• Use format specifiers to print output values
• Use compiler command line arguments to warn about incorrect results when unsigned and signed integers are compared

 

Use format specifiers to print output values

What you should not do

#include<limits.h>
#include<stdio.h>

void main(){

      unsigned long foo = ULONG_MAX;
      printf(foo);

}

When the above program is run with or without the "-Wformat-security" command line option, a warning is given by the compiler. The warning is "warning: format not a string literal and no format arguments [-Wformat-security]". This cause the program to crash. When the above C program is run, the output is "Segmentation fault (core dumped)".

What you should do

#include<limits.h>
#include<stdio.h>

void main(){

      unsigned long foo = ULONG_MAX;
      printf("%lu", foo);

}

The above program when run would print the value, 18446744073709551615 which is maximum value of unsigned long integer.

Concept Map

This maps to E in the Concept Map.

 Use compiler command line arguments to warn about incorrect results when unsigned and signed integers are compared

In the program below a signed and an unsigned integer are compared.

#include<stdio.h>

bool foo (int x, int unsigned y)
{
     printf("%u\n", y);
     printf("%x\n", y);
     if (x<=y){
        return true;
     }

     return false;

}

void main(){

     bool res = foo(4,-5);
     printf("%s\n", res ? "true" : "false");
}

What you should not do

When the above program is compiled without the -Wsign-compare option, no warning is given about  the comparison of an unsigned integer, y and signed integer x.

What you should do

-Wsign-compare option gives a warning  about the comparison of an unsigned integer, y and signed integer x. The warning is, "comparison between signed and unsigned integer expressions [-Wsign-compare]"

The following should be done to compile a program in gcc compiler using the -Wsign-compare flag.

gcc -Wsign-compare  -o greaterThan greaterThan.c

Concept Map

This maps to E in the Concept Map.