C Error (alignment)

ignoring_align.c

// The MIT License (MIT)

// Copyright (c) 2015 Leonardo de Oliveira Lourenço

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#include <stdio.h>
#include <stdlib.h>

typedef struct
{
    char c;
    int d;
} dummy_t;

void print_struct( void );
void print_sizes( void );
void print_title( char const *title );
void wait_enter( void );

int main( void )
{
    dummy_t dummy;
    dummy_t *error = (dummy_t *)malloc( sizeof( char ) + sizeof( int ) );
    unsigned long address;

    print_title( "Struct definition and sizes:" );

    print_struct();
    print_sizes();

    wait_enter();

    print_title( "Correct use:" );

    printf( "dummy_t dummy;\n" );

    printf( "\n      &dummy: %p", &dummy );
    printf( "\n    &dummy.c: %p", &dummy.c );
    printf( "\n    &dummy.d: %p", &dummy.d );
    printf( "\n  (&dummy)+1: %p", ( &dummy ) + 1 );
    printf( "\n(&dummy.d)+1: %p", ( &dummy.d ) + 1 );
    printf( "\n\n" );

    wait_enter();

    print_title( "Error use (ignoring alignment):" );

    printf( "dummy_t *error = (dummy_t *)malloc( sizeof( char ) + sizeof( int ) );\n" );

    address = (unsigned long)&error->d;
    printf( "\n          error: %p", error );
    printf( "\n      &error->c: %p", &error->c );
    printf( "\n      &error->d: %p", &error->d );
    printf( "\nAllocated space: from %p to %p",
            error,
            (void *)( ( (unsigned long)error ) + sizeof( char ) + sizeof( int ) - 1 ) );

    printf( "\n\nSo, using d on this wrongfully allocated struct"
            "\nwould access memory that doesn't belong to it,"
            "\nsince it would use:"
            "\n%p , %p , %p and %p\n",
            (void *)address,
            (void *)address + 1,
            (void *)address + 2,
            (void *)address + 3 );

    free( error );

    printf( "\n" );
    wait_enter();

    return 0;
}

void print_struct( void )
{
    printf( "typedef struct\n"
            "{\n"
            "    char c;\n"
            "    int  d;\n"
            "} dummy_t;\n" );
}

void print_sizes( void )
{
    printf( "\ndummy_t size: %2lu bytes", sizeof( dummy_t ) );
    printf( "\n   char size: %2lu bytes", sizeof( char ) );
    printf( "\n    int size: %2lu bytes", sizeof( int ) );
    printf( "\n" );
}

void print_title( char const *title )
{
    printf( "---------------------------------------\n"
            "       %s\n"
            "---------------------------------------\n",
            title );
}

void wait_enter( void )
{
    printf( "\nPress enter to continue\n" );
    getchar();
}