Skip to content

Instantly share code, notes, and snippets.

@thatseeyou

thatseeyou/0-pointer.c

Last active April 7, 2017 07:41
Show Gist options
  • Star 0 You must be signed in to star a gist
  • Fork 0 You must be signed in to fork a gist
  • Save thatseeyou/4187f79b0df1717f1ff3368c6238820a to your computer and use it in GitHub Desktop.
Save thatseeyou/4187f79b0df1717f1ff3368c6238820a to your computer and use it in GitHub Desktop.
Download ZIP
Various C pointer examples
Raw
0-pointer.c
/*
* C pointer examples
*
* 2017-07-17 (sangyoung@gmail.com)
* - initial write
*
* Reference: http://unixwiz.net/techtips/reading-cdecl.html
*/
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[], char *envp[])
{
// Pointer to int
{
{
int a = 1, b = 2;
int *pA = &a, *pB = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
}
{
int (a) = 1, (b) = 2;
int (*pA) = &a, (*pB) = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
*pA = 10;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
pA = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
}
}
// Pointer to const int
{
{
const int a = 1, b = 2;
const int *pA = &a, *pB = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
// ERROR
// a = 10;
// ERROR
// *pA = 10;
pA = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
}
{
const int (a) = 1, (b) = 2;
const int (*pA) = &a, (*pB) = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
// ERROR
//a = 10;
// ERROR
//*pA = 10;
pA = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
}
{
// int const === const int
int const a = 1, b = 2;
int const *pA = &a, *pB = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
// ERROR
// a = 10;
// ERROR
// *pA = 10;
pA = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
}
}
// Pointer to const int ( -> non-const int)
{
{
int a = 1, b = 2;
const int *pA = &a, *pB = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
a = 10;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
// ERROR
//*pA = 10;
pA = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
}
}
// const pointer to int
{
{
int a = 1, b = 2;
int * const pA = &a, * const pB = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
a = 10;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
*pA = 10;
*pB = 20;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
// ERROR
//pA = &b;
//pB = &a;
}
{
int a = 1, b = 2;
int (* const pA) = &a, (* const pB) = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
a = 10;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
*pA = 10;
*pB = 20;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
// ERROR
//pA = &b;
//pB = &a;
}
}
// const pointer to const int
{
{
const int a = 1, b = 2;
const int * const pA = &a, * const pB = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
// ERROR
// a = 10;
// ERROR
//*pA = 10;
// ERROR
// pA = &b;
}
{
const int a = 1, b = 2;
const int (* const pA) = &a, (* const pB) = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
// ERROR
// a = 10;
// ERROR
//*pA = 10;
// ERROR
// pA = &b;
// pB = &a;
}
{
// int const === const int
int const a = 1, b = 2;
int const * const pA = &a, * const pB = &b;
printf("%3d: %d, %d\n", __LINE__, *pA, *pB);
// ERROR
// a = 10;
// ERROR
// *pA = 10;
// ERROR
// pA = &b;
}
}
// array of pointer to int
{
{
int a = 1, b = 2;
int *pA[] = {&a, &b}, *pB[] = {&b, &a};
}
{
int a = 1, b = 2;
int *(pA[]) = {&a, &b}, *(pB[]) = {&b, &a};
}
{
int a = 1, b = 2;
int (*(pA[])) = {&a, &b}, (*(pB[])) = {&b, &a};
}
}
// pointer to array of int
{
{
int a[3] = {1, 2, 3}, b[2] = {4, 5};
int (*pA)[3] = &a, (*pB)[2] = &b;
printf("%3d: %d, %d, %d\n", __LINE__, a[0], a[1], a[2]);
a[0] = 10;
(*pA)[1] = 20;
*pA[2] = 30; // UNEXPECTED BEHAVIOR, equivalent to *(pA[2]) = 30;
printf("%3d: %d, %d, %d\n", __LINE__, a[0], a[1], a[2]);
}
{
// the number of array is inferenced
int a[3] = {1, 2, 3}, b[] = {4, 5};
int (*pA)[] = &a, (*pB)[2] = &b;
// ERROR
// int (*pC)[4] = &a;
}
// cf: pointer to first element of array
{
int a[] = {1, 2, 3}, b[2] = {4, 5};
int *pA = a, (*pB) = b;
int *pA2 = &a[0], (*pB2) = &b[0];
}
}
// pointer to array of const int
{
{
const int a[] = {1, 2, 3}, b[] = {4, 5, 6};
const int (*pA)[] = &a, (*pB)[] = &b;
printf("%3d: %d, %d, %d\n", __LINE__, a[0], a[1], a[2]);
// ERROR
// a[0] = 10;
// (*pA)[1] = 20;
pA = &b;
printf("%3d: %d, %d, %d\n", __LINE__, (*pA)[0], (*pA)[1], (*pA)[2]);
}
}
// const pointer to array of const int
{
{
const int a[] = {1, 2, 3}, b[] = {4, 5, 6};
const int (* const pA)[] = &a, (* const pB)[] = &b;
printf("%3d: %d, %d, %d\n", __LINE__, a[0], a[1], a[2]);
// ERROR
// a[0] = 10;
// (*pA)[1] = 20;
// ERROR
// pA = &b;
}
}
{
{
char a[] = "abc";
char (*pA)[] = &a;
printf("%3d: %s(%c-%c-%c)\n", __LINE__, a, a[0], a[1], a[2]);
a[0] = 'A';
printf("%3d: %s(%c-%c-%c)\n", __LINE__, a, a[0], a[1], a[2]);
// ERROR: overflow
// a[4] = 'D';
(*pA)[1] = 'B';
printf("%3d: %s(%c-%c-%c)\n", __LINE__, (*pA), (*pA)[0], (*pA)[1], (*pA)[2]);
// ERROR: overflow
//(*pA)[4] = 'X';
}
{
char a[4] = "abc";
char (*pA)[4] = &a;
}
{
char *a = "abc";
char *pA = a;
printf("%3d: %s(%c-%c-%c)\n", __LINE__, a, a[0], a[1], a[2]);
// ERROR: Bus error
// a[0] = 'A';
}
{
const char a[] = "abc";
const char (*pA)[] = &a;
}
}
// pointer to array of array(= two dimensional array) of int
{
{
int a[][2] = { {1,2}, {3,4} }, b[][2] = { {5,6}, {7,8} };
int (*pA)[][2] = &a, (*pB)[][2] = &b;
printf("%3d: (%d,%d), (%d,%d)\n", __LINE__, a[0][0], a[0][1], a[1][0], a[1][1]);
// ( ) is necessary
(*pA)[0][0] = 10;
printf("%3d: (%d,%d), (%d,%d)\n", __LINE__, a[0][0], a[0][1], a[1][0], a[1][1]);
pA = &b;
printf("%3d: (%d,%d), (%d,%d)\n", __LINE__, (*pA)[0][0], (*pA)[0][1], (*pA)[1][0], (*pA)[1][1]);
}
{
int a[][2] = { {1,2}, {3,4} }, b[][2] = { {5,6}, {7,8} };
int (*pA)[2] = a, (*pB)[2] = b;
printf("%3d: (%d,%d), (%d,%d)\n", __LINE__, a[0][0], a[0][1], a[1][0], a[1][1]);
pA[0][0] = 10;
printf("%3d: (%d,%d), (%d,%d)\n", __LINE__, a[0][0], a[0][1], a[1][0], a[1][1]);
pA = b;
printf("%3d: (%d,%d), (%d,%d)\n", __LINE__, pA[0][0], pA[0][1], pA[1][0], pA[1][1]);
}
}
// array of pointer to array of int
{
{
int a1[] = {1,2}, a2[] = {3,4}, a3[] = {5,6};
int (*(pA[]))[2] = { &a1, &a2, &a3 };
// a1 == &a1
printf("%3d: a1 = %p, &a1 = %p\n", __LINE__, a1, &a1);
printf("%3d: (%d,%d), (%d,%d), (%d,%d)\n", __LINE__, a1[0], a1[1], a2[0], a2[1], a3[0], a3[1]);
a1[0] = 10;
(*(pA[0]))[1] = 20;
(*pA[1])[0] = 30;
printf("%3d: (%d,%d), (%d,%d), (%d,%d)\n", __LINE__, (*pA[0])[0], (*pA[0])[1], (*pA[1])[0], (*pA[1])[1], (*pA[2])[0], (*pA[2])[1]);
// ERROR
//(*pA[0])[2] = 100;
//
// NO ERROR but Segmentation fault
//(*pA[3])[0] = 1000;
}
// array of pointer to first element of array of int
{
int a1[] = {1,2}, a2[] = {3,4}, a3[] = {5,6};
int *(pA[]) = { a1, a2, a3 };
// a1 == &a1
printf("%3d: a1 = %p, &a1 = %p\n", __LINE__, a1, &a1);
printf("%3d: (%d,%d), (%d,%d), (%d,%d)\n", __LINE__, a1[0], a1[1], a2[0], a2[1], a3[0], a3[1]);
a1[0] = 10;
(pA[0])[1] = 20;
pA[1][0] = 30;
printf("%3d: (%d,%d), (%d,%d), (%d,%d)\n", __LINE__, pA[0][0], pA[0][1], pA[1][0], pA[1][1], pA[2][0], pA[2][1]);
// NO CHECK FOR INDEX OVERFLOW
pA[0][2] = 100;
// NO ERROR but segmentation fault
// pA[3][0] = 1000;
printf("%3d: (%d,%d), (%d,%d), (%d,%d)\n", __LINE__, pA[0][0], pA[0][1], pA[1][0], pA[1][1], pA[2][0], pA[2][1]);
}
}
// array of pointer to array of int which is malloced
{
{
const int numCol = 2;
const int numRow = 3;
int (*a1)[numCol] = (int (*)[numCol])malloc(numCol * sizeof(int));
int (*a2)[numCol] = (int (*)[numCol])malloc(numCol * sizeof(int));
int (*a3)[numCol] = (int (*)[numCol])malloc(numCol * sizeof(int));
int (*((*pA)[]))[numCol] = (int (*((*)[]))[numCol])malloc(numRow * sizeof(int *));
(*a1)[0] = 1; (*a1)[1] = 2; (*a2)[0] = 3; (*a2)[1] = 4; (*a3)[0] = 5; (*a3)[1] = 6;
printf("%3d: (%d,%d), (%d,%d), (%d,%d)\n", __LINE__, (*a1)[0], (*a1)[1], (*a2)[0], (*a2)[1], (*a3)[0], (*a3)[1]);
(*pA)[0] = a1; (*pA)[1] = a2; (*pA)[2] = a3;
(*((*pA)[0]))[0] = 10;
(*((*pA)[0]))[1] = 20;
printf("%3d: (%d,%d), (%d,%d), (%d,%d)\n", __LINE__, (*a1)[0], (*a1)[1], (*a2)[0], (*a2)[1], (*a3)[0], (*a3)[1]);
// ERROR CHECKED
// (*a1)[2] = 3;
}
{
const int numCol = 2;
const int numRow = 3;
int *a1 = (int *)malloc(numCol * sizeof(int));
int *a2 = (int *)malloc(numCol * sizeof(int));
int *a3 = (int *)malloc(numCol * sizeof(int));
int *(*pA) = (int **)malloc(numRow * sizeof(int *));
a1[0] = 1; a1[1] = 2; a2[0] = 3; a2[1] = 4; a3[0] = 5; a3[1] = 6;
printf("%3d: (%d,%d), (%d,%d), (%d,%d)\n", __LINE__, a1[0], a1[1], a2[0], a2[1], a3[0], a3[1]);
pA[0] = a1; pA[1] = a2; pA[2] = a3;
pA[0][0] = 10;
pA[0][1] = 20;
printf("%3d: (%d,%d), (%d,%d), (%d,%d)\n", __LINE__, a1[0], a1[1], a2[0], a2[1], a3[0], a3[1]);
// NOT CHECKED
// a1[2] = 3;
}
}
}
Raw
1-functionpointer.c
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[], char *envp[])
{
// array of pointers to function returning int
{
int plus(int arg1, int arg2) {
return arg1 + arg2;
}
int minus(int arg1, int arg2) {
return arg1 - arg2;
}
int x2(int arg1) {
return arg1 * 2;
}
// function input type is ignored
{
int (*fp[])() = {plus, minus, x2};
int result[] = {fp[0](1, 2), fp[1](2, 1), fp[2](3)};
printf("%3d: %d, %d, %d\n", __LINE__, result[0], result[1], result[2]);
// MISSING ARGUMENT -> CHECKED
// result[0] = plus(1);
// MISSING ARGUMENT -> NOT CHECKED
result[0] = fp[0](1);
}
// function of (int, int)
{
int (*fp[])(int, int) = {plus, minus};
int result[] = {fp[0](1, 2), fp[1](2, 1)};
printf("%3d: %d, %d\n", __LINE__, result[0], result[1]);
// MISSING ARGUMENT -> CHECKED
// result[0] = plus(1);
// MISSING ARGUMENT -> CHECKED
// result[0] = fp[0](1);
}
}
return 0;
}
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment