16. System and process information

/proc file system

Many modern UNIX The implementation provides a /proc Of Virtual file system . This file system is mainstream in /proc Directory , Contains various files for displaying kernel information , And it allows you to use regular files I/O To easily read , Sometimes you can also modify this information .

It is called virtual , Because the files and subdirectories it contains are not stored on disk , Instead, the kernel accesses such information when the process , Dynamically created .
Here are Linux Proprietary details .

Get information about the process ：/proc/PID

For each process in the system , The kernel is named /proc/PID（ among PID It's a process ID） The catalog of . Various files and subdirectories in this directory contain information about the process .
for example ：
/proc/1 The files in the directory can be obtained init Information about the process , Of the process ID Always 1.

$cat /proc/1/status
Name:	systemd
Umask:	0000
State:	S (sleeping)
Tgid:	1
Ngid:	0
Pid:	1
PPid:	0
...

Every /proc/PID An extract of the files in the directory

file	describe （ Process properties ）
cmdline	With \0 Delimited command line arguments
cwd	Symbolic link to the current working directory
Environ	NAME=value Key value pair environment list , With \0 Separate
exe	Symbolic link to the file being executed
fd	File directory , Contains symbolic links to files opened by the process
maps	Memory mapping
mem	Process virtual memory （ stay I/O Must call... Before operation lseek() Move to valid offset ）
mounts	Installation point of process
root	Symbolic link to the root directory
status	All kinds of information （ such as , process ID、 voucher 、 Memory usage 、 The signal ）
task	Include a subdirectory for each thread in the process （ Began in Linux2.6）

/proc/PID/fd Catalog

/proc/PID/fd Each file descriptor opened by the directory for the process contains a symbolic link , The name of each symbolic link matches the value of the descriptor . for example ,/proc/1968/1 yes ID by 1968 Symbolic links to standard output in the process of .
For convenience , Any process can use symbolic links /proc/self To access their own /proc/PID Catalog .

Threads ：/proc/PID/task Catalog

Linux2.4 Added the concept of thread group , Formal support POSIX Threading model .
Because some properties in the thread group are unique to threads , therefore Linux2.4 stay /proc/PID Added... To the directory
One task subdirectories .
For each thread of the process , The kernel provides /proc/PID/task/TID Named subdirectories , among TID Is the thread of this thread ID（ This value is equivalent to calling gettid() The return value of the function ）.

Every /proc/PID/task/TID Have a set similar to /proc/PID Files and directories of directory contents . Because threads share multiple properties , So much information in these files is the same for each thread .

/proc System information under directory

Catalog	Information expressed by files in the directory
/proc	All kinds of system information
/proc/net	Status information about networks and sockets
/proc/sys/fs	File system related settings
/proc/sys/kernel	Various general kernel settings
/proc/sys/net	Network and socket settings
/proc/sys/vm	Memory management settings
/proc/sysvipc	of System V IPC Object information

visit /proc file

Usually use shell Script to access /proc A file in a directory .
for example ：

# echo 100000 > /proc/sys/kernel/pid_max
# cat /proc/sys/kernel/pid_max
1000000

You can also use regular... From the program I/O System call to access /proc A file in a directory . Ann has some restrictions on access ：

• /proc Some files in the directory are read-only , That is, these files are only used to display kernel information , But it cannot be modified ./proc/PID Most files in the directory belong to this type .
• /proc Some files in the directory can only be used by the file owner （ Or privilege level process ） Read . for example ,/proc/PID All files in the directory belong to the user with the corresponding process , And even the owner of the file , Some of these documents （ Such as ：/proc/PID/environ file ） Only read permission is granted .
• except /proc/PID Files in subdirectories ,/proc Most of the other files in the directory belong to root user , And only root Users can modify files that can be modified .

visit /proc/PID Files in directory

/proc/PID The contents of the catalogue vary , With the corresponding process ID Created by the process of , With the termination of the process . During the interview , Deal with the situation that the process has been completed .

The sample program

/*proc.c*/
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>

#define MAX_LINE 100

int main(int argc, char **argv)
{
    
    int fd;
    char line[MAX_LINE];
    ssize_t n;
    fd = open("/proc/sys/kernel/pid_max", (argc > 1) ? O_RDWR : O_RDONLY);
    if( fd == -1)
    {
    
        printf("open");
        exit(EXIT_FAILURE);
    }
    n = read(fd, line, MAX_LINE);
    if( n == -1)
    {
    
        printf("read");
        exit(EXIT_FAILURE);
    }
    if(argc > 1)
        printf("Old value:");
    printf("%.*s", (int)n, line);
    if(argc > 1)
    {
    
        if(write(fd, argv[1], strlen(argv[1]) != strlen(argv[1])))
        {
    
            printf("write() failed");
            exit(EXIT_FAILURE);
        }
        system("echo /proc/sys/kernel/pid_max now contains"
                "cat /proc/sys/kernel/pid_max");
    }
    exit(EXIT_SUCCESS);
}

System identification : uname()

uname() The system call returns a series of identification information about the host system , Store in utsbuf In the structure pointed to .

#include <sys/utsname.h>
int uname(struct utsname *utsbuf);
	Returns 0 on success, or -1 on error

utsbuf The parameter is a point to utsname Pointer to structure , Its definition is as follows ：

#define _UTSNAME_LENGTH 65
struct utsname{
    
	char sysname[_UTSNAME_LENGTH]; /*Implementation name*/
	char nodename[_UTSNAME_LENGTH]; /*Node name on network*/
	char release[_UTSNAME_LENGTH]; /*Implementation release level*/
	char version[_UTSNAME_LENGTH]; /*Release version level*/
	char machine[_UTSNAME_LENGTH]; /*Haedware on which system is running*/
	
#ifdef _GNU_SOURCE /*Following is Linux-specific*/
	char domainname[_UTSNAME_LENGTH];/*NIS domain name of host*/
#endif
};

among ：

• sysname、release、version、machine Automatically set by the kernel .
• nodename from sethostname() System call settings . Usually this value is similar to the system DNS Prefix hostname in domain name .
• domainname from setdomainname() System call settings . This value is the network information service of the host
  （NIS） domain name （ Different from the host domain name ）.

The sample program ：

/*uname.c*/
#define _GNU_SOURCE
#include <sys/utsname.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
    
    struct utsname uts;
	if(uname(&uts) == -1)
	{
    
		printf("uname");
		exit(EXIT_FAILURE);
	}
    printf("Node name: %s\n", uts.nodename);
    printf("System name: %s\n", uts.sysname);
    printf("Release: %s\n", uts.release);
    printf("Version: %s\n", uts.version);
    printf("Machine: %s\n", uts.machine);
#ifdef _GNU_SOURCE
    printf("Domain name: %s\n", uts.domainname);
#endif
    exit(EXIT_SUCCESS);
}

$gcc uname.c -o uname
$./uname
Node name:  sixqaq
System name:    Linux
Release:    5.13.0-27-generic
Version:    #29~20.04.1-Ubuntu SMP Fri Jan 14 00:32:30 UTC 2022
Machine:    x86_64
Domain name:    (none)

Advanced

About /proc In depth information about file systems can be found in proc(5) Manual page 、 Kernel source files Documentation/filesystems/proc.txt as well as Documentation/sysctl Various files in the directory .