Name

The setgrent return the value 1 if successful, otherwise the value 0 is returned. The endgrent API has no return value.

We refer to the structure group found in the include file

#include <grp.h> struct group {         char    *gr_name;       /* group name */         char    *gr_passwd;     /* group password */         gid_t   gr_gid;         /* group id */         char    **gr_mem;       /* group members */ }; The functions getgrnam and getgrgid returns the default values for a single user system i.e., gr_name,gr_passwd,gr_gid set to root,NULL and 0 respectively and gr_mem has just one entry,that of root as a single user in case the input parameter gid/name is 0/ root else the error number is set and NULL is returned by the APIs. The getgrent when called the first time by an application returns the pointer to a valid group, that of the simulated single-user group.Subsequent calls return NULL until setgrent is called The functions getgrnam_r, and getgrgid_r are thread-safe versions of getgrnam, and getgrgid respectively. The caller must provide storage for the results of the search in the grp, buffer, bufsize, and result arguments. When these functions are successful, the grp argument will be filled-in, and a pointer to that argument will be stored in result. If an entry is not found or an error occurs, result will be set to NULL. The setgrent function sets the library such that subsequent getgrent call returns a filled group structure. The endgrent function sets up the library such that the subsequent getgrent call returns NULL. The setgroups API returns success status just for the sake of build support.The actual function cannot be supported.

#include <grp.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
/*A user function that takes gid as parameter to call getgrgid
Can be replaced by function that takes char* grnam to call getgrnam*/
void grgid_user(int gid)
{
        struct group* gr_gid;   /*This struct is filled by the getgrgid call*/
        setgrent ();            /*Set read to start of database*/
        /*Call to getgrgid with gid as parameter..could also be replaced with getgrnam
        with grnam as parameter*/
        gr_gid = getgrgid (gid);
        if(gr_gid == NULL)      /*End of database reached*/
                return;
        else
                printf ("For gid = %d ,group name = %s", gid, gr_gid->gr_name); /*Print a valid group name*/
        endgrent ();            /*Set the read pointer to the end of database*/
        /*In case of our library,the user has to free the resources allocated
        by the API call itself*/
        free (gr_gid->gr_mem);
        free (gr);
}

Output

For gid = 0 ,group name = root

#include <grp.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
/*A user function that takes char* grnam as parameter to call getgrnam_r
Can be replaced by function that takes int gid to call getgrgid_r*/
void grgid_r_user(char* grnam)
{
        /*We have to allocate memory for the structures for the getgrnam_r thread-safe call*/
        struct group* gr_nam = (struct group*)malloc (sizeof (struct group));
        gr_nam->gr_mem = (char**)malloc (sizeof(char**));
        char* buffer=(char*)malloc(sizeof(struct group));
        size_t bufsize=sizeof(buffer);
        struct group* tempresult;
        int result;     
        setgrent ();            /*Set read to start of database*/
        /*Call to getgrnam_r with grnam as parameter..could also be replaced with getgrgid_r
        with grgid as parameter*/
        int result=getgrnam_r(grnam, gr_nam, buffer, bufsize, &tempresult);
        if(result != 0) /*End of database reached*/
                return;
        else
                printf ("For grnam = %s ,group id = "%d", grnam, gr_nam->gr_gid); /*Print a valid group name*/
        endgrent ();            /*Set the read pointer to the end of database*/
        /*The user has to free the resources allocated*/
        free(buffer);   
        free (gr_gid->gr_mem);
        free (gr);
}

Output

For grnam = root ,group id = 0

#include <grp.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
/*A user function that takes calls getgrent API*/
void getgrent_user()
{
        struct group* gr_ent;   /*This struct is filled by the getgrent call*/
        setgrent ();            /*Set read to start of database*/
        /*Call to getgrent */
        gr_ent = getgrent ();
        if(gr_ent == NULL)      /*End of database reached*/
                return;
        else
                /*Print a valid group name*/
                printf ("gid = %d ,group name = %s", gr_ent->gid, gr_ent->gr_name);
        endgrent ();            /*Set the read pointer to the end of database*/
        /*In case of our library,the user has to free the resources allocated
        by the API call itself*/
        free (gr_ent->gr_mem);
        free (gr_ent);
}

Output

gid = 0 ,group name = root

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