#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);
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.
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.
The getgrgid_r() function may fail if:
The following sections are informative.
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);
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); ...
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.
The Base Definitions volume of POSIX.1-2017, <grp.h>, <sys_types.h>
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/man-pages/reporting_bugs.html .