c - C内联函数和 “undefined external”错误

Question

我试图用内联函数替换一些宏子例程，以便编译器可以优化它们，以便调试器可以进入它们，依此类推。如果我将它们定义为普通函数，则可以正常工作:

void do_something(void)
{
  blah;
}

void main(void)
{
  do_something();
}

但是如果我将它们定义为内联:

inline void do_something(void)
{
  blah;
}

void main(void)
{
  do_something();
}

它说“错误:外部未定义”。这意味着什么？我在黑暗中刺了一下

static inline void do_something(void)
{
  blah;
}

void main(void)
{
  do_something();
}

没有更多的错误。函数定义和对该函数的调用位于同一.c文件中。

有人可以解释为什么一个有效而另一个无效的原因吗？

(第二个相关问题:如果要在多个.c文件中使用内联函数，应在哪里放置内联函数？)

Answer 1

首先，编译器并不总是内联标记为inline的函数；例如，如果您关闭所有优化，则可能不会内联它们。

定义内联函数时

inline void do_something(void)
{
  blah
}

并使用该函数，即使在同一文件中，对该函数的调用也要由链接程序而不是编译器来解决，因为它是隐式的“外部”。但是，仅此定义并不能提供功能的外部定义。

如果包含不带inline的声明

void do_something(void);

在可以看到inline定义的C文件中，编译器将提供函数的外部定义，并且错误应消失。
static inline起作用的原因是，它使函数仅在该编译单元中可见，因此允许编译器解析对该函数的调用(并对其进行优化)，并在该编译单元中发出该函数的代码。然后，链接器不必解析它，因此不需要外部定义。

放置内联函数的最佳位置是在头文件中，并将它们声明为static inline。这消除了对外部定义的任何需要，因此可以解决链接器问题。但是，这会导致编译器在使用该函数的每个编译单元中为该函数发出代码，因此可能导致代码膨胀。但是由于该函数是内联函数，所以无论如何它可能都很小，因此这通常不是问题。

另一种选择是在 header 中将其定义为extern inline，并在一个C文件中提供和extern声明，而无需使用inline修饰符。

gcc手册对此进行了解释:

By declaring a function inline, you can direct GCC to make calls to that function faster. One way GCC can achieve this is to integrate that function's code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included. The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case. You can also direct GCC to try to integrate all "simple enough" functions into their callers with the option -finline-functions.

GCC implements three different semantics of declaring a function inline. One is available with -std=gnu89 or -fgnu89-inline or when gnu_inline attribute is present on all inline declarations, another when -std=c99, -std=c1x, -std=gnu99 or -std=gnu1x (without -fgnu89-inline), and the third is used when compiling C++.

To declare a function inline, use the inline keyword in its declaration, like this:

 static inline int
 inc (int *a)
 {
   return (*a)++;
 }

If you are writing a header file to be included in ISO C90 programs, write __inline__ instead of inline.

The three types of inlining behave similarly in two important cases: when the inline keyword is used on a static function, like the example above, and when a function is first declared without using the inline keyword and then is defined with inline, like this:

 extern int inc (int *a);
 inline int
 inc (int *a)
 {
   return (*a)++;
 }

In both of these common cases, the program behaves the same as if you had not used the inline keyword, except for its speed.

When a function is both inline and static, if all calls to the function are integrated into the caller, and the function's address is never used, then the function's own assembler code is never referenced. In this case, GCC does not actually output assembler code for the function, unless you specify the option -fkeep-inline-functions. Some calls cannot be integrated for various reasons (in particular, calls that precede the function's definition cannot be integrated, and neither can recursive calls within the definition). If there is a nonintegrated call, then the function is compiled to assembler code as usual. The function must also be compiled as usual if the program refers to its address, because that can't be inlined.

Note that certain usages in a function definition can make it unsuitable for inline substitution. Among these usages are: use of varargs, use of alloca, use of variable sized data types , use of computed goto, use of nonlocal goto, and nested functions. Using -Winline will warn when a function marked inline could not be substituted, and will give the reason for the failure.

As required by ISO C++, GCC considers member functions defined within the body of a class to be marked inline even if they are not explicitly declared with the inline keyword. You can override this with -fno-default-inline.

GCC does not inline any functions when not optimizing unless you specify the always_inline attribute for the function, like this:

 /* Prototype.  */
 inline void foo (const char) __attribute__((always_inline));

The remainder of this section is specific to GNU C90 inlining.

When an inline function is not static, then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-static inline function is always compiled on its own in the usual fashion.

If you specify both inline and extern in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it.

This combination of inline and extern has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking inline and extern) in a library file. The definition in the header file will cause most calls to the function to be inlined. If any uses of the function remain, they will refer to the single copy in the library.