进程的内存空间布局
进程的内存空间布局
- Kernel space：内核态内存空间，用户代码无法读写，否则会导致“段错误”的异常
- Random stack offset
- Stack：用户态的堆栈，大小不固定，可以用 ulimit -s 进行调整，默认一般为 8M，从高地址向低地址增长
- Random mmap offset
- Memory Mapping Segment（mmap）：共享内存，进程茫茫内存空间里的主要部分，既可以从高地址到低地址延伸(所谓 flexible layout)，也可以从低到高延伸(所谓 legacy layout)，看进程具体情况
- Program break（brk）
- Heap：程序运行过程中动态申请的内存
- Random brk offset
- BSS Segment：数据段，主要是进程里未初始化的全局数据总和，当然还有编译器生成一些辅助数据结构等等)，大小取决于具体进程，其位置紧贴着代码段
- Data Segment：数据段，主要是进程里已初始化的全局数据总和，当然还有编译器生成一些辅助数据结构等等)，大小取决于具体进程，其位置紧贴着代码段
- Text Segment（ELF， Executable and Linkable Format）：代码段，主要是进程的指令，包括用户代码和编译器生成的辅助代码，其大小取决于具体程序
一个C程序所使用的内存分析
//main.cpp
int a = 0; 全局初始化区
char *p1; 全局未初始化区
main()
{
int b; 栈
char s[] = "abc"; 栈
char *p2; 栈
char *p3 = "123456"; 123456\0在常量区，p3在栈上。
static int c =0； 全局（静态）初始化区
p1 = (char *)malloc(10);
p2 = (char *)malloc(20);
分配得来得10和20字节的区域就在堆区。
strcpy(p1, "123456"); 123456\0放在常量区。编译器可能会将它与p3所指向的"123456"优化成一个地方。
}
相关概念
- 虚拟内存 / Virtual Memory
- 页表 / Page Table
- 物理内存 / Physical Memory
- 驻留内存 / Resident Memory
- 共享内存 / Shared Memory
查看进程的内存占用
[root@055e4e6dc6db /]# top
top - 21:31:26 up 18 min, 0 users, load average: 0.08, 0.08, 0.07
Tasks: 4 total, 1 running, 3 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 2047036 total, 132976 free, 322092 used, 1591968 buff/cache
KiB Swap: 1048572 total, 1048572 free, 0 used. 1540976 avail
虚拟内存 驻留内存 共享内存
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 20 0 11784 2852 2560 S 0.0 0.1 0:00.12 bash
44 root 20 0 11784 3000 2576 S 0.0 0.1 0:00.05 bash
67 root 20 0 11784 3024 2596 S 0.0 0.1 0:00.23 bash
197 root 20 0 51912 3636 3112 R 0.0 0.2 0:00.00 top
[root@055e4e6dc6db /]# cat /proc/$$/maps
# 代码段 / Text Segment
00400000-004dd000 r-xp 00000000 08:01 131482 /usr/bin/bash
006dc000-006dd000 r--p 000dc000 08:01 131482 /usr/bin/bash
006dd000-006e6000 rw-p 000dd000 08:01 131482 /usr/bin/bash
# 数据段 / BSS Segment + Data Segment
006e6000-006ec000 rw-p 00000000 00:00 0
# 堆 / Heap
01eaa000-01eec000 rw-p 00000000 00:00 0 [heap]
# mmap
7f1df94cc000-7f1df94d8000 r-xp 00000000 08:01 923657 /usr/lib64/libnss_files-2.17.so
7f1df94d8000-7f1df96d7000 ---p 0000c000 08:01 923657 /usr/lib64/libnss_files-2.17.so
7f1df96d7000-7f1df96d8000 r--p 0000b000 08:01 923657 /usr/lib64/libnss_files-2.17.so
7f1df96d8000-7f1df96d9000 rw-p 0000c000 08:01 923657 /usr/lib64/libnss_files-2.17.so
7f1df96d9000-7f1df96df000 rw-p 00000000 00:00 0
7f1df96df000-7f1df9897000 r-xp 00000000 08:01 923591 /usr/lib64/libc-2.17.so
7f1df9897000-7f1df9a97000 ---p 001b8000 08:01 923591 /usr/lib64/libc-2.17.so
7f1df9a97000-7f1df9a9b000 r--p 001b8000 08:01 923591 /usr/lib64/libc-2.17.so
7f1df9a9b000-7f1df9a9d000 rw-p 001bc000 08:01 923591 /usr/lib64/libc-2.17.so
7f1df9a9d000-7f1df9aa2000 rw-p 00000000 00:00 0
7f1df9aa2000-7f1df9aa4000 r-xp 00000000 08:01 923601 /usr/lib64/libdl-2.17.so
7f1df9aa4000-7f1df9ca4000 ---p 00002000 08:01 923601 /usr/lib64/libdl-2.17.so
7f1df9ca4000-7f1df9ca5000 r--p 00002000 08:01 923601 /usr/lib64/libdl-2.17.so
7f1df9ca5000-7f1df9ca6000 rw-p 00003000 08:01 923601 /usr/lib64/libdl-2.17.so
7f1df9ca6000-7f1df9ccb000 r-xp 00000000 08:01 133586 /usr/lib64/libtinfo.so.5.9
7f1df9ccb000-7f1df9ecb000 ---p 00025000 08:01 133586 /usr/lib64/libtinfo.so.5.9
7f1df9ecb000-7f1df9ecf000 r--p 00025000 08:01 133586 /usr/lib64/libtinfo.so.5.9
7f1df9ecf000-7f1df9ed0000 rw-p 00029000 08:01 133586 /usr/lib64/libtinfo.so.5.9
7f1df9ed0000-7f1df9ef1000 r-xp 00000000 08:01 923580 /usr/lib64/ld-2.17.so
7f1dfa0e9000-7f1dfa0ec000 rw-p 00000000 00:00 0
7f1dfa0ee000-7f1dfa0f1000 rw-p 00000000 00:00 0
7f1dfa0f1000-7f1dfa0f2000 r--p 00021000 08:01 923580 /usr/lib64/ld-2.17.so
7f1dfa0f2000-7f1dfa0f3000 rw-p 00022000 08:01 923580 /usr/lib64/ld-2.17.so
# Random mmap offset
7f1dfa0f3000-7f1dfa0f4000 rw-p 00000000 00:00 0
# 栈 / Stack
7ffd8e07b000-7ffd8e09c000 rw-p 00000000 00:00 0 [stack]
7ffd8e0a6000-7ffd8e0a8000 r--p 00000000 00:00 0 [vvar]
7ffd8e0a8000-7ffd8e0aa000 r-xp 00000000 00:00 0 [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
# Random stack offset
# Kernel space
[root@055e4e6dc6db /]# cat /proc/1/smaps
00400000-004dd000 r-xp 00000000 08:01 131482 /usr/bin/bash
Size: 884 kB
Rss: 820 kB
Pss: 273 kB
Shared_Clean: 820 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 0 kB
Referenced: 820 kB
Anonymous: 0 kB
AnonHugePages: 0 kB
ShmemPmdMapped: 0 kB
Shared_Hugetlb: 0 kB
Private_Hugetlb: 0 kB
Swap: 0 kB
SwapPss: 0 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Locked: 0 kB
VmFlags: rd ex mr mw me dw
01a72000-01ab4000 rw-p 00000000 00:00 0 [heap]
Size: 264 kB
Rss: 208 kB
Pss: 208 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 208 kB
Referenced: 208 kB
Anonymous: 208 kB
AnonHugePages: 0 kB
ShmemPmdMapped: 0 kB
Shared_Hugetlb: 0 kB
Private_Hugetlb: 0 kB
Swap: 0 kB
SwapPss: 0 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Locked: 0 kB
VmFlags: rd wr mr mw me ac
7ffed73f5000-7ffed7416000 rw-p 00000000 00:00 0 [stack]
Size: 132 kB
Rss: 20 kB
Pss: 20 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 20 kB
Referenced: 20 kB
Anonymous: 20 kB
AnonHugePages: 0 kB
ShmemPmdMapped: 0 kB
Shared_Hugetlb: 0 kB
Private_Hugetlb: 0 kB
Swap: 0 kB
SwapPss: 0 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Locked: 0 kB
VmFlags: rd wr mr mw me gd ac
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
Size: 4 kB
Rss: 0 kB
Pss: 0 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 0 kB
Referenced: 0 kB
Anonymous: 0 kB
AnonHugePages: 0 kB
ShmemPmdMapped: 0 kB
Shared_Hugetlb: 0 kB
Private_Hugetlb: 0 kB
Swap: 0 kB
SwapPss: 0 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Locked: 0 kB
VmFlags: rd ex
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