Question

Using linux; how would you construct a void function using strfttime() to display time. Also a function using user_from_uid and group_from_gid to show the users name or group name. Trying to recreate the ls -l command.

Thank you in advance!

Answer 1

#include <stdio.h>
#include <time.h>// to use time_t,struct tm,time(),localtime(),strftime()
#include <pwd.h>// to use struct passwd,getpwuid()
#include <grp.h>// to use struct group,getgrgid()

// here we can return a string instead of printing it in the function,
// since you requested to make it void, i have done so
void get_time(void);

// returns the user name from user id
char* user_from_uid(uid_t);

// returns the group name from group id
char* group_from_gid(int);

int main () {
   get_time();
   printf("%s\n",user_from_uid(1000));
   printf("%s\n",group_from_gid(1000));
   return(0);
}

void get_time(void){
   // to store raw time
   time_t rawtime;
   // to store local time
   struct tm *time_info;
   // to store formated time string
   char time_formated_string[80];
   // gets the rawtime
   time( &rawtime );
   // converts to localtime
   time_info = localtime( &rawtime );
   // converts to ls -l's format Eg: Oct 28 14:07
   strftime(time_formated_string,80,"%b %d %H:%M", time_info);
   // prints the formatted time
   printf("%s\n", time_formated_string );
}

char* user_from_uid(uid_t uid)
{
    // to store current pws
    struct passwd *pws;
    // gets current pws from given pid
    pws = getpwuid(uid);
    // prints the pw_name of current pws
    return pws->pw_name;
}

char* group_from_gid(int gid)
{
    // to store current group
    struct group *g;
    // gets the current group from given gid
    g = getgrgid(gid);
    // returns the group name
    return g->gr_name;
}

Code Screenshot:

Output:(blured the username and groupname due to some privacy issues)

Edited code(changed the get_time() function to return string):

#include <stdio.h>
#include <time.h>// to use time_t,struct tm,time(),localtime(),strftime()
#include <pwd.h>// to use struct passwd,getpwuid()
#include <grp.h>// to use struct group,getgrgid()
#include <stdlib.h>// to use malloc

// returns the time string
char* get_time(void);

// returns the user name from user id
char* user_from_uid(uid_t);

// returns the group name from group id
char* group_from_gid(int);

int main () {
   printf("%s\n",get_time());
   printf("%s\n",user_from_uid(1000));
   printf("%s\n",group_from_gid(1000));
   return(0);
}

char* get_time(void){
   // to store raw time
   time_t rawtime;
   // to store local time
   struct tm *time_info;
   // to store formated time string
   char *time_formated_string = malloc(80*sizeof(char));
   // gets the rawtime
   time( &rawtime );
   // converts to localtime
   time_info = localtime( &rawtime );
   // converts to ls -l's format Eg: Oct 28 14:07
   strftime(time_formated_string,80,"%b %d %H:%M", time_info);
   // returns the formatted time as string
   return time_formated_string;
}

char* user_from_uid(uid_t uid)
{
    // to store current pws
    struct passwd *pws;
    // gets current pws from given pid
    pws = getpwuid(uid);
    // prints the pw_name of current pws
    return pws->pw_name;
}

char* group_from_gid(int gid)
{
    // to store current group
    struct group *g;
    // gets the current group from given gid
    g = getgrgid(gid);
    // returns the group name
    return g->gr_name;
}

Code screenshot:

PS: If you have any problems/doubts please comment below.Thank You