Key C Topics: Tutorial Pointers, Dynamic Memory allocation, Valgrind and Makefile CS370

Transcription

1 Key C Topics: Tutorial Pointers, Dynamic Memory allocation, Valgrind and Makefile CS370

2 Outline Pointers in C & and * operators Pointers with Arrays and Strings Dynamic memory allocation malloc() and free() Valgrind tool Makefile

3 What is a Pointer? int k; k =2; k is the name given to the memory location which stores the value 2. As the data type used is int, 4 bytes of memory is reserved for k. Internally the memory is identified by an address, which is what k refers to. This address of k can be accessed by using the & operator, i.e. &k int *p = &k; Here p is a pointer variable, hence the dereferencing (*) operator before it. The pointer variable p holds the address where the k holds the value.

4 What is a Pointer? Hence *p = 2 is equivalent to k = 2

5 Data Type of a Pointer *(p+1) =? C increments the pointer based on the type of variable it points to. In the above case as it points to int, p is incremented by 4, so we have (p+1)=0x100 and *(p+1)=7

6 p = &arr[0]; Pointers and Arrays The symbol arr itself is a pointer to the first element of the array. Hence, arr[i] can also be written as *(arr+i)

7 Pointers and Arrays int arr[3]= {1,2,3}; int *p = arr; p is pointing to arr[0](=1); Step1: printf( %d,*p++) Get value at p: 1 (output) Increment p: p is now pointing to arr[1] Step2: printf( %d,*++p) Increment p: p is now pointing to arr[2] Get value at p: 3 (output) Step3: printf( %d,++*p) Increment value at p and then print it arr[2]=3+1=4(output)

8 Pointers and Strings A string in C is simply an array of char values So the functioning of pointers with strings is same as that with the arrays. char stra[] = Ping ; char strb[] = Pong ; char *pa = stra; char *pb = strb; while (*pa) *pb++ = *pa++; *pb = '\0 ;;

9 Pointers and Strings Put the reverse of str1 into str2: char str1[] = "Pointers are fun. Yeah right!"; char str2[30], *p1, *p2; p1 = str1 + strlen(str1) - 1; p2 = str2; while(p1 >= str1) *p2++ = *p1--; *p2 = '\0';

10 Pointers and Multi-Dimensional Arrays arr[2] is the pointer to the third row So, we can access arr[2][1] as *(arr[2]+1) But arr[2] is again same as *(arr + 2) So, arr[2][1] is same as *(*(arr+2)+1) In general, arr[i][j] is same as *(*(arr+i)+j)

11 Dynamic Memory Allocation int arr[1000]; - sets aside 1000*sizeof(int) bytes of memory irrespective of whether you use it or not Instead, use malloc() to allocate memory at runtime depending on requirement And then when you are done using it, use free() to deallocate it malloc ed memory will not be automatically freed until process exits

12 malloc() int *p = (int *)malloc(sizeof(int)*n) allocates enough memory to hold N int values returns the starting address of the memory block we store the address in the pointer p malloc() returns NULL if memory could not be allocated We can use *p, *(p+1),..., *(p+n-1) to refer to the integers in the memory block But, *(p+i) is same as p[i] Effectively, we just dynamically allocated an array to hold N integers

13 free(p) - - free() deallocates the memory addressed by p It s good practice to set the pointer to NULL: p=nu

14 Dynamic Memory for 2D Arrays Recall that each row of a 2D array can be referenced by a pointer to a 1D array So for 2D arrays, we need a 1D array of pointers int **p; p = (int **) malloc(nrow * sizeof(int *)) We just allocated memory to hold Nrow pointers, accessed as *(p+i) (or p[i]) for (i=0; i<nrow; i++) p[i] = (int *)malloc(ncol * sizeof(int)) Each of those pointers now points to a block of memory of size (Ncol*sizeof(int))

15 Dynamic Memory for 2D Arrays To access the integer at row I and column j, use - *(*(p+i)+j) or p[i][j] Each *(p+i) need not point to a memory block of same size - therefore, each column of the array can be of different size Tofree the memory: for (i=0; i<nrow; i++) free(p[i]); free(**p); p=null; You may also free only certain columns: free(p[2]); p[2]=null;

16 malloc() example #include <stdio.h> #include <stdlib.h> int main() { int n, i, *ptr, sum=0; prinm("enter number of elements: "); scanf("%d",&n); ptr=(int*)malloc(n*sizeof(int)); if(ptr==null) { } prinm("error! memory not allocated."); exit(0); } prinm("enter elements of array: "); for(i=0;i<n;++i) { scanf("%d",ptr+i); sum+=*(ptr+i); } prinm("sum=%d",sum); free(ptr); return 0; // number of elements entered by user //memory allocated using malloc // // array values entered by user don t forgetfree()!

17 Valgrind Valgrind is a program that is useful for tracking memory errors (such as segmentation faults) and memory leaks. If working on your own machine you may have to install valgrind using: sudo apt install valgrind You may run Valgrind with any of your C programs with the following: valgrind --leak-check=yes./<executable name> If you want to see the line numbers where memory was allocated you must compile with the g flag. Ex. gcc o test g test.c

18 Valgrind Example Below is a simple C program to demonstrate valgrind. I ran the program twice with valgrind and included the valgrind output in later slides. // header files #include <stdio.h> #include <stdlib.h> // main functions int main() { // give memory leak example // allocate an array of 10 integers int *numptr = malloc(sizeof(int)*10); int temp = numptr[1]; // access one value in the array free(numptr); // free memory return 0; }

19 Valgrind Example Cont. Below are the results of running valgrind with the program when memory is properly freed and accessed. ==3447== Memcheck, a memory error detector ==3447== Copyright (C) , and GNU GPL'd, by Julian Seward et al. ==3447== Using Valgrind and LibVEX; rerun with -h for copyright info ==3447== Command:./valTest ==3447== ==3447== ==3447== HEAP SUMMARY: ==3447== in use at exit: 0 bytes in 0 blocks ==3447== total heap usage: 1 allocs, 1 frees, 40 bytes allocated ==3447== ==3447== All heap blocks were freed -- no leaks are possible ==3447== ==3447== For counts of detected and suppressed errors, rerun with: -v ==3447== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0) Notice there is no memory in use at exit Notice the number of allocs is equal to number of frees

20 Valgrind Example Cont. Below are the results of running valgrind when memory is not freed and is accessed out of bounds. I removed some unnecessary output from valgrind to save space. ==3518== Invalid read of size 4 ==3518== at 0x108664: main (valtest.c:11) ==3518== Address 0x521c1d0 is 288 bytes inside an unallocated block of size 4,194,096 in arena "client" ==3518== ==3518== ==3518== HEAP SUMMARY: ==3518== in use at exit: 40 bytes in 1 blocks ==3518== total heap usage: 1 allocs, 0 frees, 40 bytes allocated ==3518== ==3518== 40 bytes in 1 blocks are definitely lost in loss record 1 of 1 ==3518== at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==3518== by 0x10865B: main (valtest.c:10) Accessing an array out of bounds line 11 Notice there is still memory in use at exit and the number of allocs does not equal number of frees There was a memory leak of 40 bytes in one block. Allocated on line 10.

21 Valgrind Example Cont. Below is a continuation of the last slide. Valgrind provides a summary of memory leaks. ==3518== LEAK SUMMARY: ==3518== definitely lost: 40 bytes in 1 blocks ==3518== indirectly lost: 0 bytes in 0 blocks ==3518== possibly lost: 0 bytes in 0 blocks ==3518== still reachable: 0 bytes in 0 blocks ==3518== suppressed: 0 bytes in 0 blocks

22 Makefile basics A makefile is simply a way of associating short names, called targets, with a series of commands to execute when the action is requested - - Default target: make Alternate target: make clean

23 Makefile Basic macro: CC=gcc Convert a macro to its value in a target: $(CC) - Ex: $(CC) a_source_file.c gets expanded to gcc a_source_file.c Basic makefile: CC = gcc FILES = in_one.c in_two.c OUT_EXE = out_executable build: $(FILES) $(CC) -o $(OUT_EXE) $(FILES) To execute: make build

24 Make clean CC = gcc FILES = in_one.c in_two.c OUT_EXE = out_executable build: $(FILES) $(CC) -o $(OUT_EXE) $(FILES) clean: rm -f *.o $(OUT_EXE) The target make clean will remove all.o files and the executable

25 References A Tutorial on Pointers And Arrays in C: Essential C: Reading C Type Declarations: C Programming Dynamic Memory Allocation Makefiles