получить запись в базе данных группы для идентификатора группы (get group database entry for a group ID)
Пролог (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.
Имя (Name)
getgrgid, getgrgid_r — get group database entry for a group ID
Синопсис (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);
Описание (Description)
The getgrgid() function shall search the group database for an
entry with a matching gid.
The getgrgid() function need not be thread-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 non-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.
Возвращаемое значение (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.
Ошибки (Error)
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.
Примеры (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", grp->gr_name);
else
printf(" %-8d", statbuf.st_gid);
...
Использование в приложениях (Application usage)
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.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
endgrent(3p), getgrnam(3p), sysconf(3p)
The Base Definitions volume of POSIX.1‐2017, grp.h(0p),
sys_types.h(0p)
