How can I print all PID from /proc/ using C code?-CodePudding

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <ctype.h>
#include <fcntl.h>

#define PATH_LINE 128

void get_pid(){
    char full_path[PATH_LINE]="";
    
    int fd;
    char pid[6];

    for(int i=0 ;i <99999; i  ){
        fd =0;
        memset(full_path,0,PATH_LINE);

        strcat(full_path,PROC_PATH);
        sprintf(pid, "%d", i);
        strcat(full_path,pid);

        fd = open(full_path, O_RDONLY);
        if(fd != -1){
            printf("%s - fd [%d]\n", full_path, fd);
        }
    }
}

int main(void){
    get_pid();

    return 0;
}

I expected this code to print all the PIDs I have on my computer.

That is, it will try to open the folder /proc/PID and if it does not return an error, then it will print the PID.

But, he prints me a lot of PIDs that don't really run on my computer...

Is there an error in my code? I'm trying to do this without built-in structs.

terminal output:

CodePudding user response：

I saw one serious problem here. First of all, it is easier for us to view all processes with "find /proc/ -maxdepth 1 -type d 2>/dev/null" this command will show you all the processes in /proc/. I ran your code and saw that you are opening files and never close them. I added one line "close(fd);" inside the if statement and I got all the processes that run on my machine. In my opinion, this will fix your problem.

P.S. There is a limit to opened files from your program. Not closing opened files in for loop is what you missed here. You can check what is it with: "ulimit -n".

CodePudding user response：

I would not loop from 0 to 99998 and try to open all those. It takes time and it misses processes with greater pids than 99998.

I suggest that you use opendir, readdir and closedir in order to read all directory entries in /proc and to filter out those you are interested in.

Example:

#include <dirent.h>     // opendir, readdir, closedir
#include <sys/types.h>

#include <ctype.h>
#include <stdbool.h>
#include <stdio.h>

// a function to check if a string consists of all digits
bool all_digits(const char *str) {
    for(; *str != '\0';   str) {
        if(!isdigit((unsigned char)*str)) {
            return false;
        }
    }
    return true;
}

int main(void) {
    DIR* dp = opendir("/proc");             // open /proc
    if(dp) {
        struct dirent *curr;                // see: man 3 readdir

        while((curr = readdir(dp))) {       // read next directory entry
            if(all_digits(curr->d_name)) {  // d_name holds the leaf name
                puts(curr->d_name);         // only print if it only has digits
            }
        }

        closedir(dp);                       // finally close the directory
    }
}