# Chapter 7 Linking

- satic linking
- dynamic linking of shared libraries at load time
- dynamic linking of shared libraries at run time

C preprocessor(cpp)
```sh
cpp main.c tmp/main.i
gcc -E main.c
```
C compiler(cc1)
```sh
gcc -S main.c tmp/main.s
```
Assembler(as)
```sh
as  -o tmp/main.o tmp/main.s
gcc -c sum.c -o tmp/sum.o
```
Linker(ld)
```sh
ld -o tmp/prog  tmp/main.o tmp/sum.o
gcc -o tmp/prog tmp/main.o tmp/sum.o
```

# Chapter 10 System-Level IO

Input/output(I/O) is the process of copying data between main memory and external devices such as disk drives, terminals, and networks.
**An input operaton** copies data from an I/O device to main memory
**An output operation** copies data from memory to a device
```
					 Unix I/O
						|
						|derived from
						|
  ANSI C standrad I/O library      C++ standrand I/O library
  e.g. printf scanf                  overloaded >> << operator
```

Each Linux **file** has a **type**
- A *regular file* contains arbitrary data. Applications often distingush between *text files* and *binary files*. To the kernel, there is no difference.
- A *directory* is a file consisting of an array of links, where each link maps a *filename* to a file, which maybe another directory.
- A *socket* is a file that is used to communicate with another process across a network
- other *named pipes, symbolic links, and characterfiles* beyond our scope

The kernel represents open files using three related data structures:
- Descriptor table
- File table
- v-node table
```
                                        Open file table           v-node table
      Descriptor table                  (shared by                (shared by
      (one table per process)           all processes)            all processes)

                                          File A                    +-----------+
stdin  fd0+---+                          +--------+                 |File access|
stdout fd1|----------------------------> |        +---------------->+File size  |
stderr fd2|---|                          |File pos|                 |File type  |
       fd3|---|                          |refcnt=1|                 |...        |
       fd4|--------------+               |        |                 +-----------+
       fd5+---+          |               |        |
                         |               +--------+
                         |
                         +---------------v File B
                                         +--------+                 +-----------+
                                         |        +---------------->+File access|
                                         |File pos|                 |File size  |
                                         |refcnt=1|                 |File type  |
                                         |        |                 |...        |
                                         |        |                 +-----------+
                                         +--------+
```
```
                       +--------------+
                       |fopen fdopen  |
                       |fread fwrite  |
                       |fscanf fprintf|
Standard I/O functoins +--------------+

          ^
          |
          |         +------------+
                    |open read   |
 Unix I/O functions |write lseek |
                    |stat close  |
                    +------------+
```
# chapter 11 Network Programming