getgroups

NAME top

getgroups, setgroups - get/set list of supplementary group IDs(取得/设置补充进程组列表ID)

SYNOPSIS top

#include <sys/types.h>
#include <unistd.h>

int getgroups(int size, gid_t list[]);

#include <grp.h>

int setgroups(size_t size, const gid_t *list);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

setgroups(): _BSD_SOURCE

DESCRIPTION top

getgroups() returns the supplementary group IDs of the calling

(getgroups()返回)
process in list.  The argument size should be set to the maximum
number of items that can be stored in the buffer pointed to by list.
If the calling process is a member of more than size supplementary
groups, then an error results.  It is unspecified whether the
effective group ID of the calling process is included in the returned
list.  (Thus, an application should also call getegid(2) and add or
remove the resulting value.)

If size is zero, list is not modified, but the total number of
supplementary group IDs for the process is returned.  This allows the
caller to determine the size of a dynamically allocated list to be
used in a further call to getgroups().

setgroups() sets the supplementary group IDs for the calling process.
Appropriate privileges (Linux: the CAP_SETGID capability) are
required.  The size argument specifies the number of supplementary
group IDs in the buffer pointed to by list.

RETURN VALUE top

On success, getgroups() returns the number of supplementary group
IDs.  On error, -1 is returned, and errno is set appropriately.

On success, setgroups() returns 0.  On error, -1 is returned, and
errno is set appropriately.

ERRORS top

EFAULT list has an invalid address.

getgroups() can additionally fail with the following error:

EINVAL size is less than the number of supplementary group IDs, but
is not zero.

setgroups() can additionally fail with the following errors:

EINVAL size is greater than NGROUPS_MAX (32 before Linux 2.6.4; 65536
since Linux 2.6.4).

ENOMEM Out of memory.

EPERM  The calling process has insufficient privilege (it does not
have the CAP_SETGID capability).

EPERM (since Linux 3.19)
The use of setgroups() is denied in this user namespace.  See
the description of /proc/[pid]/setgroups in
user_namespaces(7).

CONFORMING TO top

SVr4, 4.3BSD.  The getgroups() function is in POSIX.1-2001 and
POSIX.1-2008.  Since setgroups() requires privilege, it is not
covered by POSIX.1.

NOTES top

A process can have up to NGROUPS_MAX supplementary group IDs in
addition to the effective group ID.  The constant NGROUPS_MAX is
defined in <limits.h>.  The set of supplementary group IDs is
inherited from the parent process, and preserved across an execve(2).

The maximum number of supplementary group IDs can be found at run
time using sysconf(3):

long ngroups_max;
ngroups_max = sysconf(_SC_NGROUPS_MAX);

The maximum return value of getgroups() cannot be larger than one
more than this value.  Since Linux 2.6.4, the maximum number of
supplementary group IDs is also exposed via the Linux-specific read-
only file, /proc/sys/kernel/ngroups_max.

The original Linux getgroups() system call supported only 16-bit
group IDs.  Subsequently, Linux 2.4 added getgroups32(), supporting
32-bit IDs.  The glibc getgroups() wrapper function transparently
deals with the variation across kernel versions.

C library/kernel differences
At the kernel level, user IDs and group IDs are a per-thread
attribute.  However, POSIX requires that all threads in a process
share the same credentials.  The NPTL threading implementation
handles the POSIX requirements by providing wrapper functions for the
various system calls that change process UIDs and GIDs.  These
wrapper functions (including the one for setgroups()) employ a
signal-based technique to ensure that when one thread changes
credentials, all of the other threads in the process also change
their credentials.  For details, see nptl(7).

SEE ALSO top

getgid(2), setgid(2), getgrouplist(3), group_member(3),
initgroups(3), capabilities(7), credentials(7)

COLOPHON top

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