GETGRGID(3P) POSIX Programmer's Manual GETGRGID(3P)PROLOG
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.
getgrgid, getgrgid_r — get group database entry for a group ID
struct group *getgrgid(gid_t gid);
int getgrgid_r(gid_t gid, struct group *grp, char *buffer,
size_t bufsize, struct group **result);
The getgrgid() function shall search the group database for an entry
with a matching gid.
The getgrgid() function need not be thread-safe.
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 data‐
base 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 found.
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. 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 getgr‐
If successful, the getgrgid_r() function shall return zero; otherwise,
an error number shall be returned to indicate the error.
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
ENFILE The maximum allowable number of files is currently open in the
The getgrgid_r() function may fail if:
ERANGE Insufficient storage was supplied via buffer and bufsize to con‐
tain the data to be referenced by the resulting group structure.
The following sections are informative.
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);
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)
while ((e = getgrgid_r(42, &result, buffer, len, &resultp)) == ERANGE)
size_t newlen = 2 * len;
if (newlen < len)
len = newlen;
char *newbuffer = realloc(buffer, len);
if (newbuffer == NULL)
buffer = newbuffer;
if (e != 0)
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 pro‐
gram prints the numeric value of the group for the entry.
struct stat statbuf;
struct group *grp;
if ((grp = getgrgid(statbuf.st_gid)) != NULL)
printf(" %-8.8s", grp->gr_name);
printf(" %-8d", statbuf.st_gid);
Applications wishing to check for error situations should set errno to
0 before calling getgrgid(). If errno is set on return, an error
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.
SEE ALSOendgrent(), getgrnam(), sysconf()
The Base Definitions volume of POSIX.1‐2008, <grp.h>, <sys_types.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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 http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
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IEEE/The Open Group 2013 GETGRGID(3P)