Malloc Memory Questions
c, gcc, linux, malloc
Solution
1- If you are working on a 32-bit machine you can't have a variable with more than 2GBs allocated to it.
2- If you are working on a 64-bit machine you can allocate as much as RAM+Swap memory in total, however, allocating all for one variable requires a big consequent chunk of memory which might not be available. Try it with a linked list, where each element has only 1 MB assigned and you can achieve a higher memory allocated in total.
3- As noted by you and Sharth, unless you use your memory, linux won't allocate it.
Problem
First of all I noticed when I malloc memory vs. calloc the memory footprint is different. I am working with datasets of several GB. It is ok for this data to be random. I expected that I could just malloc a large amount of memory and read whatever random data was in it cast to a float. However, looking at the memory footprint in the process viewer the memory is obviously not being claimed (vs. calloc where I see a large foot print). I ran a loop to write data into the memory and then I saw the memory footprint climb. Am I correct in saying that the memory isn't actually claimed until I initialize it? Finally after I passed 1024*1024*128 bytes (1024 MB in the process viewer) I started getting segfaults. Calloc however seems to initialize the full amount up to 1 GB. Why do I get segfaults when initializing memory in a for loop with malloc at this number 128MB and why does the memory footprint show 1024MB? If malloc a large amount from memory and then read from it what am I getting (since the process viewer shows almost no footprint until I initialize it)? Finally is there any way for me to alloc more than 4GB? I am testing memory hierarchy performance. Code for #2: ``` long long int i; long long int *test=(long long int*)malloc(1024*1024*1024); for (i=0;i<1024*1024*128;i++) test[i]=i; sleep(15); ```