1, / proc file system

The / proc virtual file system provides convenience for accessing kernel information. The file system resides in the / proc directory, contains various files used to display kernel information, and allows processes to easily read and sometimes modify these information through regular file I/O system calls. The reason why the / proc file

The system is called virtual because the files and subdirectories it contains are not stored on disk, but are dynamically created by the kernel when the process accesses such information.

For each process in the system, the kernel provides a corresponding directory named / proc/PID, where PID is the ID of the process. Various files and subdirectories in this directory contain information about the process. For example, by viewing the files in the / proc/1 directory, you can obtain the information of the init process, whose ID is always 1.

book@100ask:/proc/1$ sudo ls
[sudo] password for book: 
arch_status	 cpuset   loginuid    numa_maps      sched	   status
attr		 cwd	  map_files   oom_adj	     schedstat	   syscall
autogroup	 environ  maps	      oom_score      sessionid	   task
auxv		 exe	  mem	      oom_score_adj  setgroups	   timers
cgroup		 fd	  mountinfo   pagemap	     smaps	   timerslack_ns
clear_refs	 fdinfo   mounts      patch_state    smaps_rollup  uid_map
cmdline		 gid_map  mountstats  personality    stack	   wchan
comm		 io	  net	      projid_map     stat
coredump_filter  limits   ns	      root	     statm

There is a file named status in each / proc/PID directory, which provides a series of information about the process.

/Common files in proc/pid Directory:

To access the proc file:

Shell scripts are usually used to access files in the / proc directory (using scripting languages such as Python or Perl, it is easy to parse most files with multiple values in the / proc directory). For example, you can modify and view the file contents in the / proc directory by using the following shell command:

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

You can also use regular I/O system calls from the program to access files in the / proc directory. However, there are some restrictions when accessing these files.

① Some files in the / proc directory are read-only, that is, these files are only used to display kernel information, but they cannot be modified/ Most files in the proc/PID directory belong to this type.

② Some files in the / proc directory can only be read by the file owner (or privilege level process). For example, all files in the / proc/PID directory belong to the user with the corresponding process, and even for the owner of the file, some files (such as the proc/PID/environ file) are only granted read permission.

③ In addition to the files in the / proc/PID subdirectory, most of the other files in the / proc directory belong to the root user, and only the root user can modify those modifiable files.

int fd；
char line[100];
fd= open("/proc/sys/kernel/pid_max",O_RDWR);
read(fd,line,100);
write(fd,"1000",5);

System ID: uname

uname system call returns a series of identification information about the host system, which is stored in the structure pointed to by buf.

#include <sys/utsname.h>
int uname(struct utsname *buf);

struct utsname {
    char sysname[];    /* Operating system name (e.g., "Linux") */
    char nodename[];   /* Name within "some implementation-defined
                                     network" */
    char release[];    /* Operating system release (e.g., "2.6.28") */
    char version[];    /* Operating system version */
    char machine[];    /* Hardware identifier */
    #ifdef _GNU_SOURCE
    char domainname[]; /* NIS or YP domain name */
    #endif
};

Note: refer to the Linux UNIX system programming manual