C Programming Tutorial
Description
In the C Programming Language, the #error directive causes preprocessing to stop at the location where the directive is encountered. Information following the #error directive is output as a message prior to stopping preprocessing.
The #error preprocessor directive indicates error. The compiler gives fatal error if #error directive is found and skips further compilation process.
C #error example
Let's see a simple example to use #error preprocessor directive.
#include<stdio.h> #ifndef __MATH_H #error First include then compile #else void main() { float a; a=sqrt(7); printf("%f",a); } #endif
Output:
Compile Time Error: First include then compile
But, if you include math.h, it does not gives error.
#include<stdio.h> #include<math.h> #ifndef __MATH_H #error First include then compile #else void main(){ float a; a=sqrt(7); printf("%f",a); } #endif
Output:
2.645751
Example
Let's look at how to use #error directives in your C program.
The following example shows the output of the #error directive:
/* Example using #error directive by c programming */ #include <stdio.h> #include <limits.h> /* * Calculate the number of milliseconds for the provided age in years * Milliseconds = age in years * 365 days/year * 24 hours/day, 60 minutes/hour, 60 seconds/min, 1000 millisconds/sec */ #define MILLISECONDS(age) (age * 365 * 24 * 60 * 60 * 1000) int main() { /* The age of C programming in milliseconds */ int age; #if INT_MAX < MILLISECONDS(12) #error Integer size cannot hold our age in milliseconds #endif /* Calculate the number of milliseconds in 12 years */ age = MILLISECONDS(12); printf("TechOnTheNet is %d milliseconds old\n", age); return 0; }
In this example, we are using the int data type to hold the age of c programming in milliseconds. The int data type has a maximum value of INT_MAX which is defined in the limits.h header file and holds a value of 2^31 - 1 on both 32 and 64 bit systems. (See here for other variable types and their sizes.)
The statement #if INT_MAX < MILLISECONDS(12) evaluates whether or not the int data type is large enough to store the age in milliseconds. If it is not, the processor will output the following error message:
error: Integer size cannot hold our age in milliseconds
Since an error occurred, the program compilation will not complete and therefore no executable program will be created.
A typical use of the #error directive is to prevent compilation if a known condition exists that would cause the program to not function properly if the compilation completed. (For example, the C source was designed for a specific platform and was being compiled on an incompatible platform.)