malloc question, why one irrelevant pointer changes when I call malloc!

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It is a very wierd problem:

In my program there is a function called update_Index:

C++

int update_Index(int documentID,char *_word,INVERTED_INDEX *index)
{
  char *hashValue=_word;
  unsigned long hashKey=hash1(hashValue)%MAX_NUMBER_OF_SLOTS;//get hash key
  if (index->hash[hashKey]->word==NULL)//case that the hash for this word not exists
    {
      /*
        add documentNode
       */
      DocumentNode *docNode=NULL;
      docNode=(DocumentNode *)malloc(sizeof(DocumentNode));
      docNode->next=NULL;
      docNode->document_id=documentID;
      docNode->page_word_frequency=1;
    }
    ...
}

Here is part of it, I define a data structure called DocumentNode as following:

C++

typedef struct _DocumentNode{
  struct _DocumentNode *next;
  int document_id;
  int page_word_frequency;
} __DocumentNode;
typedef struct _DocumentNode DocumentNode;

hash1 is a function to general hashKey of a string, there is no problems about that~

problem is everytime when I try to malloc memory to DocumentNode *docNode,
the input argument _word will change!!!

I use gdb to detect the code, it really happens..
Plz help me~

Thanks!

ps:

C#

unsigned long hash1(char* str) {
  unsigned long hash = 5381;
  int c;
  while ((c = *str++) != 0)
    hash = ((hash << 5) + hash) + c; /* hash * 33 + c */
  return hash;
}

more strangely, when I add the following one line code, the _word is not changing anymore.

<pre lang="c++">
char s[MAX_WORD_LENGTH];
strcpy(s,_word);

Posted 23-Feb-12 17:13pm

zhaoyilong1

Updated 24-Feb-12 5:46am

v3

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Comments

Chuck O'Toole 24-Feb-12 0:11am

You really need to show the hash1() function.

When you say "input argument _word will change", you need to be more specific. Does the binary value of the pointer "_word" change so that is has a different hexidecimal value or does the string of characters pointed to by "_word" change to a different or clobbered string? Makes a difference.

zhaoyilong1 24-Feb-12 10:01am

only the string change, the memory is the same ~
I post my hash1 function

2 solutions

Solution 1

It seems you're not supplying all of the relevant code.
As nonsensical as your assertion sounds, that malloc modifies unrelated memory - I whipped up a quick test. There is no such behaviour exhibited.

I notice that you set hashValue to be equal to _word. Can't help but wonder if you're modifying *hashValue (which obviously or not, will modify *_word)

Here's the code I compiled and the output:

Code:

C#

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

typedef struct _DocumentNode
{
    struct _DocumentNode *next;
    int document_id;
    int page_word_frequency;
} __DocumentNode;

typedef struct _DocumentNode DocumentNode;

int update_Index(int documentID,char *_word) //,INVERTED_INDEX *index)
{
//  char *hashValue=_word;
//  unsigned long hashKey=hash1(hashValue)%MAX_NUMBER_OF_SLOTS;//get hash key
//  if (index->hash[hashKey]->word==NULL)//case that the hash for this word not exists
    {
      /*
        add documentNode
       */
      DocumentNode *docNode=NULL;
      printf("_word = 0x%X\n", (int)_word);
      printf("*_word = %s\n\n", _word);

      docNode=(DocumentNode *)malloc(sizeof(DocumentNode));

      printf("_word = 0x%X\n", _word);
      printf("_word[0] = %c\n\n", _word[0]);


      docNode->next=NULL;
      docNode->document_id=documentID;
      docNode->page_word_frequency=1;
    }
}


int main()
{
    update_Index(0 , "inputWord");
    char *tmp1, *tmp2;
    tmp1 = (char*)malloc(10);
      printf("tmp1 = 0x%X\n", tmp1);
    tmp2 = (char*)malloc(10);
      printf("tmp2 = 0x%X\n", tmp2);
}

Result:

_word = 0x40305F
*_word = inputWord

_word = 0x40305F
_word[0] = i

tmp1 = 0x9F3D48
tmp2 = 0x9F3D60

Process returned 0 (0x0)   execution time : 0.054 s
Press any key to continue.

Posted 23-Feb-12 18:23pm

enhzflep

Comments

zhaoyilong1 24-Feb-12 9:59am

Thanks a lot, but I still have this problem, here is the test of the code:

235 docNode=(DocumentNode *)malloc(sizeof(DocumentNode));
(gdb) p _word
$3 = 0x7fffffffdf20 "www"
(gdb) n
236 docNode->next=NULL;
(gdb) p _word
$4 = 0x7fffffffdf20 "["

zhaoyilong1 24-Feb-12 10:01am

the input argument for _word is a char **, is that will be a problem ~?

kamendula 22-Nov-12 3:08am

i come up with the same problem and that really confused me...could u tell me how u fix the proble?? plz...i really dont konw how to fix it...thank u very very much.....

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ShengbenZhu · Accepted Answer · 2012-02-24T15:02:00

<pre lang="c++">
char s[MAX_WORD_LENGTH];
strcpy(s,_word);

Here are the problem:
----char *hashValue=_word;
At this line,the two pointer hashValue and _word point to the same memory.If you change one ,the other changes too.
As with why if you add this
----char s[MAX_WORD_LENGTH];
----strcpy(s,_word);
that would be fine? Here's the strcpy:
<pre lang="c++">
char *strcpy(char *strDestination, const char *strSource)
　　{
　　assert(strDestination && strSource);
　　char *strD=strDestination;
　　while ((*strDestination++=*strSource++)!='\0')
　　NULL;
　　return strD;
　　}</pre></pre>
As you can see,it creates a new string ,but not just give the point to it.
So the use "char *hashValue=_word" would be very dangerous