realloc() fails

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# 1  
Old 08-16-2011
realloc() fails

Not sure in which forum to post this. I'm trying here, in Programming.

I'm working on a PC with Intel Duo processor & 2GB of ram. OS is Ubuntu 10.04.

I'm having problems with a C++ program that makes extensive use of realloc(). It happens that as soon as the overall memory allocated(OS + user) reaches ~60% (tracked with System Monitor), realloc() fails to allocate additional memory and the program stops.

If the memory used stays below 60%, everything goes just fine.

Again, that 60% is reported by the System Monitor.

I also run Memtest86. It reports no problems with the ram.

Did someone ever experienced something similar? Any hint about where to look at?

Any feedback will be appreciated.
# 2  
Old 08-16-2011
realloc() doesn't check physical memory. What matters is virtual memory availability.
There are several factors that might prevent realloc (or malloc for that matter) to fail including:
  • 32 bit process addressing space
  • Memory overcommitting configuration
  • Area requested larger than free space available (obvious)
  • Memory fragmentation (malloc/realloc must return a contiguous area of virtual memory).
  • Bugs in your program leading to heap corruption
# 3  
Old 08-16-2011
Inside your code are you checking and printing the error returned by realloc when it fails to allocate memory...and can you post that error message here.
# 4  
Old 08-16-2011
How much memory does your system have?
# 5  
Old 08-16-2011
The OP poster already stated 2GB of RAM was installed. Unfortunately, he doesn't tell how much swap is set if any, how memory overcommiting is configured, how large is the segment he tries to reallocate, to what size, and if this is a 32 or 64 bit program.
# 6  
Old 08-16-2011
Thanks everyone. Your responses made me think more.
void * realloc ( void * ptr, size_t size );

In case that ptr is NULL, the function behaves exactly as malloc, assigning a new block of size bytes and returning a pointer to the beginning of it.
In case that the size is 0, the memory previously allocated in ptr is deallocated as if a call to free was made, and a NULL pointer is returned.

I'm saying it fails because it returns NULL although size is *not* zero.

And, again, it fails when the overall ram usage goes just above 60%.

The C++ program through realloc() is storing a few thousands text files in ram for postprocessing. Being the files size unknown, the ram for each individual file is re-allocated in increments of 500KB.

Although I cannot apriori exclude bugs in the C++ code, it works very well as long as the overall allocated memory stays below 60% of available ram (2GB).

About the system: it is 32b Ubuntu 10.04 on Intel Duo Core. Swap is ~6GB. RAM is 2GB. I'm not familiar with "memory over-committing config"...

I'm digging more into it. If you guys have any hint, I'll be glad to hear.
# 7  
Old 08-16-2011
My best advice would be for you to switch to a 64 bit OS and application. You still don't give a clue about the allocated area sizes but I suspect that your process virtual memory is too fragmented for a contiguous segment large enough to be available.
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