Thread: can you tell me why11?

Code:

#include <stdio.h>
#include <string.h>
int main(int argc,char * argv[])
{int i;
char buf[4];
strcpy (buf,"aaaa");
printf("%d\n",i);
return 0;
}

Run-Time Check Failure #3-the variable i is being used without being defined
why ........

Code:

#include <stdio.h>
#include <string.h>
int main(int argc,char * argv[])
{int i=3;
char buf[5];
strcpy (buf,"aaaa");
printf("%d\n",i);
return 0;
}

like this ??..

Code:

#include <stdio.h>
main()
{ char buff[4];
 strcpy(buff,"aaaa");
 printf("%s",buff);
}

why these codes can run without mistake??

Originally Posted by pacific

Code:

#include <stdio.h>
main()
{ char buff[4];
 strcpy(buff,"aaaa");
 printf("%s",buff);
}

why these codes can run without mistake??

Since you have written into memory where you weren't supposed to, the behavior is undefined. Undefined means that the program might or might not give the appearance of running as expected (maybe the memory you wrote to was not being used for anything else at that time). It might do anything. If you are lucky it will give a run-time error so that you know that something is wrong. If you are not lucky, the program will appear to run OK and will give expected output. Then, sometime in the future, this program, or a program that incorporates this code will cause user problems that you didn't forsee.

Nothing in the C language checks for memory accesses out of bounds. It is the programmer's responsibility.

This is extremely important, and it is an illustration that you can't prove a program is correct by testing it.

When I ran the following, I could see the consequences of out-of-range array access:

Code:

#include <stdio.h>
#include <string.h>
int main(void)
{ 
  int x=-1;
  char buff[4];
  int y=-1;

  printf("Before strcpy:\n");
  printf("x = %d\n", x);
  printf("y = %d\n\n", y);

  strcpy(buff,"aaaa");
  printf("buff: <%s>\n\n",buff);

  printf("After  strcpy:\n");
  printf("x = %d\n", x);
  printf("y = %d\n", y);

  return 0;
}

With my compilers I get this (no compiler error or warning messages; no run-time messages):

Before strcpy:
x = -1
y = -1

buff: <aaaa>

After strcpy:
x = -256
y = -1

What do you get?

Regards,

Dave