在 C 程序中使用 fabs、fmin、fmax 时，如果不包含 math.h，为什么编译器/链接器不报告错误？

Question

When compiling a C program in Visual Studio 2013 the following may produce different results:

#include <math.h>

void bar(void) {
    double f = fabs(-1.0);
    /* f is 1.0 */
}

and

void foo(void) {
    double f = fabs(-1.0);
    /* f is 0 */
}

and the same snippet without including math.h . When omitting the include, the compiler does not report an error and assumes fabs has the following signature int fabs() .

Is there anyway to force the compiler to report this as an error or even a warning?

Answer 1

In older C standards if a function is used before it's declared then it's assumed to receive an unknown number of arguments (under default promotion like in variadic functions such as printf ) and return int . The compiler will expect the symbol to be fixed later during linking stage so no errors should be reported

So without including math.h the compiler doesn't know the prototype of fabs and will assume that it accepts any kinds of parameters and returns int . Then when linking the linker really found a symbol with the expected name and resolves to that function, but due to the mismatch in the function signature the behavior is undefined . Turn on all warnings and you'll see something about implicit return type

In VS2013 I got the below warning even without increasing the warning level

Warning 1   warning C4013: 'fabs' undefined; assuming extern returning int

For more information read

• Why can I call a function in C without declaring it but not in C++?
• Can we call functions before defining it?
• Implicit declaration in C language
• Are prototypes required for all functions in C89, C90 or C99?
• Why the error of - calling the function before being declared, is not shown?
• C language - calling functions without function prototype