Why did my program crash when I tried the perror function?

Question

I'm testing the perror function in C, and according to this page it prints a default message when a null pointer is passed:

int main(void)
{
    int *p;
    perror(p); //crashes
}

Answer 1

Cause int* p contains a random/garbage value.

It is not an NULL pointer. You need to explicitly initialize it with p = NULL; .

Using an uninitialised variable is Undefined behaviour.

main() also needs to return 0; .

Answer 2

因为在C中p不会自动初始化为0（这不是Java）。

int *p = 0;

Answer 3

Passing an invalid pointer to perror is undefined behavior.

(C99, 7.1.4p1) "Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow: If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer, or a pointer to non-modifiable storage when the corresponding parameter is not const-qualified) or a type (after promotion) not expected by a function with variable number of arguments, the behavior is undefined."