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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

Books covering this topic
Yes. We have over 700 books in our library. Books covering C and C++ are listed here and when you've selected a relevant book we'll link you on to Amazon to order.

Other Examples
This example comes from our "Revision of C" training module. You'll find a description of the topic and some other closely related examples on the "Revision of C" module index page.

Full description of the source code
You can learn more about this example on the training courses listed on this page, on which you'll be given a full set of training notes.

Many other training modules are available for download (for limited use) from our download centre under an Open Training Notes License.

Other resources
• Our Solutions centre provides a number of longer technical articles.
• Our Opentalk forum archive provides a question and answer centre.
The Horse's mouth provides a daily tip or thought.
• Further resources are available via the resources centre.
• All of these resources can be searched through through our search engine
• And there's a global index here.

Purpose of this website
This is a sample program, class demonstration or answer from a training course. It's main purpose is to provide an after-course service to customers who have attended our public private or on site courses, but the examples are made generally available under conditions described below.

Web site author
This web site is written and maintained by Well House Consultants.

Conditions of use
Past attendees on our training courses are welcome to use individual examples in the course of their programming, but must check the examples they use to ensure that they are suitable for their job. Remember that some of our examples show you how not to do things - check in your notes. Well House Consultants take no responsibility for the suitability of these example programs to customer's needs.

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