GENERAL: #DEFINE Generates Wrong Results

Information in this article applies to:

• C51 All Versions
• C166 All Versions
• C251 All Versions

QUESTION

I have found a serious problem when I use simple constants in a #define statement. In the following code the constant is not divided:

if (value >= (MAX_MSG_LEN/4))

The value will not be (MAX_MSG_LEN/4) it is just MAX_MSG_LEN.

MAX_MSG_LEN is defined as follows:

#define MAX_LEN      16
#define MAX_MSG_LEN  MAX_LEN+3

ANSWER

The #define is a pure text replacement utility. So you have effectively written if (value >= (16+3/4)). The ANSI C compiler calculations plain numbers with the arithmetic rules for int variables. Therefore, 3/4 is calculated first which results in 0 and the result of the complete calculation is just 16. This lets you believe that the compiler does not divide.

As a general rule, it is recommended that you use parenthesis in #define statements. For example:

#define MAX_MSG_LEN (MAX_LEN+3)

This avoids simple programming mistakes as you had above.

MORE INFORMATION

• Refer to General Programming Book like The C Programming Language by Kernigan & Ritchie.

SEE ALSO

• GENERAL: #DEFINEs With Arithmetic Don't Work
• GENERAL: Multiplying Two Ints Gives Incorrect Result
• GENERAL: Logical Not ('~') Gives Incorrect Results
• C166: Multiplication Bug
• ARM: Wrong Result With Binary NOT and UNSIGNED CHAR
• C251: Pointer Arithmetic Delivers Unexpected Results

Last Reviewed: Thursday, February 25, 2021