14. Memory API. Operating System: Three Easy Pieces

Transcription

1 14. Memory API Oerating System: Three Easy Pieces 1

2 Memory API: malloc() #include <stdlib.h> void* malloc(size_t size) Allocate a memory region on the hea. w Argument size_t size : size of the memory block(in bytes) size_t is an unsigned integer tye (defined in C standard). w Return Success : a void tye ointer to the memory block by malloc Fail : a null ointer 2

3 sizeof() Routines and macros are utilized for size in malloc instead tying in a number directly. Two tyes of results of sizeof with variables w The actual size of x is known at run-time. int *x = malloc(10 * sizeof(int)); rintf( %d\n, sizeof(x)); 4 w The actual size of x is known at comile-time. int x[10]; rintf( %d\n, sizeof(x)); 40 3

4 Memory API: free() #include <stdlib.h> void free(void* tr) Free a memory region by a call to malloc. w Argument void *tr : a ointer to a memory block with malloc w Return none 4

5 Memory Allocating ointer hea int *i; // local variable 16KB *i i = (int *)malloc(sizeof(int)* 4); 16KB *i 5

6 Memory Freeing freed freed freed freed free(i); 16KB (invalid) *i hea 16KB (invalid) *i 6

7 Forgetting To Allocate Memory Incorrect code char *src = hello ; //character string constant char *dst; //un strcy(dst, src); //segfault and die Hello\0 strcy(dst, src); hea un *dst *src 7

8 Forgetting To Allocate Memory(Cont.) Correct code char *src = hello ; //character string constant char *dst (char *)malloc(strlen(src) + 1 ); // strcy(dst, src); //work roerly hello\0 hello\0 hello\0 strcy(dst, src); hea hea *dst *src *dst *src 8

9 Not Allocating Enough Memory Incorrect code, but work roerly char *src = hello ; //character string constant char *dst (char *)malloc(strlen(src)); // too small strcy(dst, src); //work roerly \0 is omitted 6 bytes strlen 5 bytes h el l o \0 hello\0 strcy(dst, src); hea *dst *src

10 Forgetting to Initialize Encounter an uninitialized read int *x = (int *)malloc(sizeof(int)); // rintf( *x = %d\n, *x); // uninitialized memory access value used before with value used before hea hea *x *x 10

11 Memory Leak A rogram runs out of memory and eventually dies. unused : unused, but not freed unused unused unused hea hea unused hea *d *c *b *b *a *a *a run out of memory 11

12 Dangling Pointer Freeing memory before it is finished using w A rogram accesses to memory with an invalid ointer *b *b free() unreachable *a *a dangling ointer 3KB 3KB 3KB 4KB 4KB NULL Hea free(b) 3KB 4KB freed NULL Hea *b *a Stack 3KB *b *a Stack 3KB 12

13 Double Free Free memory that was freed already. int *x = (int *)malloc(sizeof(int)); // free(x); // free memory free(x); // free reeatedly freed Hea free(x) Hea free(x) Undefined Error Stack Stack 16KB *x 16KB (invalid) *x 13

14 Other Memory APIs: calloc() #include <stdlib.h> void *calloc(size_t num, size_t size) Allocate memory on the hea and zeroes it before returning. w Argument size_t num : number of blocks to allocate size_t size : size of each block(in bytes) w Return Success : a void tye ointer to the memory block by calloc Fail : a null ointer 14

15 Other Memory APIs: realloc() #include <stdlib.h> void *realloc(void *tr, size_t size) Change the size of memory block. w A ointer returned by realloc may be either the same as tr or a new. w Argument void *tr: Pointer to memory block with malloc, calloc or realloc size_t size: New size for the memory block(in bytes) w Return Success: Void tye ointer to the memory block Fail : Null ointer 15

16 System Calls #include <unistd.h> int brk(void *addr) void *sbrk(inttr_t increment); malloc library call use brk system call. w brk is called to exand the rogram s break. break: The location of the end of the hea in address sace w sbrk is an additional call similar with brk. w Programmers should never directly call either brk or sbrk. 16

17 System Calls(Cont.) #include <sys/mman.h> void *mma(void *tr, size_t length, int ort, int flags, int fd, off_t offset) w mma system call can create an anonymous memory region Handled like hea w Mas files/devices into memory (memory maed I/O) E.g. use: shared access to files across rocesses/threads E.g. use: oen really small files 17

18 Disclaimer: Disclaimer: This lecture slide set is used in AOS course at University of Cantabria. Was initially develoed for Oerating System course in Comuter Science Det. at Hanyang University. This lecture slide set is for OSTEP book written by Remzi and Andrea Araci- Dusseau (at University of Wisconsin) 18