Fast but unchecked - C

Revision of C example from a Well House Consultants training course
More on Revision of C [link]

This example is described in the following article(s):
• C - why is slow to write and debug) but fast to run? - [link]

Source code: overflow.c Module: C230

#include <stdio.h>
/* Some features of C that make it fast to run but you have to be
very sure that your code is right!*/

/* Note for reader - please see associated blog article. This shows you things that
can go wrong and you should not do! */

int main() {

/* Overflow of integers */

        short int a = 40000;
        short int b = 30000;
        short int c;
        c = a+b;
        printf("First Value is %d\n",c);

/* failing to initialise */

        float empty;
        printf("Unset value is %f\n",empty);

/* Accessing an array beyond its scope */

        int k;
        int d1[] = {666,777};
        int numbers[] = {4,7,9,15};
        int d2[] = {888,999};

        for (k=-2;k<7;k++) {
                numbers[k] = numbers[k+1];
        }
        for (k=-2;k<7;k++) {
                printf("Array contains is %d\n",numbers[k]);
        }

/* Uninitialised pointers */

        int *somewhere;
        printf("Final Value is %d\n",*somewhere);

/* Output using the wrong format (at least you often get a warning) */

        float pie = 3.1415;
        printf("Floated pie value is %d\n",pie);

/* Accessing character strings out of scope */

        char *wordy = "Fishpaste";
        wordy[9] = '*';
        printf("Overshooting %s\n",wordy);

}

/* Sample output

WomanWithCat:harwell grahamellis$ ./overflow
First Value is 4464
Unset value is 0.000000
Array contains is 999
Array contains is 4
Array contains is 7
Array contains is 9
Array contains is 15
Array contains is 0
Array contains is 0
Array contains is 0
Array contains is 666
Final Value is -17958193
Floated pie value is 3072
Bus error: 10
WomanWithCat:harwell grahamellis$

*/

Learn about this subject

This module and example are covered on the following public courses:
* Learning to program in C and C++
* C++ for C Programmers
* C and C++ Programming
* Learning to program in C and C++
* C and C++ Programming
Also available on on site courses for larger groups

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