Setting the Group IDs
=====================
This section describes the functions for altering the group IDs (real
and effective) of a process. To use these facilities, you must include
the header files `sys/types.h' and `unistd.h'.
- Function: int setegid (gid_t NEWGID)
This function sets the effective group ID of the process to
NEWGID, provided that the process is allowed to change its group
ID. Just as with `seteuid', if the process is privileged it may
change its effective group ID to any value; if it isn't, but it
has a file group ID, then it may change to its real group ID or
file group ID; otherwise it may not change its effective group ID.
Note that a process is only privileged if its effective _user_ ID
is zero. The effective group ID only affects access permissions.
The return values and error conditions for `setegid' are the same
as those for `seteuid'.
This function is only present if `_POSIX_SAVED_IDS' is defined.
- Function: int setgid (gid_t NEWGID)
This function sets both the real and effective group ID of the
process to NEWGID, provided that the process is privileged. It
also deletes the file group ID, if any.
If the process is not privileged, then `setgid' behaves like
`setegid'.
The return values and error conditions for `setgid' are the same
as those for `seteuid'.
- Function: int setregid (gid_t RGID, gid_t EGID)
This function sets the real group ID of the process to RGID and
the effective group ID to EGID. If RGID is `-1', it means not to
change the real group ID; likewise if EGID is `-1', it means not
to change the effective group ID.
The `setregid' function is provided for compatibility with 4.3 BSD
Unix, which does not support file IDs. You can use this function
to swap the effective and real group IDs of the process.
(Privileged processes are not limited to this usage.) If file IDs
are supported, you should use that feature instead of using this
function. Note:Enable/Disable Setuid.
The return values and error conditions for `setregid' are the same
as those for `setreuid'.
`setuid' and `setgid' behave differently depending on whether the
effective user ID at the time is zero. If it is not zero, they behave
like `seteuid' and `setegid'. If it is, they change both effective and
real IDs and delete the file ID. To avoid confusion, we recommend you
always use `seteuid' and `setegid' except when you know the effective
user ID is zero and your intent is to change the persona permanently.
This case is rare--most of the programs that need it, such as `login'
and `su', have already been written.
Note that if your program is setuid to some user other than `root',
there is no way to drop privileges permanently.
The system also lets privileged processes change their supplementary
group IDs. To use `setgroups' or `initgroups', your programs should
include the header file `grp.h'.
- Function: int setgroups (size_t COUNT, gid_t *GROUPS)
This function sets the process's supplementary group IDs. It can
only be called from privileged processes. The COUNT argument
specifies the number of group IDs in the array GROUPS.
This function returns `0' if successful and `-1' on error. The
following `errno' error conditions are defined for this function:
`EPERM'
The calling process is not privileged.
- Function: int initgroups (const char *USER, gid_t GROUP)
The `initgroups' function sets the process's supplementary group
IDs to be the normal default for the user name USER. The group
GROUP is automatically included.
This function works by scanning the group database for all the
groups USER belongs to. It then calls `setgroups' with the list it
has constructed.
The return values and error conditions are the same as for
`setgroups'.
If you are interested in the groups a particular user belongs to,
but do not want to change the process's supplementary group IDs, you
can use `getgrouplist'. To use `getgrouplist', your programs should
include the header file `grp.h'.
- Function: int getgrouplist (const char *USER, gid_t GROUP, gid_t
*GROUPS, int *NGROUPS)
The `getgrouplist' function scans the group database for all the
groups USER belongs to. Up to *NGROUPS group IDs corresponding to
these groups are stored in the array GROUPS; the return value from
the function is the number of group IDs actually stored. If
*NGROUPS is smaller than the total number of groups found, then
`getgrouplist' returns a value of `-1' and stores the actual
number of groups in *NGROUPS. The group GROUP is automatically
included in the list of groups returned by `getgrouplist'.
Here's how to use `getgrouplist' to read all supplementary groups
for USER:
gid_t *
supplementary_groups (char *user)
{
int ngroups = 16;
gid_t *groups
= (gid_t *) xmalloc (ngroups * sizeof (gid_t));
struct passwd *pw = getpwnam (user);
if (pw == NULL)
return NULL;
if (getgrouplist (pw->pw_name, pw->pw_gid, groups, &ngroups) < 0)
{
groups = xrealloc (ngroups * sizeof (gid_t));
getgrouplist (pw->pw_name, pw->pw_gid, groups, &ngroups);
}
return groups;
}