(also determines size)


9

10,000

10,004

10,012

10,016

10,020

…

The “&” Operator:Reads “Address of”

Value1

Value2

Value3

Value4

Value5

x

y

z

p

d

Name

Value

&y

Memory


10

10,000

10,004

10,012

10,016

10,020

…

Pointers

Value1

Value2

Value3

10,004

Value5

x

y

z

p

d

Name

Value

A pointer is a variable whose value is a memory address:

p = &y;

Memory


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10,000

10,004

10,012

10,016

10,020

…

The “*” OperatorReads “Location pointed to by”

Value1

Value2

Value3

10,004

Value5

x

y

z

p

d

Name

Value

A pointer is a variable whose value is the address of another:

p = &y;

*p

Memory


12

What is the Output?

main() {int *p, q, x;x=10;p=&x;*p=x+1;q=x;printf (“q = %d\n“, q);

}


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What is the Output?


}

#@*%!

#@%$!

@*%^

p

q

x


14

What is the Output?


}

#@*%!

#@%$!

10

p

q

x


15

What is the Output?


}

#@%$!

10

p

q

x


16

What is the Output?


}

#@%$!

11

p

q

x


17

What is the Output?


}

11

11

p

q

x


18

Cardinal Rule: Must Initialize Pointers before Using them

int *p;*p = 10;

BAD


19


int *p;*p = 10;

#@*%!p??

Pointing somewhererandom


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int *p;*p = 10;

#@*%!p

#@*%!10


21

How to Initialize Pointers


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How to Initialize Pointers Set pointer equal to address of

known variable

int *p;int x;…p=&x;


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How to Initialize Pointers Use malloc()

int *p;…p=(int*) malloc (sizeof (int));

malloc() allocates memory dynamically: “heap”sizeof(T) gives the size of a program type T.


24

How to Initialize Pointers Copy another initialized pointer

value

int *p, *q;…q = (int*) malloc(sizeof(int));p = q;


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How to Initialize Pointers Create an Array

int p[10];

Same as:int *p;p=(int*) malloc (10*sizeof (int));


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Arrays

int p[5];

p[0]

p[1]

p[2]

p[3]

p[4]

Name of array (is a pointer)

p

Shorthand:*(p+1) is called p[1]*(p+2) is called p[2]etc..


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Example

int y[4];

y[1]=6;

y[2]=2; 62

y[0]

y[1]

y[2]

y[3]

y


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Array Name as Pointer What’s the difference between the examples below:

Example 1:int z[8];int *q;q=z;

Example 2:int z[8];int *q;q=&z[0];


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Array Name as Pointer What’s the difference between the examples below:

Example 1:int z[8];int *q;q=z;

Example 2:int z[8];int *q;q=&z[0];

NOTHING!!

z (the array name) is a pointer to the beginning of the array, which is &z[0]


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Example:

How much is y at the end:int y, x, *p;

x = 20;

*p = 10;

y = x + *p;


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Example:

How much is y at the end:int y, x, *p;

x = 20;

*p = 10;

y = x + *p;

BAD!! Dereferencing an unitialized pointerwill likely segfault or overwrite something!

Segfault = unauthorized memory access


32

Question: What’s the difference betweenint* q;

int q[5];

What’s wrong with:int ptr[2];

ptr[1] = 1;

ptr[2] = 2;


33

Question: What is the value of b[2] at the end?

int b[3];int* q;

b[0]=48; b[1]=113; b[2]=1;q=b;*(q+1)=2;b[2]=*bb[2]=b[2]+b[1];


34

Strings (Null-terminated Arrays of char)

Examplechar s[5]; Strings are arrays that contain byte values

ending in a “Null” character to indicate end of string. Do not forget to leave room for the null

character

s[0]

s[1]

s[2]

s[3]

s[4]

s


35

String and char constants

String constants “wassup” // how many bytes? “c” // how many bytes?

Character constants ‘c’ ‘\0’ // the Null character


36

String Operations

strcpy strlen strcat strcmp


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strcpy, strlenSyntax: strcpy(ptr1, ptr2);

where ptr1 and ptr2 are pointers to char value = strlen(ptr);

where value is an integer and ptr is a pointer to char

Example:int len; char str[15]; strcpy (str, "Hello, world!"); len = strlen(str);


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strcpy, strlen

What’s wrong with

char str[5];

strcpy (str, "Hello");


39

strncpy

Syntax: strncpy(ptr1, ptr2, num);

where ptr1 and ptr2 are pointers to char num is the number of characters to be copied

Example:int len; char str1[15], str2[15]; strcpy (str1, "Hello, world!"); strncpy (str2, str1, 5);


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strncpy

Syntax: strncpy(ptr1, ptr2, num);

where ptr1 and ptr2 are pointers to char num is the number of characters to be copied

Example:int len; char str1[15], str2[15]; strcpy (str1, "Hello, world!"); strncpy (str2, str1, 5);

Caution: strncpy blindly copies the characters. It does not voluntarily append the string-terminating null character.


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strcat Syntax: strcat(ptr1, ptr2);

where ptr1 and ptr2 are pointers to char

Concatenates the 2ndargument to the 1st (both must be null terminated strings or havoc results!).

char S[25] = "world!"; char D[25] = "Hello, "; strcat(D, S);


42

strcatExample

What’s wrong with:

char S[25] = "world!";

strcat(“Hello, ”, S);


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strcmp Syntax: diff = strcmp(ptr1, ptr2);

where diff is an integer and ptr1 and ptr2 are pointers to char

Returns zero if strings are identical (or >0 or <0)

int diff;char s1[25] = "pat"; char s2[25] = "pet"; diff = strcmp(s1, s2);

Formatted I/O

printf, fprintf: Write data to fileprintf( “Lose %d pounds in %d %s\n”,

N, D, “weeks”);

scanf, fscanf: Read data from filescanf( “%d %f %80s\n”,

&N, &F, buf); // buf must be char*


44


45

Math: Increment and Decrement Operators

Example 1:int x, y, z, w;y=10; w=2;x=++y;z=--w;

Example 2:int x, y;y=10; w=2;x=y++;z=w--;


46

Math: Increment and Decrement Operators

Example 1:int x, y, z, w;y=10; w=2;x=++y;z=--w;

Example 2:int x, y;y=10; w=2;x=y++;z=w--;

First increment/decrement then assign resultx is 11, z is 1

First assign then increment/decrementx is 10, z is 2


47

Math: Increment and Decrement Operators on Pointers

Example 1:int a[2];int number1, number2, *p;a[0]=1; a[1]=10; a[2]=100; p=a;number1 = *p++;number2 = *p;

What will number1 and number2 be at the end?


48

Math: Increment and Decrement Operators on Pointers

Example 1:int a[2];int number1, number2, *p;a[0]=1; a[1]=10; a[2]=100; p=a;number1 = *p++;number2 = *p;

What will number1 and number2 be at the end?

Hint: ++ increments pointer p not variable *p


49

Logic: Relational (Condition) Operators

== equal to != not equal to > greater than < less than >= greater than or equal to <= less than or equal to


50

Logic Example

if (a == b) printf (“Equal.”);

else printf (“Not Equal.”);

Question: what will happen if I replaced the above with:

if (a = b) printf (“Equal.”);

else printf (“Not Equal.”);

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Copyright ©: Nahrstedt, Angrave, Abdelzaher1 C Basics Tarek Abdelzaher and Vikram Adve