Exercise Session 3 Systems Programming and Computer Architecture

Transcription

1 Systems Group Department of Computer Science ETH Zürich Exercise Session 3 Systems Programming and Computer Architecture Herbstsemester 2016

2 Agenda Review of Exercise 2 More on C-Programming Outlook to Exercise 3 Quiz

3 Checking your Arguments Make sure that you check your arguments before using them. Especially pointers: void (*f)(int); f = null; callfptr(f, 123); int *data = null; reverse(data, -3); Is your code still valid with these invocation? Systems Programming and Computer Architecture 3

4 Initializing Memory It may not always be the case that your allocated memory is zeroed out: #include <string.h> int *data = malloc(10 * (sizeof(int))); if (data == null) { printf( No memory ); } // ensure memory is zeroed out memset(data, 0, 10 * sizeof(int))); Systems Programming and Computer Architecture 4

5 Choose Your Types Wisely When you define your interface to your function, always have in mind which values you want to accept void reverse_array(int array[], int length); void reverse_array(int array[], size_t length); Remember: The bit representation stays the same, but the interpretation of 0xFFFFFFFF may be different.. Systems Programming and Computer Architecture 5

6 Solution of Exercise 2

7 1. Reverse an array Write a C program that has a function that: accepts an array of 32-bit unsigned integers and a length reverses the elements of the array in place returns void (nothing)

8 Solution void ReverseArray(uint32_t *arr, unsigned int len) { int i; // check the obvious corner case of no reversal needed if (len <= 1) return; // make sure the caller passed us a valid array assert(arr!= NULL); // do the reversal // think through the termination condition carefully for (i=0; i<(len/2); i++) { uint32_t tmp; } } tmp = arr[i]; arr[i] = arr[len-i-1]; arr[len-i-1] = tmp;

9 4. Little vs. big endian Write a C program that prints out whether the computer it is running on is little endian or big endian. (hint: pointer arithmetic from the lecture)

10 Solution int main(int argc, char **argv) { int x = 1; char *charptr = (char *) &x; if (*charptr == 1) { printf("little endian\n"); } else { printf("big endian\n"); } } return 0;

11 C-Programming

12 Cleanup code with Goto struct person * alloc_person(){ struct person * ret = malloc(sizeof(struct person)); ret->first_name = malloc(100 * sizeof(char)); ret->last_name = malloc(100 * sizeof(char)); return ret; } malloc() can fail, returns NULL. Correct code must not leak memory.

13 Cleanup code with Goto Perform operation 1 If failure, goto exit 1 Perform operation 2 If failure, goto exit 2 Perform operation 3 If failure, goto exit 3 Return success Exit 3: Exit 2: Exit 1: Undo operation 3 Undo operation 2 Return an error Not easy to represent scalably in structured programming Best effort: nested if statements But can get arbitrarily deep in a complex function goto version is highly stylized (undos can be paired with operations in reverse order) Can also be modified to handle cases where one undo deals with several operations

14 Cleanup code with Goto struct person * alloc_person(){ struct person * ret = malloc(sizeof(struct person)); if(!ret) goto err_3; ret->first_name = malloc(100 * sizeof(char)); if(!ret->first_name) goto err_2; ret->last_name = malloc(100 * sizeof(char)); if(!ret->last_name) goto err_1; return ret; err_1: err_2: err_3: } free(ret->first_name); free(ret); return NULL;

15 File I/O File Descriptors, Read, Write Systems Programming and Computer Architecture 15

16 Files in Linux Pretty much everything is a file: Files on hard drive Network Sockets Pipes Std IO (stdin, stderr and stdout) Systems Programming and Computer Architecture 16

17 Accessing Files In general you cannot access the file directly You need support from the operating system to open/read/write/close a file. This is called a system call (syscall) -> Lecture Operating System & Network All file related stuff is in the stdio.h Systems Programming and Computer Architecture #include <stdio.h> 17

18 The File Descriptor To access a file you need some reference to it FILE *fp; Current offset in the file (cursor) File Descriptor Pointer to the file File FOO File BAR Systems Programming and Computer Architecture 18

19 The File Descriptor To get a file descriptor you have to open a file FILE *fopen(const char *filename, const char *mode); The opening may fail. Check return value! Close the file in the end int fclose(file *fp); Systems Programming and Computer Architecture 19

20 (Text) File Opening Modes Mode Read Write File Not Exists File Exists r Yes, from beginning w No Yes, from the beginning a No Yes, from the end (append) r+ Yes, from beginning w+ Yes, from beginning a+ Yes, from beginning Systems Programming and Computer Architecture No Error FILE* descriptor returned Yes, from beginning Yes, From beginning Yes, from the end (append only) New file created, FILE* Descriptor returned New file crated, FILE* descriptor returned Error.. b Binary flag that can be added for binary IO New file created, FILE* descriptor returned New file crated, FILE* descriptor returned FILE* descriptor returned FILE* descriptor returned FILE* descriptor returned. Delete file contents (overwrite), File* descriptor returned FILE* descriptor returned 20

21 Reading a Text File Reading a single char int fgetc(file *fp); Returns EOF on file end or error Reading one line in the text file of max count-1 length char *fgets(char *str, int count, FILE *fp); New line \n will terminate reading, \0 appended Direct file access size_t fread(void *buf, size_t size, size_t count, FILE *fp); Read (size * count) bytes from the file Returns the number of bytes read Systems Programming and Computer Architecture 21

22 Reading a Text File (Example) FILE *fp; int c; int n = 0; fp=fopen("myfile.txt","r"); if (fp ==NULL) { printf("error opening file"); } else { do { c = fgetc (fp); if (c == A') { n++; } } while (c!= EOF); fclose (fp); } printf( %i, n); What s this program doing? Can you do the same program using fread() or fgets()? What are the potential benefits / drawbacks? Systems Programming and Computer Architecture 22

23 Writing a Text File Writing a single char int fputc(int character, FILE *fp); Returns EOF on error, or the character written if success Reading one line in the text file of max count-1 length int *puts(char *str, FILE *fp); Copy contents of *str until \0 is reached. Returns non-negative vaue on success Direct file access size_t write(void *ptr, size_t size, size_t count, FILE *fp); Write (size * count) bytes from the file Returns the number of bytes written Systems Programming and Computer Architecture 23

24 Writing a Text File (Example) FILE *fp; char name]; printf( Enter your name: "); fgets (name, 256, stdin); What s this program doing? fp=fopen("myfile.txt", a"); if (fp ==NULL) { printf("error opening file"); } else { if ( fputs (name, fp) < 0) { printf("error writing file"); } fclose (fp); } Can you do the same program using fwrite() or fputc()? What are the potential benefits / drawbacks? Systems Programming and Computer Architecture 24

25 Always keep in mind All those function will change the state of the file descriptor by advancing the cursor position Sometimes not all data requested is read/written Keep track of how many bytes are processed Loop until finished Systems Programming and Computer Architecture 25

26 Navigating the Text File Getting the position of the cursor in the file Using an offset from the beginning long int ftell(file *fp); Reposition the file cursor Seeking: (offset = 0 or the return value of ftell) int fseek(file *fp, long int offset, int origin); Reset to the beginning void rewind(file *fp); Systems Programming and Computer Architecture 26

27 Formatting int fscanf(file *stream, const char *format,...); Dual of printf int a; char b[10]; fscanf(stdin, %d %s, &a, &b); Also sscanf may be useful Format string: %s Read string %d Read integer Systems Programming and Computer Architecture 27

28 Managing Files Create file FILE *fopen(const char *filename, const char *mode); Create directory Mode specifies the permission bits (R/W/X) int mkdir(const char *filename, const char *mode); Rename file / directory Returns 0 if successfully renamed int rename(const char *oldname, const char *newname); Deleting Returns 0 if successfully renamed int remove(const char *filename); Systems Programming and Computer Architecture 28

29 File IO: Resources Reference and Examples Read the man pages! man 3 getc man 3 isspace man 3 scanf Systems Programming and Computer Architecture 29

30 Heap The heap Large pool of unused memory, used for dynamically allocating data structures malloc() allocates chunks of memory in the heap, free() returns them malloc() maintains bookkeeping data in the heap to track allocated blocks. Address Space Kernel virtual memory User stack shared libraries Run-time heap Read/write segment (.data,.bss) Read-only segment (.init,.text,.rodata) Unused

31 Memory API // declared in stdlib.h typedef unsigned int size_t; void *malloc(size_t sz); // declared in stdlib.h void *calloc(size_t nm, size_t sz); // declared in stdlib.h void free(void *); // declared in stdlib.h void *realloc(void *ptr, size_t size);

32 structs struct typename { type name; type name; type name; }; struct list_el { unsigned long val; struct list_el *next; }; struct list_el my_list; Use "." to refer to fields in a struct p->x is nicer than (*p).x Use "->" to refer to fields through a pointer to a struct structs are passed by value

33 Struct tags and typedefs You can: typedef struct list_el el_t; Or even: Or even: typedef struct list_el { unsigned long val; struct list_el *next; } el_t; struct list_el my_list; el_t my_other_list; typedef struct list_el { unsigned long val; struct list_el *next; } list_el; struct list_el my_list; list_el my_other_list; Confusing. Better to stay with always using tags. Some projects (e.g. Linux kernel) prohibit typedefs of structs in their style guide Dynamic Memory Allocation AS

34 Outlook to Exercise 3

35 1. Implement a Complex number module consisting of the following source files: complex.c, complex.h includes a typedef to define a complex number a + bi, where a and b are doubles includes functions to: add, subtract, multiply, and divide complex numbers implement a test driver in test_complex.c, which contains main( ).

36 Recap: Complex Numbers Addition / Subtraction: (a + bi) ± (c + di) = (a ± c) + (b ± d)i Multiplication: (a + bi) (c + di) = (ac bd) + (ad + bc)i Division: a + bi c + di = (ac + bd) + (bc ad)i c 2 + d 2

37 Complex numbers create a struct complex_set (a set of complex numbers) implement alloc_set( ) and free_set( ): alloc_set( ) needs to use malloc twice: once to allocate a new complex_set, and once to allocate the points field inside it free_set( ) needs to use free twice

38 3. word-count (wc) wc = word count. Linux utility to count lines, words and characters. Not related to toilet. you can use template wc.c and only fill in the missing part Exercise is about reading c-code and understand it (because you do not have to implement much Use the function getc(fp) to retrieve the next character in a file

39 4. File I/O Read a text-file, parse it and write a simple report better do this on Linux (in Linux new lines are \n while on windows they are \r\n) you only have to have a look at lines where the first column starts with «25»

40 5. Function pointer In this problem, we will use and create function that utilizes function pointers. The file callback.c contains an array of records consisting of a fictitious class of celebrities. Each record consists of the first name, last name and age of the student. fill in the gaps you will write functions, use function pointers and do some other cool stuff nothing new, but quite a bit of work

41 Quiz Questions on handout.

42 Quiz 1: Pointer and Strings char *c[] = { ENTER, NEW, POINT, FIRST }; char **cp[] = { c+3, c+2, c+1, c }; char ***cpp = cp; main() { printf( %s, **++cpp); printf( %s, *--*++cpp+3); printf( %s, *cpp[-2]+3); printf( %s\n, cpp[-1][-1]+1); } Operator Precedence: (),[],., ->, expr++, expr-- (left-to-right) then *, &, ++expr, --expr (right-to-left) then + - * / & >> << ==!=

43 printf( %s, *++cpp); ++cpp *(++cpp) *(*(++cpp)) => POINT cpp cp cp E N P F N E O I T W I R E 0 N S R T T 0 0 0

44 printf( %s, *--*++cpp+3); *(++cpp) --(*(++cpp)) *(--(*(++cpp)))+3 => ENTER cpp cp cp E N P F N E O I T W I R E 0 N S R T T 0 0 0

45 printf( %s, *cpp[-2]+3); cpp[-2] *(cpp[-2]) (*(cpp[-2]))+3 => FIRST cpp cp cp E N P F N E O I T W I R E 0 N S R T T 0 0 0

46 printf( %s\n, cpp[-1][-1]+1); (cpp[-1]) (cpp[-1])[-1] ((cpp[-1])[-1])+1 => NEW cpp cp cp E N P F N E O I T W I R E 0 N S R T T 0 0 0

47 Quiz 1: Pointer and Strings printf( %s, **++cpp); (*(*(++cpp))) => POINT printf( %s, *--*++cpp+3); *(--(*(++cpp)))+3 => ENTER printf( %s, *cpp[-2]+3); (*(cpp[-2]))+3 => FIRST printf( %s\n, cpp[-1][-1]+1); ((cpp[-1])[-1])+1 => NEW POINTER STEW

48 Quiz 2: Memory API main() { int* ip = malloc(sizeof(int)); printf( %d\n, *ip); int* ip2 = calloc(1, sizeof(int)); printf( %d\n, *ip2); } free(ip); free(ip2);

49 Quiz 2: Memory API int* ip = malloc(sizeof(int)); printf( %d\n, *ip); Memory region returned by malloc not guaranteed to be zeroed (can print any value). If not enough memory will return NULL, possible NULL pointer dereference! int* ip2 = calloc(1, sizeof(int)); printf( %d\n, *ip2); Prints 0, calloc memory is guaranteed to be zeroed If not enough memory will return NULL, possible NULL pointer dereference! free(ip); free(ip2); If ip or ip2 is NULL no operation is performed (no check required for free).

50 Quiz 3: Structures #define PRINT(fmt, val) printf(#val = % #fmt \t, (val)) #define PRINT2(fmt, v1, v2) PRINT(fmt, v1); PRINT(fmt, v2); static struct S1 { char c[3], *s; } s1 = { abc, def }; static struct S2 { char *cp; struct S1 ss1; } s2 = { ghi, { jkl, mno } }; PRINT2(c, s1.c[0], *s1.s); PRINT2(s, s2.cp, s2.ss1.s); PRINT2(s, ++s2.cp, ++s2.ssl.s); (),[],., ->, expr++, expr-- (left-to-right) then *, &, ++expr, --expr (right-to-left) then + - * / & >> << ==!=

51 Quiz 3: Structures PRINT2(c, s1.c[0], *s1.s); s1.c[0] = a *s1.s = d PRINT2(s, s2.cp, s2.ss1.s); s2.cp = ghi s2.ssl.s = mno PRINT2(s, ++s2.cp, ++s2.ssl.s); ++s2.cp = hi ++s2.ssl.s = no

52 Obligatory XKCD