The Address Space

The traditional organization of the virtual address space (as seen from user space, on x86 systems) is as shown in the diagram to the right. The very bottom part of the address space is unused; it is there to catch NULL pointers and such. Starting at 0x8000000 is the program text - the read-only, executable code. The text is followed by the heap region, being the memory obtainable via the brk() system call. Typically functions like malloc() obtain their memory from this area; non-automatic program data is also stored there.

The heap differs from the first two regions in that it grows in response to program needs. A program like cat will not make a lot of demands on the heap (one hopes), while running a yum update can grow the heap in a truly disturbing way. The heap can expand up to 1GB (0x40000000), at which point it runs into the mmap area; this is where shared libraries and other regions created by the mmap() system call live. The mmap area, too, grows upward to accommodate new mappings.

Meanwhile, the kernel owns the last 1GB of address space, up at 0xc0000000. The kernel is inaccessible to user space, but it occupies that portion of the address space regardless. Immediately below the kernel is the stack region, where things like automatic variables live. The stack grows downward. On a really bad day, the stack and the mmap area can run into each other, at which point things start to fail.

(Thanks to http://lwn.net/Articles/91829/).