malloc

[rinjo]

Hi All,

Have a few questions regarding malloc.

1. How does "malloc" system call attocate memory?
2. Are there any env variables that can affect the allocation of memory in malloc?
3. Is fragmentation of heap and non availability of heap the only reasons for malloc failure?

[tthtlc]

write one line program that use malloc, and then use strace on it.

[matt_kleviar]

1. How does "malloc" system call attocate memory?
-> kernel does not allocate memory as soon as you do malloc, but it only prepares vmas. and when you actually access it it allocates page because of page fault exception. if it was a read access then kernel allocates special page called zero page, which is called demand paging
2. Are there any env variables that can affect the allocation of memory in malloc?
I think no.
3. Is fragmentation of heap and non availability of heap the only reasons for malloc failure?
there is nothing like fragamentation of heap, because your process understands virtual address space,not physical. it is kernel who understands it.
so for malloc process may sleep or other pages could be swapped out, if the swap area is full and as physical memory fragamented (internal fragmentation) then malloc may fail.

[rinjo]

Hi Matt,

Thanks for your detailed answer to my query.

[the gooey kablooie]

For detailed explanation on how malloc allocates memory visit these sites:
CS360 Lecture notes -- Malloc Lecture #1
CS360 Lecture notes -- Malloc Lecture #2