System Call 扒开系统调用的三层皮[未完待续~]

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System Call 扒开系统调用的三层皮

刘子健 +

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《Linux内核分析》MOOC课程http://mooc.study.163.com/course/USTC-1000029000 ”

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首先，特别感谢孟宁老师提供了这么好的课程资源．

同样对所有contribute to Mykernel的前辈致敬！

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先推荐本书吧 <<LKD>> Linux kernel development -- by Robert Love

In any modern operating system, the kernel provides a set of interfaces by which processes running in user-space can interact with the system.These interfaces give applications controlled access to hardware, a mechanism with which to create new processes and communicate with existing ones, and the capability to request other operating system resources.

The interfaces act as the messengers between applications and the kernel, with the applications issuing various requests and the kernel fulfilling them (or returning an error).The existence of these interfaces, and the fact that applications are not free to directly do whatever they want, is key to providing a stable system.

本文打算扒一扒大家熟悉的一些库函数,比方说printf(及时间再扒别的)

Typically, applications are programmed against an Application Programming Interface (API) implemented in user-space, not directly to system calls.This is important because no direct correlation is needed between the interfaces that applications make use of and the actual interface provided by the kernel. An API defines a set of programming interfaces used by applications.

在"大探险"之前,先讲讲我们的"目的地" -- 系统调用 system call

在include/linux/syscalls.h 里面你会看到有这么个宏定义.

这是链接系统调用和库函数的关键宏定义

开扒

...

骚年,千万不要浪费了三个小时去看内核里面的那个printf...我去...

你调用的是C标准库的函数啊!!不是内核函数啊! 虽然内核也实现了...

但是...我们还是按照实际的调用路线一层层的扒吧....

没有GNU libc代码的去download一份.

http://www.gnu.org/software/libc/

然后你会看到lib c的目录如下.

这里还有个很爽的工具, strace 命令. 可以看看下面这个链接, 怎么玩strace

http://blog.csdn.net/cinmyheart/article/details/38945847

但是strace也只能大概的跟踪到调用了哪些动态库,以及系统调用API....不过话说这已经很不错了.

认真分析下面的这幅贴图!!

a.out就是一个很简单的hello world可执行程序. 会发现, 首先调用了API, execve去执行这个可执行文件,

接着调用brk()申请了0块大小的堆, 于是,这里堆地址在0x73f00.

后面access这里我不是很清楚,应该是检查链接动态库的权限是否OK

然后mmap进行内存的映射 .

重温一下mmap():

 #include <sys/mman.h>void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset);int munmap(void *addr, size_t length)

mmap() creates a new mapping in the virtual address space of the calling process. The starting address for the new mapping is specified in addr. The length argument specifies the length of the mapping.