Size definition of strcat() function

Question

The question is why should I define size of string (string[] should be string[some-number]) When the program is as following it gives me Abort trap: 6:

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

int main(void)
{
  char buffer1[] = "computer";
  char string[]="program";
  strcat( buffer1, string );
  printf( "buffer1 = %s\n", buffer1 );

}

This is the program from http://www.tutorialspoint.com/cprogramming/c_data_types.htm it works fine:

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

int main ()
{
   char str1[12] = "Hello";
   char str2[12] = "World";
   char str3[12];
   int  len ;

   /* copy str1 into str3 */
   strcpy(str3, str1);
   printf("strcpy( str3, str1) :  %s\n", str3 );

   /* concatenates str1 and str2 */
   strcat( str1, str2);
   printf("strcat( str1, str2):   %s\n", str1 );

   /* total lenghth of str1 after concatenation */
   len = strlen(str1);
   printf("strlen(str1) :  %d\n", len );

   return 0;
}

What is the mistake? Even if I define all of the sizes of strings in my program, my code still gives Abort trap:6?

Answer 1

From the man page of strcat:

DESCRIPTION The strcat() function appends the src string to the dest string, overwriting the termi? nating null byte ('\0') at the end of dest, and then adds a terminating null byte. The strings may not overlap, and the dest string must have enough space for the result. If dest is not large enough, program behavior is unpredictable; buffer overruns are a favorite avenue for attacking secure programs.

When you declare your string, the compiler allocate the size of your initial string to be 9 (resp. 8) for the buffer1 (resp. string) (includin '\0').

Thus, strcat will result in 9 - 1 + 8 (i.e. 16 bytes) but only 9 are available.

Answer 2

The first parameter of strcat is used to store the result, so it must have enough space for the concatenated string.

In your code:

char buffer1[] = "computer";

is equivalent to:

char buffer1[9] = "computer";

defines a char array with just enough space for the string "computer", but not enough space for the result.

Answer 3

char buffer1[] = "computer";

Creates a buffer big enough to hold 9 characters (strlen("Hello" + 1 byte for \0)). If you write anymore data to it what you end up with is Undefined behavior (UB). This is what happens when you do a strcat.
UB means the program might crash or show literally any behavior. You are rather lucky that a program with UB crashes because it does not need to, but if it does atleast there is a indication of something wrong in it. Most of the times programs with UB will continue running correctly and crash when you least expect or want them to.