How/where is sbrk used within malloc.c?

Question

I have read in Advanced Unix Programming (and also in a few other books) that Linux malloc() uses the Linux system call sbrk() to request memory from the operating system.

I am looking at the glibc malloc.c code and I can see many mentions of sbrk() in the comments, but not referred to directly in the code.

How/where is sbrk() referred to/used when malloc() requests memory from the OS?

(This could be a general misunderstanding on my part of how system calls are made from the C runtime library. If so, I would be interested to know how they are made??)

Girth

While I'm not sure about the actual location of sbrk, there's nothing different between a system call in C and a function, with the exception that control is completely managed by the operating system until the system call completes. – Hemstitch

sbrk(2) tend to become rusty and obsolete. There are good reasons (multi-threading) to use mmap(2) only. – Sheelah

@BasileStarynkevitch could you briefly elaborate on such reasons? – Girth

Read the man page. sbrk is removed from latest Posix standard. – Sheelah

Answer 1

Glibc's malloc.c requests more memory by calling the function stored in the __morecore global function pointer (the call actually uses the macro MORECORE which expands to __morecore). By default, this holds the address of function __default_morecore, which is defined in morecore.c. This function calls sbrk.

Note that some malloc implementations may use mmap to get more memory instead of sbrk.

Doubletongue

Cool- have found it. The comment in morecore.c says: "Allocate INCREMENT more bytes of data space, and return the start of data space". So if I want 100 bytes of data I call malloc() which ends up calling the function in morecore.c, which calls sbrk() and this function returns a pointer to the beginning of the 100 bytes? Or does sbrk() get used before main() is called to allocate the whole heap for the process? – Girth

@Girth The heap size is not known in advance, so sbrk will be called by malloc. But it won't translate 1 to 1, because malloc will probably request larger blocks of memory from sbrk and then divide them, and it can also return previously freed memory to the caller without needing sbrk. – Doubletongue

So all sbrk does is to accept a request for memory, given a size and provide a pointer to it. Isnt this VERY similar to what malloc does()? In other words, what is the purpose of malloc() if sbrk does this? Just to put a load of error-handling in? – Girth

sbrk() doesn't know which parts of the memory are still in use, as there's no equivalent to free; malloc handles recycling of memory blocks that have been malloc()ed and free()d. – Isobar