# ftok

## 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.
    


## NAME



           ftok — generate an IPC key
    


## SYNOPSIS



           #include <sys/ipc.h>
    
           key_t ftok(const char *_path_ , int _id_);
    


## DESCRIPTION



           The _ftok_() function shall return a key based on _path_ and _id_ that
           is usable in subsequent calls to _msgget_(), _semget_(), and _shmget_().
           The application shall ensure that the _path_ argument is the
           pathname of an existing file that the process is able to _stat_(),
           with the exception that if _stat_() would fail with **[EOVERFLOW]** due
           to file size, _ftok_() shall still succeed.
    
           The _ftok_() function shall return the same key value for all paths
           that name the same file, when called with the same _id_ value, and
           should return different key values when called with different _id_
           values or with paths that name different files existing on the
           same file system at the same time. It is unspecified whether
           _ftok_() shall return the same key value when called again after the
           file named by _path_ is removed and recreated with the same name.
    
           Only the low-order 8-bits of _id_ are significant. The behavior of
           _ftok_() is unspecified if these bits are 0.
    


## RETURN VALUE



           Upon successful completion, _ftok_() shall return a key. Otherwise,
           _ftok_() shall return (**key_t**)-1 and set _errno_ to indicate the error.
    


## ERRORS



           The _ftok_() function shall fail if:
    
           **EACCES** Search permission is denied for a component of the path
                  prefix.
    
           **EIO** An error occurred while reading from the file system.
    
           **ELOOP** A loop exists in symbolic links encountered during
                  resolution of the _path_ argument.
    
           **ENAMETOOLONG**
                  The length of a component of a pathname is longer than
                  {NAME_MAX}.
    
           **ENOENT** A component of _path_ does not name an existing file or _path_
                  is an empty string.
    
           **ENOTDIR**
                  A component of the path prefix names an existing file that
                  is neither a directory nor a symbolic link to a directory,
                  or the _path_ argument contains at least one non-<slash>
                  character and ends with one or more trailing <slash>
                  characters and the last pathname component names an
                  existing file that is neither a directory nor a symbolic
                  link to a directory.
    
           The _ftok_() function may fail if:
    
           **ELOOP** More than {SYMLOOP_MAX} symbolic links were encountered
                  during resolution of the _path_ argument.
    
           **ENAMETOOLONG**
                  The length of a pathname exceeds {PATH_MAX}, or pathname
                  resolution of a symbolic link produced an intermediate
                  result with a length that exceeds {PATH_MAX}.
    
           _The following sections are informative._
    


## EXAMPLES



       **Getting an IPC Key**
           The following example gets a key based on the pathname **/tmp** and
           the ID value _a_.  It also assigns the value of the resulting key to
           the _semkey_ variable so that it will be available to a later call
           to _semget_(), _msgget_(), or _shmget_().
    
               #include <sys/ipc.h>
               ...
               key_t semkey;
    
               if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
                   perror("IPC error: ftok"); exit(1);
               }
    


## APPLICATION USAGE



           For maximum portability, _id_ should be a single-byte character.
    
           Applications should not assume that the resulting key value is
           unique.
    


## RATIONALE



           None.
    


## FUTURE DIRECTIONS



           Future versions of this standard may add new interfaces to provide
           unique keys.
    


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