System Databases [POSIX Section 9]

struct group *getgrgid( gid_t gid ); 
int getgrgid( gid_t gid, struct group *grp, char *buffer, 
	      size_t bufsize, struct group **result ); 
struct group *getgrname( const char *name ); 
int getgrname_r( const char *name, struct group *grp,
                 char *buffer, size_t bufsize, struct group **result ); 
struct passwd *getpwuid( uid_t uid ); 
int getpwuid_r( uid_t uid, struct passwd *pwd,
                char *buffer, size_t bufsize, struct passwd **result ); 
struct passwd *getpwnam( const char *name ); 
int getpwnam_r( const char *name, struct passwd *pwd,
                char *buffer, size_t bufsize, struct passwd **result );