为什么strcmp（）返回'0'虽然我没有提供空终止字符串？

Question

In the below code I was expecting some weird behavior(like infinite loop) as I am not providing null terminated string as an input to strcmp() function.

#include <stdio.h>

int main() 
{
  char string1[20];
  char string2[20] = {'H','e','l','l','o',};

  strcpy(string1, "Hello");
 // strcpy(string2, "Hellooo");
  printf("Return Value is : %d\n", strcmp( string1, string2));

  return 0;
}

The output is:

 Return Value is : 0

Why it is showing me both strings are equal?

My guess is when I initialized array (string2)rest elements are filled with zero.But as the array is local this should not be the case.

Answer 1

If an initialiser to an array or struct only provides data for parts of the array or struct , the rest of the array or struct is set to 0 . This is the case for string2 .

Answer 2

I dumped the string2 and I got

72 101 108 108 111 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

The rest of the string2 is filled with 0 s

for (i = 0; i < 20; i++)
    printf("%d ", string2[i]);

And when I did this, I got

char string2[20];

string2[0] = 'H';
string2[1] = 'e';
string2[2] = 'l';
string2[3] = 'l';
string2[4] = 'o';

72 101 108 108 111 127 0 0 -27 5 64 0 0 0 0 0 -1 -78 -16 0 Return Value is : 127

When you initialize the string in the definition part, the rest of the string is filled with 0 s.

But, when you just define it and then assign values to them, it is filled with junk values

Answer 3

Partially initialised array will filled or initiaslised with Zeroes of remaining uninitialised memory

eg

int a[10]={1,2,3}

remaining index will be filled (initialised)with zeros(0)

Answer 4

As already everybody has enlightened you with the correct answer. This example is to make it more clear::

int main( int argc, char** argv )
{
 char temp[100] = {'A'} ;
 return 0 ;
}

The disassembly for array initializing part is::

//char temp[100] = {'A'} ;
// 41h is 'A', so this instruction fills the first index with 'A'
mov         byte ptr [ebp-6Ch],41h 
push        63h  
push        0    
lea         eax,[ebp-6Bh] 
push        eax 

// Then it calls memset which in turn fills the rest of array with zeroes.
call        @ILT+115(_memset) (0B11078h) 
add         esp,0Ch

Answer 5

Whether the scope of string2 array is global or local, it will be initialized to zero, in this case.

The only difference you will observe is when you declare string2 locally and do not initialize it at all, ie char string2[20]; .

Doing this way locally allocates string2 on stack and no explicit initilaization takes place, but if you do the same globally the array will be initialized to zero (all indices).

Therefore, your guess is partially correct!!

Answer 6

C11 (n1570), §7.24.2.3 The strcpy function

The strcpy function copies the string pointed to by s2 ( including the terminating null character ) into the array pointed to by s1.

The first 6 elements in your string1 and string2 become the same. Terminator is also at the same position.