getgrgid(3p) — Linux manual page


GETGRGID(3P)            POSIX Programmer's Manual           GETGRGID(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME         top

       getgrgid, getgrgid_r — get group database entry for a group ID

SYNOPSIS         top

       #include <grp.h>

       struct group *getgrgid(gid_t gid);
       int getgrgid_r(gid_t gid, struct group *grp, char *buffer,
           size_t bufsize, struct group **result);

DESCRIPTION         top

       The getgrgid() function shall search the group database for an
       entry with a matching gid.

       The getgrgid() function need not be thread-safe.

       Applications wishing to check for error situations should set
       errno to 0 before calling getgrgid().  If getgrgid() returns a
       null pointer and errno is set to non-zero, an error occurred.

       The getgrgid_r() function shall update the group structure
       pointed to by grp and store a pointer to that structure at the
       location pointed to by result.  The structure shall contain an
       entry from the group database with a matching gid.  Storage
       referenced by the group structure is allocated from the memory
       provided with the buffer parameter, which is bufsize bytes in
       size. A call to sysconf(_SC_GETGR_R_SIZE_MAX) returns either -1
       without changing errno or an initial value suggested for the size
       of this buffer.  A null pointer shall be returned at the location
       pointed to by result on error or if the requested entry is not

RETURN VALUE         top

       Upon successful completion, getgrgid() shall return a pointer to
       a struct group with the structure defined in <grp.h> with a
       matching entry if one is found. The getgrgid() function shall
       return a null pointer if either the requested entry was not
       found, or an error occurred. If the requested entry was not
       found, errno shall not be changed. On error, errno shall be set
       to indicate the error.

       The application shall not modify the structure to which the
       return value points, nor any storage areas pointed to by pointers
       within the structure. The returned pointer, and pointers within
       the structure, might be invalidated or the structure or the
       storage areas might be overwritten by a subsequent call to
       getgrent(), getgrgid(), or getgrnam().  The returned pointer, and
       pointers within the structure, might also be invalidated if the
       calling thread is terminated.

       If successful, the getgrgid_r() function shall return zero;
       otherwise, an error number shall be returned to indicate the

ERRORS         top

       The getgrgid() and getgrgid_r() functions may fail if:

       EIO    An I/O error has occurred.

       EINTR  A signal was caught during getgrgid().

       EMFILE All file descriptors available to the process are
              currently open.

       ENFILE The maximum allowable number of files is currently open in
              the system.

       The getgrgid_r() function may fail if:

       ERANGE Insufficient storage was supplied via buffer and bufsize
              to contain the data to be referenced by the resulting
              group structure.

       The following sections are informative.

EXAMPLES         top

       Note that sysconf(_SC_GETGR_R_SIZE_MAX) may return -1 if there is
       no hard limit on the size of the buffer needed to store all the
       groups returned. This example shows how an application can
       allocate a buffer of sufficient size to work with getgrid_r().

           long int initlen = sysconf(_SC_GETGR_R_SIZE_MAX);
           size_t len;
           if (initlen == -1)
               /* Default initial length. */
               len = 1024;
               len = (size_t) initlen;
           struct group result;
           struct group *resultp;
           char *buffer = malloc(len);
           if (buffer == NULL)
               ...handle error...
           int e;
           while ((e = getgrgid_r(42, &result, buffer, len, &resultp)) == ERANGE)
               size_t newlen = 2 * len;
               if (newlen < len)
                   ...handle error...
               len = newlen;
               char *newbuffer = realloc(buffer, len);
               if (newbuffer == NULL)
                   ...handle error...
               buffer = newbuffer;
           if (e != 0)
               ...handle error...
           free (buffer);

   Finding an Entry in the Group Database
       The following example uses getgrgid() to search the group
       database for a group ID that was previously stored in a stat
       structure, then prints out the group name if it is found. If the
       group is not found, the program prints the numeric value of the
       group for the entry.

           #include <sys/types.h>
           #include <grp.h>
           #include <stdio.h>
           struct stat statbuf;
           struct group *grp;
           if ((grp = getgrgid(statbuf.st_gid)) != NULL)
               printf(" %-8.8s", grp->gr_name);
               printf(" %-8d", statbuf.st_gid);


       The getgrgid_r() function is thread-safe and shall return values
       in a user-supplied buffer instead of possibly using a static data
       area that may be overwritten by each call.

       Portable applications should take into account that it is usual
       for an implementation to return -1 from sysconf() indicating that
       there is no maximum for _SC_GETGR_R_SIZE_MAX.

RATIONALE         top




SEE ALSO         top

       endgrent(3p), getgrnam(3p), sysconf(3p)

       The Base Definitions volume of POSIX.1‐2017, grp.h(0p),

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any
       discrepancy between this version and the original IEEE and The
       Open Group Standard, the original IEEE and The Open Group
       Standard is the referee document. The original Standard can be
       obtained online at .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see .

IEEE/The Open Group               2017                      GETGRGID(3P)

Pages that refer to this page: grp.h(0p)find(1p)endgrent(3p)getgrnam(3p)