man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

endgrent(3C)

Name

getgrnam, getgrnam_r, getgrent, getgrent_r, getgrgid, getgrgid_r, setgrent, endgrent, fgetgrent, fgetgrent_r - group database entry functions

Synopsis

#include <grp.h>

struct group *getgrnam(const char *name);
struct group *getgrnam_r(const char *name, struct group *grp,
     char *buffer, int bufsize);
struct group *getgrent(void);
struct group *getgrent_r(struct group *grp, char *buffer, int bufsize);
struct group *getgrgid(gid_t gid);
struct group *getgrgid_r(gid_t gid, struct group *grp, char *buffer,
     int bufsize);
void setgrent(void);
void endgrent(void);
struct group *fgetgrent(FILE *f);
struct group *fgetgrent_r(FILE *f, struct group *grp, char *buffer,
     int bufsize);

Standard conforming

cc [ flag... ] file... –D_POSIX_PTHREAD_SEMANTICS [ library... ]

int getgrnam_r(const char *name, struct group *grp, char *buffer,
     size_t bufsize, struct group **result);
int getgrgid_r(gid_t gid, struct group *grp, char *buffer,
     size_t bufsize, struct group **result);

Description

These functions are used to obtain entries describing user groups. Entries can come from any of the sources for group specified in the /etc/nsswitch.conf file (see nsswitch.conf(4)).

The getgrnam() function searches the group database for an entry with the group name specified by the character string parameter name.

The getgrgid() function searches the group database for an entry with the (numeric) group id specified by gid.

The setgrent(), getgrent(), and endgrent() functions are used to enumerate group entries from the database.

The setgrent() function effectively rewinds the group database to allow repeated searches. It sets (or resets) the enumeration to the beginning of the set of group entries. This function should be called before the first call to getgrent().

The getgrent() function returns a pointer to a structure containing the broken-out fields of an entry in the group database. When first called, getgrent() returns a pointer to a group structure containing the next group structure in the group database. Successive calls can be used to search the entire database.

The endgrent() function can be called to close the group database and deallocate resources when processing is complete. It is permissible, though possibly less efficient, for the process to call more group functions after calling endgrent().

The fgetgrent() function, unlike the other functions above, does not use nsswitch.conf. It reads and parses the next line from the stream f, which is assumed to have the format of the group file (see group(4)).

Reentrant Interfaces

The getgrnam(), getgrgid(), getgrent(), and fgetgrent() functions use thread-specific storage that is reused in each call to one of these functions by the same thread, making them safe to use but not recommended for multithreaded applications.

The parallel functions getgrnam_r(), getgrgid_r(), getgrent_r(), and fgetgrent_r() provide reentrant interfaces for these operations.

Each reentrant interface performs the same operation as its non-reentrant counterpart, named by removing the _r suffix. The reentrant interfaces, however, use buffers supplied by the caller to store returned results instead of using thread-specific data that can be overwritten by each call. They are safe for use in both single-threaded and multithreaded applications.

Each reentrant interface takes the same arguments as its non-reentrant counterpart, as well as the following additional parameters. The grp argument must be a pointer to a struct group structure allocated by the caller. On successful completion, the function returns the group entry in this structure. Storage referenced by the group structure is allocated from the memory provided with the buffer argument that is bufsize characters in size. The maximum size needed for this buffer can be determined with the _SC_GETGR_R_SIZE_MAX sysconf(3C) parameter. The standard-conforming versions place a pointer to the modified grp structure in the result parameter, instead of returning a pointer to this structure. A null pointer is returned at the location pointed to by result on error or if the requested entry is not found.

For enumeration in multithreaded applications, the position within the enumeration is a process-wide property shared by all threads. The setgrent() function can be used in a multithreaded application but resets the enumeration position for all threads. If multiple threads interleave calls to getgrent_r(), the threads will enumerate disjoint subsets of the group database. Like their non-reentrant counterparts, getgrnam_r() and getgrgid_r() leave the enumeration position in an indeterminate state.

group Structure

Group entries are represented by the struct group structure defined in <grp.h>:

struct group {
    char *gr_name;          /* the name of the group */
    char *gr_passwd;        /* the encrypted group password */
    gid_t gr_gid;           /* the numerical group ID */
    char **gr_mem;          /* vector of pointers to member 
                               names */
};

Return Values

The getgrnam(), getgrnam_r(), getgrgid(), and getgrgid_r() functions each return a pointer to a struct group if they successfully locate the requested entry. They return a null pointer if either the requested entry was not found or an error occurred. On error, errno is set to indicate the error. The standard-conforming functions getgrnam_r() and getgrgid_r() return 0 upon success or an error number in case of failure.

The getgrent(), getgrent_r(), fgetgrent(), and fgetgrent_r() functions each return a pointer to a struct group if they successfully enumerate an entry; otherwise they return a null pointer on end-of-file or error. On error, errno is set to indicate the error.

The getgrnam(), getgrgid(), getgrent(), and fgetgrent() functions use thread–specific data storage, so returned data must be copied before a subsequent call to any of these functions if the data are to be saved.

When the pointer returned by the reentrant functions getgrnam_r(), getgrgid_r(), getgrent_r(), and fgetgrent_r() is non-null, it is always equal to the grp pointer that was supplied by the caller.

Applications wishing to check for error situations should set errno to 0 before calling getgrnam(), getgrnam_r(), getgrent(), getgrent_r(), getgrgid(), getgrgid_r(), fgetgrent(), and fgetgrent_r(). If these functions return a null pointer and errno is non-zero, an error occurred.

Errors

The getgrent_r(), fgetgrent(), and fgetgrent_r() functions will fail if:

EIO

An I/O error has occurred.

ERANGE

Insufficient storage was supplied by buffer and bufsize to contain the data to be referenced by the resulting group structure.

The getgrent_r() function will fail if:

EMFILE

There are {OPEN_MAX} file descriptors currently open in the calling process.

ENFILE

The maximum allowable number of files is currently open in the system.

The getgrnam(), getgrnam_r(), getgrgid(), getgrgid_r(), and getgrent() functions may fail if:

EINTR

A signal was caught during the operation.

EIO

An I/O error has occurred.

EMFILE

There are {OPEN_MAX} file descriptors currently open in the calling process.

ENFILE

The maximum allowable number of files is currently open in the system.

The getgrnam_r() and getgrgid_r() functions may fail if:

ERANGE

Insufficient storage was supplied by buffer and bufsize to contain the data to be referenced by the resulting group structure.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
See below.
MT-Level
See Reentrant Interfaces in DESCRIPTION.

The endgrent(), getgrent(), getgrgid(), getgrgid_r(), getgrnam(), getgrnam_r(), and setgrent() functions are Standard.

See Also

Intro(3), getpwnam(3C), group(4), nsswitch.conf(4), passwd(4), attributes(5), standards(5)

Notes

When compiling multithreaded programs, see Intro(3).

Use of the enumeration interfaces getgrent() and getgrent_r() is discouraged; enumeration is supported for the group file and NIS, but in general is not efficient and might not be supported for all database sources. The semantics of enumeration are discussed further in nsswitch.conf(4).

Previous releases allowed the use of ``+'' and ``-'' entries in /etc/group to selectively include and exclude entries from NIS. The primary usage of these entries is superseded by the name service switch, so the ``+/-'' form might not be supported in future releases.

If required, the ``+/-'' functionality can still be obtained for NIS by specifying compat as the source for group.

Solaris 2.4 and earlier releases provided definitions of the getgrnam_r() and getgrgid_r() functions as specified in POSIX.1c Draft 6. The final POSIX.1c standard changed the interface for these functions. Support for the Draft 6 interface is provided for compatibility only and might not be supported in future releases. New applications and libraries should use the standard-conforming interface.

For POSIX.1c-conforming applications, the _POSIX_PTHREAD_SEMANTICS and _REENTRANT flags are automatically turned on by defining the _POSIX_C_SOURCE flag with a value ≥199506L.