Introduction to system programming

One 、 What is system programming ？

System programming is the use of system calls system call Programming

Two 、 System programming features ？

1. Unable to cross platform （OS）

2. Slow speed , Switching from user space to internal control space takes time

3. It's much lower , The interface is more complex

3、 ... and 、Linux Programming

1. Standard library functions and system calls

①fopen(3)

fopen(2) Open the specified file , Returns a file descriptor （ It's just one. int Type number ）, Allocate one FILE Structure , It contains the descriptor of the file ,I/O Buffer and current read / write location , Go back to this FILE The address of the structure .

②fgetc(3)

Through the introduction of FILE * Parameter to find the descriptor of the file 、I/O Buffer and current read / write location , Judge whether or not from I/O Next character read in buffer , If you can read it, you can directly return this character , Otherwise, call read(2), Pass in the file descriptor , Let the kernel read the data of this file to I/O buffer , Then return to the next character .

③fputc(3)

Judge the file I/O Whether the buffer has space for another character , If there is space, save it directly in I/O Buffer and return , If I/O Call when the buffer is full write(2), Let the kernel I/O The contents of the buffer are written back to the file .

#include <stdio.h>

int main(void) {
    int i;
    for (int i = 0; i < 1025; i++) {
        fputc('A', stdout);
    }
    return 0;
}

④fclose(4)

If I/O There is still data in the buffer that has not been written back to the file , Just call write(2) Write back the file , And then call close(2) Close file , Release FILE Structure and I/O buffer .

2. Standard library functions and system calls

Each process is in Linux There is one in the kernel task_struct Structure to maintain process related information , Called process control block (PCB,Process Control Block).taks_struct There is a pointer to files_struct Structure , It is called descriptor table , Each of these entries contains a pointer to an open file , User programs cannot directly access the descriptor table of files in the kernel , Only the index of the file descriptor table can be used , These indexes are called file descriptors , use int Type variable saving .

Handle idea .

Three files will be opened automatically when the program starts ：

① The standard input ② standard output ③ Standard error output .

stay C Used separately in the standard library FILE * The pointer stdin,stdout and stderr Express . The descriptors of these three files are 0,1,2, Save in the corresponding FILE In the structure . The header file unistd.h There is a macro definition of if in to represent this file descriptor ：

 

#include <stdio.h>
#include <unistd.h>

int main(void) {

    printf("%d\n",STDIN_FILENO);
    printf("%d\n",STDOUT_FILENO);
    printf("%d\n",STDERR_FILENO);
    return 0;
}