Editing Defensive programming (section)

== Secure programming ==
{{main|Secure coding}}

Secure programming is the subset of defensive programming concerned with [[computer security]]. Security is the concern, not necessarily safety or availability (the [[software]] may be allowed to fail in certain ways). As with all kinds of defensive programming, avoiding bugs is a primary objective; however, the motivation is not as much to reduce the likelihood of failure in normal operation (as if safety were the concern), but to reduce the attack surface – the programmer must assume that the software might be misused actively to reveal bugs, and that bugs could be exploited maliciously.

<syntaxhighlight lang="c">int risky_programming(char *input) {
  char str[1000]; 
  
  // ...
  
  strcpy(str, input);  // Copy input.
  
  // ...
}</syntaxhighlight>
The function will result in undefined behavior when the input is over 1000 characters. Some programmers may not feel that this is a problem, supposing that no user will enter such a long input. This particular bug demonstrates a vulnerability which enables [[buffer overflow]] [[exploit (computer security)|exploit]]s. Here is a solution to this example:

<syntaxhighlight lang="c">int secure_programming(char *input) {
  char str[1000+1];  // One more for the null character.

  // ...

  // Copy input without exceeding the length of the destination.
  strncpy(str, input, sizeof(str));

  // If strlen(input) >= sizeof(str) then strncpy won't null terminate. 
  // We counter this by always setting the last character in the buffer to NUL,
  // effectively cropping the string to the maximum length we can handle.
  // One can also decide to explicitly abort the program if strlen(input) is 
  // too long.
  str[sizeof(str) - 1] = '\0';

  // ...
}</syntaxhighlight>