getgrent.3

操作系统设计与实现源码

.TH GETGRENT 3
.SH NAME
getgrent, getgrnam, getgrgid, setgrent, endgrent, setgrfile \- group file routines
.SH SYNOPSIS
.ft B
.nf
#include <grp.h>

struct group *getgrent(void)
struct group *getgrnam(const char *\fIname\fP)
struct group *getgrgid(gid_t \fIgid\fP)
int setgrent(void)
void endgrent(void)
void setgrfile(const char *\fIfile\fP)
.fi
.ft P
.SH DESCRIPTION
These functions are used to obtain information from the group file.  They
return this information in a
.B struct group
as defined by <grp.h>:
.PP
.nf
.ta +4n +6n +15n
struct group {
	char	*gr_name;	/* login name */
	char	*gr_passwd;	/* encrypted password */
	gid_t	gr_gid;	/* numeric group id */
	char	**gr_mem;	/* null-terminated list of group members */
};
.fi
.PP
.B Getgrent()
reads the group file entry by entry.
.B Getgrnam()
scans the entire group file for the group with the given
.IR name .
.B Getgrgid()
looks for the first group with the given
.IR gid .
The
.B setgrent()
and
.B endgrent()
functions are used to open and later close the group file.  With
.B setgrfile()
one can specify the file to read other than the normal group file.  This
only sets the name, the next
.B setgrent()
call will open the file.  Do not touch the file name while it is active.
Use
.B setgrfile(NULL)
to revert back to the normal group file.
.PP
The usual way to scan the group file is (error checking omitted):
.PP
.RS
.nf
.DT
setgrent();
while ((gr = getgrent()) != NULL)
	if (appropriate_test(gr)) break;
endgrent();
.fi
.RE
.PP
The
.B gr
variable contains the entry that is wanted if non-NULL.  The
.B getgrnam()
and
.B getgrgid()
functions are implemented as in this example, with error checking of course.
.PP
.B Getgrent()
calls
.B setgrent()
if this has not yet been done.
.B Setgrent()
first calls
.B endgrent()
if the group file is still open.  (Other implementations may simply
rewind the file.)
.SH FILES
.TP 15
.B /etc/group
The group file database.
.SH "SEE ALSO"
.BR getgroups (2),
.BR initgroups (3),
.BR getpwent (3),
.BR passwd (5).
.SH DIAGNOSTICS
.B Setgrent()
has the same return value and error codes as the
.BR open (2)
call it uses to open the group file.  The
.BI get xxx ()
functions return NULL on end of file, entry not found, or error.  You can
set
.B errno
to zero before the call and check it after.
.SH NOTES
All
.BI get xxx ()
routines return a pointer to static storage that is overwritten in each call.
.PP
Only
.B getgrnam()
and
.B getgrgid()
are defined by \s-2POSIX\s+2.  The
.B _MINIX_SOURCE
macro must be defined before including <grp.h> to make the other functions
visible.  The
.B gr_passwd
field is also not defined by \s-2POSIX\s+2, but is always visible.
Portable code cannot reliably detect errors by setting
.B errno
to zero.  Under Minix it is better to make a
.B getgrent()
scan if you need to look up several group-id's or names, but portable code
had better use several
.B getgrgid()
or
.B getgrnam()
calls.  The
.B getgrent()
is usually available on other systems, but may be very expensive.  See
.BR initgroups (3)
if you are after supplementary group id's.
.SH AUTHOR
Kees J. Bot ([e-mail])