Copyright ©: Nahrstedt, Angrave, Abdelzaher1 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 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

Copyright ©: Nahrstedt, Angrave, Abdelzaher 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)

Copyright ©: Nahrstedt, Angrave, Abdelzaher 4 Programming in C C = Variables + Instructions

Copyright ©: Nahrstedt, Angrave, Abdelzaher 5 Programming in C C = Variables + Instructions int char float … pointer array struct { …}

Copyright ©: Nahrstedt, Angrave, Abdelzaher 6 Programming in C C = Variables + Instructions int char float struct { …} … pointer array function call assignment if switch … for while

Copyright ©: Nahrstedt, Angrave, Abdelzaher 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)

Copyright ©: Nahrstedt, Angrave, Abdelzaher 8 10,000 10,004 10,012 10,016 10,020 … Variables Value1 Value2 Value3 Value4 Value5 x y z p d Memory Address (“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)

Copyright ©: Nahrstedt, Angrave, Abdelzaher 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

Copyright ©: Nahrstedt, Angrave, Abdelzaher 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

Copyright ©: Nahrstedt, Angrave, Abdelzaher 11 10,000 10,004 10,012 10,016 10,020 … The “*” Operator Reads “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

Copyright ©: Nahrstedt, Angrave, Abdelzaher 12 What is the Output? main() { int *p, q, x; x=10; p=&x; *p=x+1; q=x; printf (“q = %d\n“, q); }

Copyright ©: Nahrstedt, Angrave, Abdelzaher 13 What is the Output? main() { int *p, q, x; x=10; p=&x; *p=x+1; q=x; printf (“q = %d\n“, q); } @*%^ p q x

Copyright ©: Nahrstedt, Angrave, Abdelzaher 14 What is the Output? main() { int *p, q, x; x=10; p=&x; *p=x+1; q=x; printf (“q = %d\n“, q); } 10 p q x

Copyright ©: Nahrstedt, Angrave, Abdelzaher 15 What is the Output? main() { int *p, q, x; x=10; p=&x; *p=x+1; q=x; printf (“q = %d\n“, q); } 10 p q x

Copyright ©: Nahrstedt, Angrave, Abdelzaher 16 What is the Output? main() { int *p, q, x; x=10; p=&x; *p=x+1; q=x; printf (“q = %d\n“, q); } 11 p q x

Copyright ©: Nahrstedt, Angrave, Abdelzaher 17 What is the Output? main() { int *p, q, x; x=10; p=&x; *p=x+1; q=x; printf (“q = %d\n“, q); } 11 p q x

Copyright ©: Nahrstedt, Angrave, Abdelzaher 18 Cardinal Rule: Must Initialize Pointers before Using them int *p; *p = 10; GOOD or BAD???

Copyright ©: Nahrstedt, Angrave, Abdelzaher 19 Cardinal Rule: Must Initialize Pointers before Using them int *p; *p = 10; p ?? Pointing somewhere random BAD!

Copyright ©: Nahrstedt, Angrave, Abdelzaher 20 Cardinal Rule: Must Initialize Pointers before Using them int *p; *p = 10; p 10 BAD!

Copyright ©: Nahrstedt, Angrave, Abdelzaher 21 How to Initialize Pointers

Copyright ©: Nahrstedt, Angrave, Abdelzaher 22 How to Initialize Pointers Set pointer equal to address of known variable int *p; int x; … p=&x;

Copyright ©: Nahrstedt, Angrave, Abdelzaher 23 How to Initialize Pointers Use void *malloc(size_t size); int *p; … p=(int*) malloc (sizeof (int)); malloc() allocates memory dynamically: “heap” sizeof(T) gives the size of a program type T.

Copyright ©: Nahrstedt, Angrave, Abdelzaher 24 How to Initialize Pointers Copy another initialized pointer value int *p, *q; … q = (int*) malloc(sizeof(int)); p = q;

Copyright ©: Nahrstedt, Angrave, Abdelzaher 25 How to Initialize Pointers Create an Array int p[10]; Same as: int *p; p=(int*) malloc (10*sizeof (int));

Copyright ©: Nahrstedt, Angrave, Abdelzaher 26 Arrays int p[5]; p[0] p[1] p[2] p[3] p[4] Name of array (is a pointer) Shorthand: *(p+1) is called ……? *(p+2) is called ……? etc..

Copyright ©: Nahrstedt, Angrave, Abdelzaher 27 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..

Copyright ©: Nahrstedt, Angrave, Abdelzaher 28 Example int y[4]; y[1]=6; y[2]=2; 6 2 y[0] y[1] y[2] y[3] y

Copyright ©: Nahrstedt, Angrave, Abdelzaher 29 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];

Copyright ©: Nahrstedt, Angrave, Abdelzaher 30 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]

Copyright ©: Nahrstedt, Angrave, Abdelzaher 31 Example: How much is y at the end? int y, x, *p; x = 20; *p = 10; y = x + *p;

Copyright ©: Nahrstedt, Angrave, Abdelzaher 32 Example: How much is y at the end? int y, x, *p; x = 20; *p = 10; y = x + *p; BAD!! Dereferencing an unitialized pointer will likely segfault or overwrite something! Segfault = unauthorized memory access

Copyright ©: Nahrstedt, Angrave, Abdelzaher 33 Question: What’s the difference between int* q; int q[5]; What’s wrong with: int ptr[2]; ptr[1] = 1; ptr[2] = 2;

Copyright ©: Nahrstedt, Angrave, Abdelzaher 34 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]=*b b[2]=b[2]+b[1];

Copyright ©: Nahrstedt, Angrave, Abdelzaher 35 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[1] = 2 b[2]=*b  b[2] = b[0] = 48 b[2]=b[2]+b[1];  b[2] =

Copyright ©: Nahrstedt, Angrave, Abdelzaher 36 Strings (Null-terminated Arrays of char) Example char 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

Copyright ©: Nahrstedt, Angrave, Abdelzaher 37 String and char constants String constants “wassup”// how many bytes? “c” // how many bytes? Character constants ‘c’ ‘\0’// the Null character

Copyright ©: Nahrstedt, Angrave, Abdelzaher 38 String Operations strcpy strlen strcat strcmp

Copyright ©: Nahrstedt, Angrave, Abdelzaher 39 strcpy, strlen Syntax: 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);

Copyright ©: Nahrstedt, Angrave, Abdelzaher 40 strcpy, strlen What’s wrong with char str[5]; strcpy (str, "Hello");

Copyright ©: Nahrstedt, Angrave, Abdelzaher 41 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); Is the example correct?

Copyright ©: Nahrstedt, Angrave, Abdelzaher 42 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.

Copyright ©: Nahrstedt, Angrave, Abdelzaher 43 strcat Syntax: strcat(ptr1, ptr2); where ptr1 and ptr2 are pointers to char Concatenates the 2 nd argument to the 1 st (both must be null terminated strings or havoc results!). char S[25] = "world!"; char D[25] = "Hello, "; strcat(D, S);

Copyright ©: Nahrstedt, Angrave, Abdelzaher 44 strcat Example What’s wrong with: char S[25] = "world!"; strcat(“Hello, ”, S);

Copyright ©: Nahrstedt, Angrave, Abdelzaher 45 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) according to the lexicographical order int diff; char s1[25] = "pat"; char s2[25] = "pet"; diff = strcmp(s1, s2);

Formatted I/O printf, fprintf: Write data to file printf( “Lose %d pounds in %d %s\n”, N, D, “weeks”); scanf, fscanf: Read data from file scanf( “%d %f %80s\n”, &N, &F, buf); // buf must be char* Copyright ©: Nahrstedt, Angrave, Abdelzaher 46

Copyright ©: Nahrstedt, Angrave, Abdelzaher 47 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--;

Copyright ©: Nahrstedt, Angrave, Abdelzaher 48 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 result x is 11, z is 1 First assign then increment/decrement x is 10, z is 2

Copyright ©: Nahrstedt, Angrave, Abdelzaher 49 Math: Increment and Decrement Operators on Pointers Example 1: int a[3]; 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?

Copyright ©: Nahrstedt, Angrave, Abdelzaher 50 Math: Increment and Decrement Operators on Pointers Example 1: int a[3]; 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? number1 =1, number2 =10. Hint: ++ increments pointer p not variable *p  check C operator precedence table!

Copyright ©: Nahrstedt, Angrave, Abdelzaher 51 Logic: Relational (Condition) Operators == equal to != not equal to > greater than < less than >= greater than or equal to <= less than or equal to

Copyright ©: Nahrstedt, Angrave, Abdelzaher 52 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.”);

Copyright ©: Nahrstedt, Angrave, Abdelzaher 53 Logic Example Question: what will happen if I replaced the above with: if (a = b) printf (“Equal.”); else printf (“Not Equal.”);  This is a perfectly LEGAL C statement (syntactically speaking) which copies the value in “b" into “a", and delivers this value, which will then be interpreted as TRUE if b is non-zero.