隐式声明警告：内置函数是什么？(implicit declaration warning: What are the built

隐式声明警告：内置函数是什么？(implicit declaration warning: What are the built-in functions?)

问题界面正在标记许多“可能已经有你答案的问题”，但我试图尽职尽责地检查是否有人问我到底在哪里。 如果这是重复，我道歉。

假设我有以下不正确的程序：

extern void undefined_function(void); int main(int argc, char **argv) { undefined_function(); undeclared_function(); exit(0); }

用gcc编译给出：

$ gcc warnings.c warnings.c: In function ‘main’: warnings.c:6:2: warning: incompatible implicit declaration of built-in function ‘exit’ [enabled by default] /tmp/ccVzjkvX.o: In function `main': warnings.c:(.text+0x15): undefined reference to `undefined_function' warnings.c:(.text+0x1f): undefined reference to `undeclared_function' collect2: ld returned 1 exit status $

我知道为什么会发出这些警告，以及如何纠正它们 - 这不是我的问题。

从输出中可以清楚地看出，gcc正在将exit()与其他未定义/未声明的函数区别对待，因为它认为它是“内置函数”

对于给定的gcc，我如何判断gcc认为是“内置函数”的函数列表是什么？ 它是c标准库函数列表还是别的？

我考虑做nm libc.so ，但是在我的Ubuntu VM上，这个glibc似乎被剥离了，所以在这方面没有有用的信息：

$ nm /lib/x86_64-linux-gnu/libc.so.6 nm: /lib/x86_64-linux-gnu/libc.so.6: no symbols $

The question-asking interface is flagging many "Questions that may already have your answer", but I have attempted to do due diligence to check if any are asking exactly what I am here. My apologies if this is a duplicate.

Suppose I have the following incorrect program:

extern void undefined_function(void); int main(int argc, char **argv) { undefined_function(); undeclared_function(); exit(0); }

Compiling with gcc gives:

$ gcc warnings.c warnings.c: In function ‘main’: warnings.c:6:2: warning: incompatible implicit declaration of built-in function ‘exit’ [enabled by default] /tmp/ccVzjkvX.o: In function `main': warnings.c:(.text+0x15): undefined reference to `undefined_function' warnings.c:(.text+0x1f): undefined reference to `undeclared_function' collect2: ld returned 1 exit status $

I know why these warnings are emitted, and how to correct them - that is not my question.

From the output, it is clear that gcc is treating exit() differently to the other undefined/undeclared functions, because it considers it a "built-in function"

For a given gcc, how can I tell what the list of functions is that gcc considers to be "built-in functions"? Is it exactly the list of c standard library functions or something else?

I considered doing nm libc.so, but on my Ubuntu VM, this glibc appears to be stripped, so there is no useful information there in this regard:

$ nm /lib/x86_64-linux-gnu/libc.so.6 nm: /lib/x86_64-linux-gnu/libc.so.6: no symbols $

最满意答案

该列表很长，而且非常特定于平台。 C标准库中的许多（但并非全部）函数（有时）被视为内置函数。 但是，还有一大堆与特定处理器指令和其他硬件功能相关的内置函数。 它们记录在从这里链接的各种页面中; 特别是，请看这里， 这里， 这里， 这里和这里。

The list is quite long, and quite platform-specific. Many (but by no means all) of the functions in the C standard library are (sometimes) treated as built-ins. But there are also a raft of built-in functions that relate to specific processor instructions and other hardware features. They're documented in various pages linked from here; in particular, see here, here, here, here, and here.