malloc导致内存泄漏

Question

I have the following code.

#include<stdio.h>
#include<string.h>
#define _CRTDBG_MAP_ALLOC
#include <stdlib.h>
#include <crtdbg.h>

int main(){
   int *N=NULL;
   char *S=NULL,input[50],*Par=NULL,T='^';
   printf("Give me the equation: ");
   scanf("%s",input);
   printf("\n%d",strlen(input));
   S=(char*)malloc(3);
   N=(int*)malloc((strlen(input)-3)*sizeof(int));
   _CrtDumpMemoryLeaks();  /* Memory leak detected! */
   free(S);
   free(N);
   return 0;
}

After malloc returns without a problem the function in the line with the comment prints in the output window in visual studio the next message:

Detected memory leaks!
Dumping objects ->
c:\users\manos\documents\visual studio 2010\projects\gcjgcjc\gcjgcjc\gdjjj.cpp(17) : {60} normal block at 0x00A343F8, 16 bytes long.
Data: <                > CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD 
c:\users\manos\documents\visual studio 2010\projects\gcjgcjc\gcjgcjc\gdjjj.cpp(16) : {59} normal block at 0x00A31B30, 3 bytes long.
Data: <   > CD CD CD 
Object dump complete.

When the program stops the visual detects heap corruption. Does anyone know what happens? As far as i know there's nothing wrong with my code so what happens with malloc? Did i do something that caused the memory leak?

Answer 1

You should not attempt to detect memory leaks prior to releasing all memory. Calling _CrtDumpMemoryLeaks(); prior to free -ing everything that you have allocated is bound to detect false "leaks" that are simply memory in active use by your program.

Moving the check to the end will fix the problem:

S=(char*)malloc(3);
N=(int*)malloc((strlen(input)-3)*sizeof(int));
free(S);
free(N);
_CrtDumpMemoryLeaks();  /* No memory leaks! */

You should also add a check for strlen(input) to be 3 or more; otherwise, you could pass a negative number to malloc , which malloc would interpret as a large positive number; this should never happen.