4.3BSD-UWisc/man/cat2/vfork.2

VFORK(2)            UNIX Programmer's Manual             VFORK(2)



NAME
     vfork - spawn new process in a virtual memory efficient way

SYNOPSIS
     pid = vfork()
     int pid;

DESCRIPTION
     _V_f_o_r_k can be used to create new processes without fully
     copying the address space of the old process, which is
     horrendously inefficient in a paged environment.  It is use-
     ful when the purpose of _f_o_r_k(2) would have been to create a
     new system context for an _e_x_e_c_v_e.  _V_f_o_r_k differs from _f_o_r_k
     in that the child borrows the parent's memory and thread of
     control until a call to _e_x_e_c_v_e(2) or an exit (either by a
     call to _e_x_i_t(2) or abnormally.) The parent process is
     suspended while the child is using its resources.

     _V_f_o_r_k returns 0 in the child's context and (later) the pid
     of the child in the parent's context.

     _V_f_o_r_k can normally be used just like _f_o_r_k. It does not work,
     however, to return while running in the childs context from
     the procedure that called _v_f_o_r_k since the eventual return
     from _v_f_o_r_k would then return to a no longer existent stack
     frame.  Be careful, also, to call __e_x_i_t rather than _e_x_i_t if
     you can't _e_x_e_c_v_e, since _e_x_i_t will flush and close standard
     I/O channels, and thereby mess up the parent processes stan-
     dard I/O data structures.  (Even with _f_o_r_k it is wrong to
     call _e_x_i_t since buffered data would then be flushed twice.)

SEE ALSO
     fork(2), execve(2), sigvec(2), wait(2),

DIAGNOSTICS
     Same as for _f_o_r_k.

BUGS
     This system call will be eliminated when proper system shar-
     ing mechanisms are implemented. Users should not depend on
     the memory sharing semantics of _v_f_o_r_k as it will, in that
     case, be made synonymous to _f_o_r_k.

     To avoid a possible deadlock situation, processes that are
     children in the middle of a _v_f_o_r_k are never sent SIGTTOU or
     SIGTTIN signals; rather, output or _i_o_c_t_ls are allowed and
     input attempts result in an end-of-file indication.








Printed 12/27/86          June 30, 1985                         1