/*************************************************************************\

*                  Copyright (C) Michael Kerrisk, 2022.                   *

*                                                                         *

* This program is free software. You may use, modify, and redistribute it *

* under the terms of the GNU General Public License as published by the   *

* Free Software Foundation, either version 3 or (at your option) any      *

* later version. This program is distributed without any warranty.  See   *

* the file COPYING.gpl-v3 for details.                                    *

\*************************************************************************/

​

/* procfs_user_exe.c

​

   Demonstrate the use of /proc/PID files to obtain information about

   processes on a Linux system. In this case, given a username, this

   program scans all /proc/PID/status files to produce a list of all

   processes running under a particular real user ID; for each process,

   the process ID and command that it executes are shown.

​

   This program is Linux-specific.

*/

#define _GNU_SOURCE

#include <limits.h>

#include <sys/stat.h>

#include <dirent.h>

#include <ctype.h>

#include <stdbool.h>

#include "ugid_functions.h"

#include "tlpi_hdr.h"

​

#define MAX_LINE 1000

​

int

main(int argc, char *argv[])

{

    if (argc < 2 || strcmp(argv[1], "--help") == 0)

        usageErr("%s username\n", argv[0]);

​

    uid_t checkedUid = userIdFromName(argv[1]);

    if (checkedUid == -1)

        cmdLineErr("Bad username: %s\n", argv[1]);

​

    DIR *dirp = opendir("/proc");

    if (dirp == NULL)

        errExit("opendir");

​

    /* Scan entries under /proc directory */

​

    for (;;) {

        errno = 0;              /* To distinguish error from end-of-directory */

        struct dirent *dp = readdir(dirp);

        if (dp == NULL) {

            if (errno != 0)

                errExit("readdir");

            else

                break;

        }

​

        /* Since we are looking for /proc/PID directories, skip entries

           that are not directories, or don't begin with a digit. */

​

        if (dp->d_type != DT_DIR || !isdigit((unsigned char) dp->d_name[0]))

            continue;

​

        char path[PATH_MAX];

        snprintf(path, PATH_MAX, "/proc/%s/status", dp->d_name);

​

        FILE *fp = fopen(path, "r");

        if (fp == NULL)

            continue;           /* Ignore errors: fopen() might fail if

                                   process has just terminated */

​

        bool gotName = false;

        bool gotUid = false;

        char line[MAX_LINE], cmd[MAX_LINE];

        uid_t uid;

        while (!gotName || !gotUid) {

            if (fgets(line, MAX_LINE, fp) == NULL)

                break;

​

            /* The "Name:" line contains the name of the command that

               this process is running */

​

            if (strncmp(line, "Name:", 5) == 0) {

                char *p;

                for (p = line + 5; *p != '\0' && isspace((unsigned char) *p); )

                    p++;

                strncpy(cmd, p, MAX_LINE - 1);

                cmd[MAX_LINE -1] = '\0';        /* Ensure null-terminated */

​

                gotName = true;

            }

​

            /* The "Uid:" line contains the real, effective, saved set-,

               and file-system user IDs */

​

            if (strncmp(line, "Uid:", 4) == 0) {

                uid = strtol(line + 4, NULL, 10);

                gotUid = true;

            }

        }

​

        fclose(fp);

​

        /* If we found a username and a UID, and the UID matches,

           then display the PID and command name */

​

        if (gotName && gotUid && uid == checkedUid)

            printf("%5s %s", dp->d_name, cmd);

    }

​

    exit(EXIT_SUCCESS);

}