Originální popis anglicky: 
getgid - get the real group ID
Návod, kniha: POSIX Programmer's Manual
#include <unistd.h>
 
 
gid_t getgid(void);
 
The 
getgid() function shall return the real group ID of the calling
  process.
The 
getgid() function shall always be successful and no return value is
  reserved to indicate an error.
No errors are defined.
The following sections are informative.
None.
None.
None.
None.
getegid() , 
geteuid() , 
getuid() , 
setegid() ,
  
seteuid() , 
setgid() , 
setregid() , 
setreuid() ,
  
setuid() , the Base Definitions volume of
  IEEE Std 1003.1-2001, 
<sys/types.h>,
  
<unistd.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
  Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
  Operating System Interface (POSIX), The Open Group Base Specifications Issue
  6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
  Engineers, Inc and The Open Group. In the event of any discrepancy between
  this version and the original IEEE and The Open Group Standard, the original
  IEEE and The Open Group Standard is the referee document. The original
  Standard can be obtained online at http://www.opengroup.org/unix/online.html
  .