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GETGROUPS(2)               Linux Programmer's ManualGETGROUPS(2)NAME       getgroups, setgroups - get/set list of supplementary group IDsSYNOPSIS       #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 (seefeature_test_macros(7)):       setgroups():           Since glibc 2.19:               _DEFAULT_SOURCE           Glibc 2.19 and earlier:               _BSD_SOURCEDESCRIPTION       getgroups()  returns the supplementary group IDs of the calling 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       callgetegid(2) and add or remove the resulting value.)       If  size is zero, list is not modified, but the total number of supple-       mentary 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  are required (see the description of the EPERM       error, below).  The size argument specifies the number of supplementary       group  IDs in the buffer pointed to by list.  A process can drop all of       its supplementary groups with the call:           setgroups(0, NULL);RETURN VALUE       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       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 (the caller  does              not  have  the  CAP_SETGID  capability  in the user namespace in              which it resides).       EPERM (since Linux 3.19)              The use of setgroups() is denied in this  user  namespace.   See              the description of /proc/[pid]/setgroups inuser_namespaces(7).CONFORMING TO       getgroups(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.       setgroups(): SVr4, 4.3BSD.  Since setgroups() requires privilege, it is       not covered by POSIX.1.NOTES       A process can have up to NGROUPS_MAX supplementary group IDs  in  addi-       tion 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 anexecve(2).       The  maximum number of supplementary group IDs can be found at run time       usingsysconf(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 supplemen-       tary 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  re-       quirements  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, seenptl(7).SEE ALSOgetgid(2),setgid(2),getgrouplist(3),group_member(3),initgroups(3),capabilities(7),credentials(7)COLOPHON       This page is part of release 5.05 of the Linux  man-pages  project.   A       description  of  the project, information about reporting bugs, and the       latest    version    of    this    page,    can     be     found     at       https://www.kernel.org/doc/man-pages/.Linux                             2019-03-06GETGROUPS(2)