What is the use of the #error directive in C?

Question

What is the #error directive in C? What is the use of it?

score 43 · Accepted Answer · answered Mar 16 '11 at 06:09

It's a preprocessor directive that is used (for example) when you expect one of several possible -D symbols to be defined, but none is.

#if defined(BUILD_TYPE_NORMAL)
# define DEBUG(x) do {;} while (0) /* paranoid-style null code */
#elif defined(BUILD_TYPE_DEBUG)
# define DEBUG(x) _debug_trace x /* e.g. DEBUG((_debug_trace args)) */
#else
# error "Please specify build type in the Makefile"
#endif

When the preprocessor hits the #error directive, it will report the string as an error message and halt compilation; what exactly the error message looks like depends on the compiler.

Wouldn't it be more appropriate to say it halts preprocessing? I guess preprocessing can be viewed as a step in compilation, but it can definitely be done as a separate step, and is internally performed as a separate step, so it fails/reports a fatal error earlier on than a compilation error. — RastaJedi, Apr 19 '16 at 19:57

score 15 · Answer 2 · answered Mar 16 '11 at 06:15

15

I may have invalid code but its something like...

#if defined USING_SQLITE && defined USING_MYSQL
#error You cannot use both sqlite and mysql at the same time
#endif

#if !(defined USING_SQLITE && defined USING_MYSQL)
#error You must use either sqlite or mysql
#endif


#ifdef USING_SQLITE
//...
#endif

#ifdef USING_MYSQL
//...
#endif

answered Mar 16 '11 at 06:15

the code works fine with this little correction #if !(defined USING_SQLITE || defined USING_MYSQL) – Alvpjh Sep 21 '20 at 18:49

score 4 · Answer 3 · answered Jan 15 '14 at 08:18

If compiler compiles this line then it shows a compiler fatal error: and stop further compilation of program:

#include<stdio.h>
#ifndef __MATH_H
#error First include then compile
#else
int main(){
    float a,b=25;
    a=sqrt(b);
    printf("%f",a);
    return 0;
}
#endif

Output:compiler error --> Error directive :First include then compile

What is the use of the #error directive in C?

3 Answers3

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