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Apr 5th, 2006
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reasons why malloc fails?

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hi,

will someone point me the reasons why a call to malloc fails even if we have enough memory ?
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prasath is offline Offline
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Re: reasons why malloc fails?

Quote originally posted by prasath ...
will someone point me the reasons why a call to malloc fails even if we have enough memory ?
show your code which is failing and make sure you are not passing -1 in malloc.
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Re: reasons why malloc fails?

Every process in Windows NT has one heap called the default heap. The Win32 subsystem uses the default heap for all global and local memory management functions, and the C run-time library uses the default heap for supporting malloc functions.

maaloc can fail as it cant allocate memory bigger than default heap. not all system memory is available to malloc.
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Re: reasons why malloc fails?

The in MS-Windows the default heap is not static -- the os will expand it as needed. But since malloc() takes an size_t integer as the parameter the largest amount of memory malloc can allocate at one time is the largest value that can be stored in the size_t integer (see limits.h). And yes, all the computer's available and unused memory can be used by malloc with the previously mentioned restriction.

malloc() normally fails today for one of two reason: (1) attempt to allocate more memory then is available, or (2) memory has been previously trashed (most common reason) such as buffer overflows and using uninitialized pointers (although there are a whole host of other causes).
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Re: reasons why malloc fails?

> will someone point me the reasons why a call to malloc fails even if we have enough memory ?
My guess is you're still using crusty old TurboC which is limited to 640K no matter how many GB of memory you have on your pentium powered, XP hosted machine.

"Hello, yes I'd like to buy a Ferrari please"
"Certainly sir, will you be wanting to replace the engine with an elastic band?"
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Re: reasons why malloc fails?

Click to Expand / Collapse  Quote originally posted by Ancient Dragon ...
The in MS-Windows the default heap is not static -- the os will expand it as needed. But since malloc() takes an size_t integer as the parameter the largest amount of memory malloc can allocate at one time is the largest value that can be stored in the size_t integer (see limits.h). And yes, all the computer's available and unused memory can be used by malloc with the previously mentioned restriction.

malloc() normally fails today for one of two reason: (1) attempt to allocate more memory then is available, or (2) memory has been previously trashed (most common reason) such as buffer overflows and using uninitialized pointers (although there are a whole host of other causes).
HI, I am also facing a same problem, " Malloc fails". Later I found the reason why malloc was failing. It was failing due to Buffer Overflow. But I am not able to understand why malloc fails due to Buffer Overflow. Can any one explain what exactly happens when there is a buffer over flow and how does it affect malloc. Thank you.
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Re: reasons why malloc fails?

>Can any one explain what exactly happens when there
>is a buffer over flow and how does it affect malloc.
No, because that's an implementation detail. It depends on how malloc works for your system as well as what you're doing in your code. Post your code that fails (preferably a small example that still fails) and I can describe what you're doing wrong and speculate about what's happening behind the scenes.
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Re: reasons why malloc fails?

Click to Expand / Collapse  Quote originally posted by Narue ...
>Can any one explain what exactly happens when there
>is a buffer over flow and how does it affect malloc.
No, because that's an implementation detail. It depends on how malloc works for your system as well as what you're doing in your code. Post your code that fails (preferably a small example that still fails) and I can describe what you're doing wrong and speculate about what's happening behind the scenes.
The code is very simple. (I am giving the example)
One buffer is defined as "char buf[1001]" and "buf" is assigned with 1000 characters.
Later inside the code I am doing the following..

char *name, *temp;

name = (char *)malloc(500);

strcpy(name, buf);

temp = (char *)malloc(sizeof(char));

In this piece of code, name is allocated with 500 bytes and 1000 bytes are copied into this memory resulting in a buffer overflow. Due to this malloc at "temp = (char *)malloc(sizeof(char));" instruction is failing. I am trying to understand how this buffer overflow is causing a problem to malloc even though there is a sufficient memory is available.
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Re: reasons why malloc fails?

>I am trying to understand how this buffer overflow is causing a problem
>to malloc even though there is a sufficient memory is available.
It's not about there being sufficient memory. If you've corrupted the memory manager's housekeeping data, it can't properly do its job. A buffer overflow can easily cause that kind of corruption.
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Re: reasons why malloc fails?

Your heap is divided into segments you can have maximum 64 segments and each segment is divided into blocks. The blocks are the actuall memory you allocate.

The first heap block is an allocation of 8 bytes. This block is also a free block.
The second heap block that follows it is an allocation of 16 bytes. This block is not free.

  1. _______________________________________________________________________________________________________________________
  2. |<--------------------------------------------------- HEAP SEGMENT ---------------------------------------------------->
  3. |<------------------- HEAP BLOCK -------------->|<------------------------------ HEAP BLOCK --------------------------->
  4. |<------- header ------>|<----- user data ----->|<------- header ------>|<----------------- user data ----------------->
  5. |s- -- p- -- s- f- u- t- d- d- d- d- d- d- d- d- s- -- p- -- s- f- u- t- d- d- d- d- d- d- d- d- d- d- d- d- d- d- d- d-
  6. |02 00 03 00 03 00 00 00 88 01 07 00 00 fa 0d 0c 03 00 02 00 03 01 0c 00 08 37 e2 7c 48 5e 77 05 01 00 00 00 00 00 00 00
  7. | 0002  0003  0003  0000  0188  0007  fa00  0c0d  0003  0002  0103  000c  3708  7ce2  5e48  0577  0001  0000  0000  0000
  8. |   00030002    00000003    00070188    0c0dfa00    00020003    000c0103    7ce23708    05775e48    00000001    00000000
  9. |
  10. |00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17
  11. |00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20 21 22 23 24 25 26 27
  12. |<------- header ------>|<----- user data ----->|<------- header ------>|<----------------- user data ----------------->
  13. |_______________________________________________________________________________________________________________________
So in a heap block, the first 8 bytes is the heap header. In my case 02 00 03 00 03 00 00 00 is the heap header of the first block
03 00 02 00 03 01 0c 00 is the heap header of the second heap block.

First four bytes are the size of a heap block Once you run your code you actually corrupt the next heap block. You just overwrite the size. This does not effect functioning of malloc but it corrups the memory, once you try to access that memory you expect a crash due to access denied error ..!!
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This thread is more than three months old

No one has posted to this discussion for at least three months. Please let old threads die and do not reply to them unless you feel you have something new and valuable to contribute that absolutely must be added to make the discussion complete. Otherwise, please start a new thread in this forum instead.
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