复制到缓冲区期间出现分段错误错误

Question

I write code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(){
        char * var=(char*) calloc(10, sizeof(char));
        strncpy(var,"123456789",9);
        strncpy(var, "abcdefghi",1000000 );
        printf("This is var == %s\n",var);

}

but after compiling and running him I have Segmentation fault error. If I change maximum copy character in the line strncpy(var, "abcdefghi",1000000) to 10 then it's work fine. But I really don't understand why segmentation fault error is occurring in my case.

Answer 1

strncpy always pads up to the end of the buffer, so this line:

 strncpy(var, "abcdefghi",1000000 );

causes the segfault.

As you can read here The remainder of the buffer is filled with \\0 .

Answer 2

Because you allocate 10 bytes:

char * var=(char*) calloc(/*HERE->*/10, sizeof(char));

and try to copy 1000000 bytes to it.