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Memory Management and Process Image Overview

Memory managementOverview with process image

existoperating systemand in programming, memory management is crucial, which directly affects the program'sperformanceand stability. The following is a summary of the memory management mechanisms of the user-layer and system-layer, as well as process images andVirtual memoryrelated concepts.

1. Memory management

The process of memory management can be divided into user layer and system layer:

User layer

  1. STL (Standard Template Library)

    • Automatically allocate/release memory: Automatic memory management through STL in C++.

  2. C++

    • new/delete: Memory allocation and release mechanism in C++, the underlying call to the C language memory management functions.

  3. C

    • malloc/free: Memory management functions in standard C library, calling POSIX or Linux kernel functions for memory allocation and release.

  4. POSIX

    • brk/sbrk: Used to adjust the end of the data segment to expand or reduce the heap memory.

  5. Linux

    • mmap/munmap: Used to map and unmap virtual memory addresses to physical memory.

System layer

  1. Kernel (Kernel)

    • kmalloc/vmalloc: Used to allocate and free memory in the kernel.

  2. drive

    • get_free_page: Used to allocate memory pages in the driver.

2. Process image

The executable file stored on the disk of the program. When the program is executed, the system loads it into memory to form a process. The memory distribution of a process is called the process image, from the low address to the high address is:

  1. text(Code Snippet): Stores binary instructions and constant data.
  2. data(Data segment): Initialized global variables and static local variables.
  3. bss(static data segment): Uninitialized global variables and static local variables, automatically cleared before the program runs.
  4. heap(Heap): A large amount of data manually managed by programmers.
  5. stack(Stack): Store local variables and block variables.
  6. environ(Environment Variable Table): Store environment variables, each process has one, and it does not affect each other.
  7. argv(Command line parameters): Stores command line parameters when the program is running.

practise

Define the data of each memory segment and print their addresses, with/proc/[process id]/mapsCompare the corresponding memory records in the file.

3. Virtual memory

Key points of virtual memory:

  1. Each process allocates 4GB of virtual memory space.
  2. Users can only use virtual memory and cannot directly access physical memory.
  3. The mapping between virtual addresses and physical memory is dynamically maintained by the operating system.
  4. Only after the virtual address is mapped to physical memory can it be used, otherwise a segfault will occur.
  5. The 4GB virtual address space is divided into user space (0-3GB) and kernel space (3-4GB).
  6. User code cannot directly access the kernel space and needs to be accessed indirectly through system calls.

4. Mapping virtual memory and physical memoryfunction

sbrk and brk

  • sbrk(intptr_t increment): Adjust the position of the pointer according to the increment, and can be mapped or unmapping.
  • brk(void *addr): Directly modify the position of the pointer for memory mapping or unmapping.

mmap and munmap

  • mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset): Map virtual memory and physical memory.
  • munmap(void *addr, size_t length): Unmap.

Summarize

  1. sbrk/brkMemory mapping and unmapping are implemented through pointer movement, which belongs to heap memory.
  2. mmap/munmapNo state is maintained, map directly or unmap.
  3. malloc/freeThe underlying call wassbrk/brkormmap/munmap。
  4. Only after the virtual address is mapped can physical memory be accessed, and the system dynamically manages the mapping.

Understanding memory management mechanisms is essential to optimizing and debugging programs, especially in high-performance or resource-constrained systems.