2.9BSD/usr/man/cat2/vfork.2

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

NAME
     vfork - spawn new process in an efficient way

SYNOPSIS
     vfork()

DESCRIPTION
     _V_f_o_r_k can be used to create new processes without fully
     copying the address space of the old process.  It is useful
     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_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 from the procedure which called _v_f_o_r_k
     while running in the child's context, since the eventual
     return from _v_f_o_r_k in the parent 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, since _e_x_i_t will
     flush and close standard I/O channels, and thereby mess up
     the parent process's standard 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.)

     Similarly when using the new signal mechanism of _s_i_g_s_e_t(3)
     mechanism be sure to call _s_i_g_s_y_s rather than _s_i_g_n_a_l(2).

DIAGNOSTICS
     Same as for _f_o_r_k.

SEE ALSO
     execve(2), fork(2), sigsys(2J), wait(2)

BUGS
     This call is not portable to most dialects of UNIX.  Users
     should not depend on the memory sharing semantics of _v_f_o_r_k
     as it could, in that case, be made synonymous to _f_o_r_k.

     To avoid a possible deadlock situation, processes which 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 5/26/83                                                 1