The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group


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


#include <grp.h>

struct group *getgrgid(gid_t
int getgrgid_r(gid_t
gid, struct group *grp, char *buffer,
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 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.


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 getgrnam().

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:

An I/O error has occurred.
A signal was caught during getgrgid().
All file descriptors available to the process are currently open.
The maximum allowable number of files is currently open in the system.

The getgrgid_r() function may fail if:

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.


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);


Applications wishing to check for error situations should set errno to 0 before calling getgrgid(). If errno is set on return, an error occurred.

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.






endgrent, getgrnam, sysconf

XBD <grp.h>, <sys/types.h>


First released in Issue 1. Derived from System V Release 2.0.

Issue 5

Normative text previously in the APPLICATION USAGE section is moved to the RETURN VALUE section.

The getgrgid_r() function is included for alignment with the POSIX Threads Extension.

A note indicating that the getgrgid() function need not be reentrant is added to the DESCRIPTION.

Issue 6

The getgrgid_r() function is marked as part of the Thread-Safe Functions option.

The Open Group Corrigendum U028/3 is applied, correcting text in the DESCRIPTION describing matching the gid.

In the DESCRIPTION, the note about reentrancy is expanded to cover thread-safety.

In the SYNOPSIS, the optional include of the <sys/types.h> header is removed.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The APPLICATION USAGE section is updated to include a note on the thread-safe function and its avoidance of possibly using a static data area.

IEEE PASC Interpretation 1003.1 #116 is applied, changing the description of the size of the buffer from bufsize characters to bytes.

Issue 7

Austin Group Interpretation 1003.1-2001 #156 is applied.

SD5-XBD-ERN-4 is applied, changing the definition of the [EMFILE] error.

SD5-XSH-ERN-166 is applied.

The getgrgid_r() function is moved from the Thread-Safe Functions option to the Base.

A minor addition is made to the EXAMPLES section, reminding the application developer to free memory allocated as if by malloc().

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0241 [75] is applied.

End of informative text.


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