CS 3411 Systems Programming

Transcription

1 CS 3411 Systems Programming Department of Computer Science Michigan Technological University C vs. C++ (cont.)

2 Examples of Pointer Use: Strings in C There is no string data type in C. Instead, a string is assumed to be a sequence of char terminated by a zero byte. A char * is generally used as a string; just a pointer to the first char in the zero-terminated sequence of chars. Careful when declaring a string: char STR; / Only memory a l l o c a t e d i s to p o i n t e r v a r i a b l e / char s t r [ 2 0 ] ; / 20 bytes a l l o c a t e d to hold contents of s t r i n g

3 String Codes Example I Some examples of string functions: #include < s t d i o. h> i n t mystrlen ( s ) char s ; { char p ; p = s ; while ( p ) p++; return p s ; strcpyv1 ( t, s ) char s, t ; { while ( ( t = s )!= \0 ) { s ++; t ++;

4 String Codes Example II strcpyv2 ( t, s ) char s, t ; { while ( ( t ++ = s ++)!= \0 ) ; strcpyv3 ( t, s ) char s, t ; { while ( t ++ = s ++); main ( ) { char t e s t _ s t r [ ] = " Hello World! " ; char copy_to_str [ 2 0 ] ; p r i n t f ( " O r i g i n a l s t r i n g : %s \ n ", t e s t _ s t r ) ; p r i n t f ( " Length of s t r i n g : %d \ n ", mystrlen ( t e s t _ s t r ) ) ; strcpyv3 ( copy_to_str, t e s t _ s t r ) ; p r i n t f ( " Copied s t r i n g : %s \ n ", copy_to_str ) ; p r i n t f ( " Length of s t r i n g : %d \ n ", mystrlen ( copy_to_str ) ) ;

5 Manual Pages Usually the most accurate source of information for the system you re working on Accessed by the man command from the terminal, followed by section number, followed by the item you want information on Sections vary from system to system. You can see this by using the command man man. Commonly, the sections are: 1. User commands 2. System calls 3. Library routines 4. Devices 5. File formats 6. Games 7. Misc. 8. System Administration The info command is another option for GNU Software

6 C Standard I/O Different from C++ Manual pages available for specific functions: man 3 stdio (An overview) man 3 printf (Formatted output) man 3 scanf (Formatted input) man 3 getc (Character-based input macros) man 3 putc (Character-based output macros) Default I/O Streams: stdin, stdout, stderr Anything you open with the fopen function is also a stream. All streams are of the (FILE *) data type.

7 Output in C++ C++ iostream methods «and» automatically format. # include <iostream > using namespace std ; main ( ) { f l o a t x ; i n t y ; char s t r ; x = 3. 1 ; y = 20; s t r = " Characters " ; cout << x << " " << y << " " << s t r << " \ n " ;

8 Output in C stdio requires a string which defines a format to be used #include < s t d i o. h> main ( ) { f l o a t x ; i n t y ; char s t r ; x = 3. 1 ; y = 20; s t r = " Characters " ; p r i n t f ( " %.2 f %d %s \ n ", x, y, s t r ) ;

9 Input in C++ C++ iostream style input: # include <iostream > using namespace std ; main ( ) { double sum = 0; i n t val, num = 0; while ( c i n >> v a l ) { num++; sum += ( double ) v a l ; cout << "Mean i s " << sum / ( double )num << " \ n " ;

10 Input in C In C, we need to pass a pointer argument to scanf to get back values #include < s t d i o. h> main ( ) { double sum = 0; i n t val, num = 0; while ( scanf ( "%d ", &v a l ) == 1) { num++; sum += ( double ) v a l ; p r i n t f ( "Mean i s %f \ n ", sum / ( double )num ) ;

11 Memory Allocation in C No new/delete in C! Memory allocation is done through malloc Freeing memory is done through free man 3 malloc for more details!

12 Malloc Example I / b i n t r e e. c / # include <malloc. h> #define NILNODE ( struct node )0 struct node { char data ; struct node l e f t, r i g h t ; ; main ( ) { struct node gimme ( ), n1, n2, n3, n4, n5, n6, n7 ; void i n o r d e r ( ) ; n1 = gimme ( a, NILNODE, NILNODE ) ; n2 = gimme ( b, NILNODE, NILNODE ) ; n3 = gimme ( c, n1, n2 ) ; n4 = gimme ( d, NILNODE, NILNODE ) ; n5 = gimme ( e, n3, n4 ) ; n6 = gimme ( f, NILNODE, NILNODE ) ; n7 = gimme ( g, n5, n6 ) ; i n o r d e r ( n7 ) ; p r i n t f ( " \ n " ) ;

13 Malloc Example II struct node gimme ( val, l, r ) char v a l ; struct node l, r ; { struct node tmp ; tmp = ( struct node ) malloc ( sizeof ( struct node ) ) ; tmp >data = v a l ; tmp > l e f t = l ; tmp >r i g h t = r ; return ( tmp ) ; void i n o r d e r ( r ) struct node r ; { i f ( r!= NILNODE) { i n o r d e r ( r > l e f t ) ; p r i n t f ( "%c ", r >data ) ; i n o r d e r ( r >r i g h t ) ;

14 A Brief Look at Program Execution Text is executable code (also some strings! Usually write-protected) Data is global data (both initialized and uninitialized) Heap is area from which dynamic allocations are made (malloc!) Stack is where function activation records pushed/popped. Pushed (created) on stack when function invoked, removed on return May contain: function parameters, function locals, return address, temporaries, saved state, control link, access link Usual to preallocate a block of storage for initial heap/stack

15 Problems to Avoid It is always important to keep system programs as bug-free as possible Errant programs running in privileged mode can: Access/modify system configuration files Erase user data Halt the system And so on!

16 Buffer Overflow Writing beyond allocated array bounds i n t getuserdata ( ) { char copy [ 6 0 ] ;... / User can i n p u t s t r i n g of ANY l e n g t h / gets ( buf ) ;... / Copies u n t i l s t r i n g t e r m i n a t i o n i n buf / s t r c p y ( copy, buf ) ; main ( ) {... char i n p u t [ 5 0 ] ; char s t r P t r ;... getuserdata ( ) ; / No s t r i n g memory a l l o c a t i o n f o r s t r P t r / s t r c p y ( s t r P t r, i n p u t ) ;

17 Memory Leak Losing access to allocated memory segment - We can t reclaim it! i n t func ( ) { void p t r ; / When f u n c t i o n returns, value of p t r i n a c c e s s i b l e / p t r = malloc ( ) ; main ( ) { char b p t r ; for ( i =1; i <10; i ++) { / Previous p t r value o v e r w r i t t e n each i t e r a t i o n / b p t r = malloc ( sizeof ( char ) ) ; b p t r = i ;

18 Dereference Invalid Pointer... i n t func ( node n ) { i f ( n >value == 0) f r e e ( n ) ; return ( 0 ) ; main ( ) { node p, q ; p = malloc ( sizeof ( node ) ) ; p >value = 10; p r i n t f ( "Node p value <%d> ",p >value ) ; func ( p ) ; / p has already been freed / p r i n t f ( " A f t e r func p value <%d >\ n ", p >value ) ; / q was never i n i t i a l i z e d / p r i n t f ( "Node q value <%d >\ n ",q >value ) ;