Pointer-to-array, malloc, and out-of-bounds access-CodePudding

Given a pointer-to-array in C, can we malloc to it enough memory for extra elements (beyond the specified array size) and then safely access those elements using either the [] operator or pointer arithmetic?

Consider this example:

int (*foo)[ 10 ]; //Declare pointer to array of ten ints
foo = malloc( sizeof( int ) * 20 ); //Allocate memory for TWENTY ints
(*foo)[ 15 ] = 100; //Access element in "extra" space via [] operator 
*( *foo   16 ) = 200; //Access element in "extra" space via pointer arithmetic
printf( "%d\n", *( *foo   15 ) ); //Prints 100
printf( "%d\n", (*foo)[ 16 ] ); //Prints 200

This code compiles fine and produces correct results in gcc. However, I'm not sure if it invokes undefined behavior.

Thanks!

CodePudding user response：

What you're doing trigger undefined behavior because you're reading/writing past the bounds of an array of int of size 10.

The proper way to access this memory is to use 2D array access. So instead of this:

(*foo)[15] = 100;

Which is equivalent to this:

foo[0][15] = 100;

Do this:

foo[1][5] = 100;

CodePudding user response：

I would allocate it a bit different way:

foo = malloc( sizeof( *foo ) * nrows );

it will allocate 2D array having 10 clomns and nrows rowas.

The best way to access the array is using indexes

foo[row][column]