C compiler - implicit function declaration

Introduction

• For beginners C Language programming , Sometimes there is an abnormal crash , as follows ：

* test.c
#include <stdio.h>

int main(){
    printf("%s\n", test());
    return 0;
}
* xxx.c
char *test(){
    return "hello world";
}
*  compile 
gcc test.c xxx.c -o test
*  perform 
xxx:~/demo$ ./test
 Segment error  ( The core has been dumped )

Question why

• stay C In language , If the function is not declared before the call , The compiler will not report errors , Just a warning , If the function is undefined , The error prompt function will be reported when linking undefined.
• If there is no function declaration , How does the compiler know the number of parameters and the return type ？ The compiler compiles with a single code file as the compilation unit , Function not declared , The compiler cannot know the number of parameters of a function , The function definition can only be found when linking , The compiler will follow the default rules , For the calling function C Code generates assembly code .

Implicit function declaration

• The compiler's default rule is implicit function declarations , If the function is not declared , The compiler makes function calls according to the following declaration by default .

int function();

Possible problems

• If the implicit function declaration may differ from the real definition , May cause an exception , And some anomalies are not easy to find .

1. The return type in the function definition is not int

• Such as the example in the primer , The real return is char * type , Convert to according to the implicit function declaration int after , Last call printf Access the value as the memory address of the string , You are visiting a wrong address , So it causes the program to crash .
• If the real return is long type , convert to int Data may be truncated after .

2. The parameter type in the function definition does not match the call

• for example ：

* test.c
#include <stdio.h>

int main(){
    test(1)
    return 0;
}
* xxx.c
int test(char *a){
    printf("%s\n", a);
    return 0;
}

• The above program will crash after compilation .

Problem avoidance

1. To avoid this problem , Programmers need to heed the compiler's warnings about implicit declarations , Eliminate this warning in a timely manner by including the necessary header files .

• In some cases ,gcc The compiler doesn't even prompt a warning , Use C Linguistic C99 In the version , Warnings will be given anyway , and C++ The compiler is more stringent , Implicit function declarations are no longer supported , If the function is not declared , Will report an error directly .