2.2 Finding errors in a simple program

As mentioned above, compiler warnings are an essential aid when programming in C and C++. To demonstrate this, the program below contains a subtle error: it uses the function printf incorrectly, by specifying a floating-point format '%f' for an integer value:

#include <stdio.h>

int
main (void)
{
  printf ("Two plus two is %f\n", 4);
  return 0;
}

This error is not obvious at first sight, but can be detected by the compiler if the warning option -Wall has been enabled.

Compiling the program above, 'bad.c', with the warning option -Wall produces the following message:

$ gcc -Wall bad.c -o bad
bad.c: In function `main':
bad.c:6: warning: double format, different 
  type arg (arg 2)

This indicates that a format string has been used incorrectly in the file 'bad.c' at line 6. The messages produced by GCC always have the form file:line-number:message. The compiler distinguishes between error messages, which prevent successful compilation, and warning messages which indicate possible problems (but do not stop the program from compiling).

In this case, the correct format specifier should be '%d' for an integer argument. The allowed format specifiers for printf can be found in any general book on C, such as the GNU C Library Reference Manual (see section Further reading).

Without the warning option -Wall the program appears to compile cleanly, but produces incorrect results:

$ gcc bad.c -o bad
$ ./bad
Two plus two is 2.585495    (incorrect output)

The incorrect format specifier causes the output to be corrupted, because the function printf is passed an integer instead of a floating-point number. Integers and floating-point numbers are stored in different formats in memory, and generally occupy different numbers of bytes, leading to a spurious result. The actual output shown above may differ, depending on the specific platform and environment.

Clearly, it is very dangerous to develop a program without checking for compiler warnings. If there are any functions which are not used correctly they can cause the program to crash or produce incorrect results. Turning on the compiler warning option -Wall will catch many of the commonest errors which occur in C programming.