GETGRGID
Section: POSIX Programmer's Manual (3P)
Updated: 2017
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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.
NAME
getgrgid,
getgrgid_r
- get group database entry for a group ID
SYNOPSIS
#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
The
getgrgid()
function shall search the group database for an entry with a matching
gid.
The
getgrgid()
function need not be threa-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 no-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 found.
RETURN VALUE
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 error.
ERRORS
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
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;
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);
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", gr->gr_name);
else
printf(" -8d", statbuf.st_gid);
...
APPLICATION USAGE
The
getgrgid_r()
function is threa-safe and shall return values in a use-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
None.
FUTURE DIRECTIONS
None.
SEE ALSO
endgrent(),
getgrnam(),
sysconf()
The Base Definitions volume of POSIX.1-2017,
<grp.h>,
<sys_types.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.-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
http://www.opengroup.org/unix/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 files to
man page format. To report such errors, see
https://www.kernel.org/doc/ma-pages/reporting_bugs.html .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- Finding an Entry in the Group Database
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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