Diagnostic directives

Shows the given error message and renders the program ill-formed, or given warning message without affect the validity of the program(since C23).

Syntax

`#error` diagnostic-message	(1)
`#warning` diagnostic-message	(2)	(since C23)

Explanation

1) After encountering the #error directive, an implementation displays the message diagnostic-message and renders the program ill-formed (the compilation stops).

2) Same as (1), except that the validity of the program is not affected and the compilation continues.

diagnostic-message can consist of several words not necessarily in quotes.

Notes

Before its standardization in C23, #warning has been provided by many compilers in all modes as a conforming extension.

Example

#if __STDC__ != 1
#  error "Not a standard compliant compiler"
#endif
 
#if __STDC_VERSION__ >= 202311L
#  warning "Using #warning as a standard feature"
#endif
 
#include <stdio.h>
int main (void)
{
    printf("The compiler used conforms to the ISO C Standard !!");
}

Possible output:

The compiler used conforms to the ISO C Standard !!

References

C17 standard (ISO/IEC 9899:2018):

6.10.5 Error directive (p: 126-127)

C11 standard (ISO/IEC 9899:2011):

6.10.5 Error directive (p: 174)

C99 standard (ISO/IEC 9899:1999):

6.10.5 Error directive (p: 159)

C89/C90 standard (ISO/IEC 9899:1990):