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.TH GETGRGID "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH 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.

.SH NAME
getgrgid,
getgrgid_r
\(em get group database entry for a group ID
.SH SYNOPSIS
.LP
.nf
#include <grp.h>
.P
struct group *getgrgid(gid_t \fIgid\fP);
int getgrgid_r(gid_t \fIgid\fP, struct group *\fIgrp\fP, char *\fIbuffer\fP,
    size_t \fIbufsize\fP, struct group **\fIresult\fP);
.fi
.SH DESCRIPTION
The
\fIgetgrgid\fR()
function shall search the group database for an entry with a matching
.IR gid .
.P
The
\fIgetgrgid\fR()
function need not be thread-safe.
.P
The
\fIgetgrgid_r\fR()
function shall update the
.BR group
structure pointed to by
.IR grp
and store a pointer to that structure at the location pointed to by
.IR result .
The structure shall contain an entry from the group database with a
matching
.IR gid .
Storage referenced by the group structure is allocated from the memory
provided with the
.IR buffer
parameter, which is
.IR bufsize
bytes in size. A call to
.IR sysconf (_SC_GETGR_R_SIZE_MAX)
returns either \(mi1 without changing
.IR errno
or an initial value suggested for the size of this buffer.
A null pointer shall be returned at the location pointed to by
.IR result
on error or if the requested entry is not found.
.SH "RETURN VALUE"
Upon successful completion,
\fIgetgrgid\fR()
shall return a pointer to a
.BR "struct group"
with the structure defined in
.IR <grp.h> 
with a matching entry if one is found. The
\fIgetgrgid\fR()
function shall return a null pointer if either the requested entry was
not found, or an error occurred. On error,
.IR errno
shall be set to indicate the error.
.P
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
\fIgetgrent\fR(),
\fIgetgrgid\fR(),
or
\fIgetgrnam\fR().
.P
If successful, the
\fIgetgrgid_r\fR()
function shall return zero; otherwise, an error number shall be
returned to indicate the error.
.SH ERRORS
The
\fIgetgrgid\fR()
and
\fIgetgrgid_r\fR()
functions may fail if:
.TP
.BR EIO
An I/O error has occurred.
.TP
.BR EINTR
A signal was caught during
\fIgetgrgid\fR().
.TP
.BR EMFILE
All file descriptors available to the process are currently open.
.TP
.BR ENFILE
The maximum allowable number of files is currently open in the system.
.P
The
\fIgetgrgid_r\fR()
function may fail if:
.TP
.BR ERANGE
Insufficient storage was supplied via
.IR buffer
and
.IR bufsize
to contain the data to be referenced by the resulting
.BR group
structure.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
Note that
.IR sysconf (_SC_GETGR_R_SIZE_MAX)
may return \(mi1 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
\fIgetgrid_r\fR().
.sp
.RS 4
.nf
\fB
long int initlen = sysconf(_SC_GETGR_R_SIZE_MAX);
size_t len;
if (initlen =\|= \(mi1)
    /* Default initial length. */
    len = 1024;
else
    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);
.fi \fR
.P
.RE
.SS "Finding an Entry in the Group Database"
.P
The following example uses
\fIgetgrgid\fR()
to search the group database for a group ID that was previously stored
in a
.BR 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.
.sp
.RS 4
.nf
\fB
#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);
else
    printf(" %-8d", statbuf.st_gid);
\&...
.fi \fR
.P
.RE
.SH "APPLICATION USAGE"
Applications wishing to check for error situations should set
.IR errno
to 0 before calling
\fIgetgrgid\fR().
If
.IR errno
is set on return, an error occurred.
.P
The
\fIgetgrgid_r\fR()
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.
.P
Portable applications should take into account that it is usual
for an implementation to return \(mi1 from
\fIsysconf\fR()
indicating that there is no maximum for _SC_GETGR_R_SIZE_MAX.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIendgrent\fR\^(\|)",
.IR "\fIgetgrnam\fR\^(\|)",
.IR "\fIsysconf\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2008,
.IR "\fB<grp.h>\fP",
.IR "\fB<sys_types.h>\fP"
.SH COPYRIGHT
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
Electrical 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 .

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