C语言 C中的#error指令？
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#error directive in C?
提问by PHP
Can you please give the information about #errordirective in C?
你能提供有关#errorC指令的信息吗？
What is #errordirective? what the use of it?
什么是#error指令？有什么用呢？
采纳答案by geekosaur
It's a preprocessor directive that is used (for example) when you expect one of several possible -Dsymbols to be defined, but none is.
这是一个预处理器指令，用于（例如）当您希望-D定义几个可能的符号之一但没有定义时。
#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 #errordirective, it will report the string as an error message and halt compilation; what exactly the error message looks like depends on the compiler.
当预处理器命中#error指令时，它会将字符串报告为错误消息并停止编译；错误消息的具体内容取决于编译器。
回答by geekosaur
I may have invalid code but its something like...
我可能有无效的代码，但它类似于...
#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
回答by Nishant Kumar
If compiler compiles this line then it shows a compiler fatal error: and stop further compilation of program:
如果编译器编译此行，则显示编译器致命错误：并停止进一步编译程序：
#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
