pointer-to-pointer (double pointer)

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Presentation transcript:

pointer-to-pointer (double pointer) "pass by pointer“ of function parameter means passing a pointer by value In most cases, no problem! Problem comes when modifying the pointer inside the function In this case, we need pointer-to-pointer. int g_One=1; void func(int* pInt); int main() { int nvar=2; int* pvar=&nvar; func(pvar); printf("%d",*pvar); // output??? return 0; } void func(int* pInt) pInt=&g_One;

Pointer-to-Pointer parameter void func(int** ppInt); int g_One=5; int main() { int nvar=2; int* pvar=&nvar; func(&pvar); // this is necessary to modify the pointer pvar in the function printf("%d",*pvar); // output??? return 0; } void func(int** ppInt) // ppInt takes the address of pointer variable //Modify the pointer, ppInt points to *ppInt=&g_One; //You can also allocate memory, depending on your requirements //*ppInt=(int*)malloc(sizeof(int)); //**ppInt=10; //Modify the variable, *ppInt points to //**ppInt=3;

example Is this OK??? #include <stdio.h> #include <stdlib.h> int main() { int* p, i; func(p); for (i=0;i<5;i++) scanf(“%d”,&p[i]); for (i=0;i<5;i++) printf(“p[%d]=%d\n”,i,p[i]); return 0; } void func(int* ptr) ptr=(int*)malloc(5*sizeof(int));