Section: POSIX Programmer's Manual (3P)
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.
--- 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);
function shall search the group database for an entry with a matching
function need not be thread-safe.
Applications wishing to check for error situations should set
to 0 before calling
returns a null pointer and
is set to non-zero, an error occurred.
function shall update the
structure pointed to by
and store a pointer to that structure at the location pointed to by
The structure shall contain an entry from the group database with a
Storage referenced by the group structure is allocated from the memory
provided with the
parameter, which is
bytes in size. A call to
returns either -1 without changing
or an initial value suggested for the size of this buffer.
A null pointer shall be returned at the location pointed to by
on error or if the requested entry is not found.
Upon successful completion,
shall return a pointer to a
with the structure defined in
with a matching entry if one is found. The
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,
shall not be changed. On error,
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
The returned pointer, and pointers within the structure, might
also be invalidated if the calling thread is terminated.
If successful, the
function shall return zero; otherwise, an error number shall be
returned to indicate the error.
functions may fail if:
An I/O error has occurred.
A signal was caught during
All file descriptors available to the process are currently open.
The maximum allowable number of files is currently open in the system.
function may fail if:
Insufficient storage was supplied via
to contain the data to be referenced by the resulting
The following sections are informative.
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
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
to search the group database for a group ID that was previously stored
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
struct stat statbuf;
struct group *grp;
if ((grp = getgrgid(statbuf.st_gid)) != NULL)
printf(" %-8.8s", grp->gr_name);
printf(" %-8d", statbuf.st_gid);
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
indicating that there is no maximum for _SC_GETGR_R_SIZE_MAX.
The Base Definitions volume of POSIX.1-2017,
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
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