Pointer for 2D array seems to be 3D in C Post: 302999243

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Top Forums Programming Pointer for 2D array seems to be 3D in C Post 302999243 by yifangt on Thursday 15th of June 2017 07:07:42 PM

06-15-2017

Registered User

Pointer for 2D array seems to be 3D in C

I am struggling with the pointer to 2D-array (cf: 2D array of pointers). Can anybody help me elaborate how the pointer x moves in the memory to access the individual of y[2][6], especially the high lighted lines?
I have talked to one of the curators of the forum, but I am still not quite clear.
Here is my code:

Code:

#include<stdio.h>

int main(void)
{
    int (*x)[2][6];                 //pointer for integers array in size of 2x6 (2 rows x 6 columns),
                                    //.i.e the array is always with size of 12?
//    int (*a[8])[5];                 //Line 9: a is a pointer array of size 8, each for integer array of size 5 

    int y[2][6] = {{11,12,13,14,15,16},
                   {21,22,23,24,25,26}};    //2D array of integers
    int *z;                      //pointer to integer
    int i;

    z = y[0];
    for(i = 0;i<6;i++)
        printf("%d ",z[i]);
    printf("\n");

    x = &y;    // More complicated situation for me!
    x = y;     // Warning: incompatible pointer type.
//    x[0][0] = y[0][0];     // won't work
        printf("   (x[0][0]): %p\n",  x[0][0]);
        printf("  *(x[0][0]): %d\n",*(x[0][0]));        //Q1a
        printf("  x[0][0][0]: %d\n",x[0][0][0]);        //Q1b
        printf("*(x[0][0]+1): %d\n",*(x[0][0]+1));      //Q1c
//        printf("*(x[0][0]+2): %d\n",*(x[0][0]+2));
//        printf("*(x[0][0]+3): %d\n",*(x[0][0]+3));
//        printf("*(x[0][0]+4): %d\n",*(x[0][0]+4));
        printf("*(x[0][0]+5): %d\n",*(x[0][0]+5));

        printf("    x[0][1]: %p\n",   x[0][1]);        //Q2a
        printf("    *x[0][1]: %d\n",*(x[0][1]));
        printf("  x[0][1][0]: %d\n",x[0][1][0]);       //Q2b 
        printf("*(x[0][1]+1): %d\n",*(x[0][1]+1));     //Q2c 
//        printf("*(x[0][1]+2): %d\n",*(x[0][1]+2));
//        printf("*(x[0][1]+3): %d\n",*(x[0][1]+3));
        printf("*(x[0][1]+4): %d\n",*(x[0][1]+4));
        printf("*(x[0][1][4]): %d\n",*(x[0][4]));
        
    printf("&y: %p\n", &y);
    printf(" y: %p\n",  y);
  
    printf(" x: %p\n",  x);
    printf("&x: %p\n", &x);
    
    return 0;
}

1) Although y and &y are the same, but x = y issues warning;
2) Q1a/Q2a is the part I think I understand which is the first element of each row of y.
3) but Q1b/c, and Q2b/c turns out to be 3-D to me.
Can anybody give me a diagram how pointer x moves in the memory for each member of y?
4) Line 9: int (*a[8])[5]; is related, and I put it here for future reference but skip it at this moment.

Thanks a lot!

yifangt

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LEARN ABOUT OSX

backtrace_symbols

backtrace(3)						   BSD Library Functions Manual 					      backtrace(3)

NAME

     backtrace, backtrace_symbols, backtrace_symbols_fd -- call stack backtrace and display functions

SYNOPSIS

     #include <execinfo.h>

     int
     backtrace(void** array, int size);

     char**
     backtrace_symbols(void* const* array, int size);

     void
     backtrace_symbols_fd(void* const* array, int size, int fd);

DESCRIPTION

     These routines provide a mechanism to examine the current thread's call stack.

     backtrace() writes the function return addresses of the current call stack to the array of pointers referenced by array.  At most, size
     pointers are written.  The number of pointers actually written to array is returned.

     backtrace_symbols() attempts to transform a call stack obtained by backtrace() into an array of human-readable strings using dladdr().  The
     array of strings returned has size elements.  It is allocated using malloc() and should be released using free().	There is no need to free
     the individual strings in the array.

     backtrace_symbols_fd() performs the same operation as backtrace_symbols(), but the resulting strings are immediately written to the file
     descriptor fd, and are not returned.

EXAMPLE

	 #include <execinfo.h>
	 #include <stdio.h>
	 ...
	 void* callstack[128];
	 int i, frames = backtrace(callstack, 128);
	 char** strs = backtrace_symbols(callstack, frames);
	 for (i = 0; i < frames; ++i) {
	     printf("%s
", strs[i]);
	 }
	 free(strs);
	 ...

HISTORY

     These functions first appeared in Mac OS X 10.5.

SEE ALSO

     dladdr(3), malloc(3)

Mac OS X							 February 15, 2007							  Mac OS X