1vfork(2)                         System Calls                         vfork(2)
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4

NAME

6       vfork, vforkx - spawn new process in a virtual memory efficient way
7

SYNOPSIS

9       #include <unistd.h>
10
11       pid_t vfork(void);
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13
14       #include <sys/fork.h>
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16       pid_t vforkx(int flags);
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18

DESCRIPTION

20       The  vfork()  and vforkx() functions create a new process without fully
21       copying the address space of the old process. These functions are  use‐
22       ful  in instances where the purpose of a fork(2) operation is to create
23       a new system context for an execve() operation (see exec(2)).
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25
26       Unlike with the fork() function, the child process borrows the parent's
27       memory  and  thread  of  control  until  a  call to execve() or an exit
28       (either abnormally or by a call to _exit() (see exit(2)). Any modifica‐
29       tion  made  during this time to any part of memory in the child process
30       is reflected in the parent process on return from vfork() or  vforkx().
31       The parent process is suspended while the child is using its resources.
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34       In  a  multithreaded application,  vfork() and vforkx() borrow only the
35       thread of control that called vfork() or vforkx() in the  parent;  that
36       is,  the child contains only one thread. The use of vfork() or vforkx()
37       in multithreaded applications, however, is unsafe due  to  race  condi‐
38       tions  that can cause the child process to become deadlocked and conse‐
39       quently block both the child and parent process from execution  indefi‐
40       nitely.
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42
43       The vfork() and vforkx() functions can normally be used the same way as
44       fork() and  forkx(),  respectively.  The  calling  procedure,  however,
45       should not return while running in the child's context, since the even‐
46       tual return from vfork() or vforkx() in the parent would be to a  stack
47       frame  that  no  longer  exists. The _exit() function should be used in
48       favor of exit(3C) if unable to perform  an  execve()  operation,  since
49       exit()  will  invoke  all  functions  registered by atexit(3C) and will
50       flush and close standard I/O channels, thereby  corrupting  the  parent
51       process's standard I/O data structures. Care must be taken in the child
52       process not to modify any global or local data that affects the  behav‐
53       ior  of  the  parent process on return from vfork() or vforkx(), unless
54       such an effect is intentional.
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56
57       Unlike fork() and forkx(), fork handlers are not run when  vfork()  and
58       vforkx() are called.
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61       The  vfork()  and vforkx() functions are deprecated. Their sole legiti‐
62       mate use as a prelude to an immediate call to a function from the  exec
63       family can be achieved safely by posix_spawn(3C) or posix_spawnp(3C).
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65   Fork Extensions
66       The  vforkx() function accepts a flags argument consisting of a bitwise
67       inclusive-OR of zero or more of the following flags, which are  defined
68       in the header <sys/fork.h>:
69         FORK_NOSIGCHLD
70         FORK_WAITPID
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72
73       See  fork(2)  for descriptions of these flags. If the flags argument is
74       0, vforkx() is identical to vfork().
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RETURN VALUES

77       Upon successful completion, vfork() and vforkx() return  0 to the child
78       process  and  returns the process ID of the child process to the parent
79       process. Otherwise, −1 is returned to  the  parent  process,  no  child
80       process is created, and errno is set to indicate the error.
81

ERRORS

83       The vfork() and vforkx() functions will fail if:
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85       EAGAIN    The  system-imposed  limit  on  the total number of processes
86                 under execution (either system-quality or by a  single  user)
87                 would  be  exceeded. This limit is determined when the system
88                 is generated.
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90
91       ENOMEM    There is insufficient swap space for the new process.
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95       The vforkx() function will fail if:
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97       EINVAL    The flags argument is invalid.
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ATTRIBUTES

101       See attributes(5) for descriptions of the following attributes:
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105
106       ┌─────────────────────────────┬─────────────────────────────┐
107       │      ATTRIBUTE TYPE         │      ATTRIBUTE VALUE        │
108       ├─────────────────────────────┼─────────────────────────────┤
109       │Interface Stability          │Obsolete                     │
110       ├─────────────────────────────┼─────────────────────────────┤
111       │MT-Level                     │Unsafe                       │
112       └─────────────────────────────┴─────────────────────────────┘
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SEE ALSO

115       exec(2),   exit(2),   fork(2),    ioctl(2),    atexit(3C),    exit(3C),
116       posix_spawn(3C),     posix_spawnp(3C),    signal.h(3HEAD),    wait(3C),
117       attributes(5), standards(5)
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NOTES

120       To avoid a possible deadlock situation, processes that are children  in
121       the  middle  of a vfork() or vforkx() are never sent SIGTTOU or SIGTTIN
122       signals; rather, output or ioctls are allowed and input attempts result
123       in an EOF indication.
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125
126       To  forestall parent memory corruption due to race conditions with sig‐
127       nal handling, vfork() and vforkx() treat signal handlers in  the  child
128       process  in the same manner as the exec(2) functions: signals set to be
129       caught by the parent process are set to the default action (SIG_DFL) in
130       the  child  process  (see signal.h(3HEAD)). Any attempt to set a signal
131       handler in the child before execve() to anything other than SIG_DFL  or
132       SIG_IGN is disallowed and results in setting the handler to SIG_DFL.
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134
135       On  some  systems, the implementation of vfork() and vforkx() cause the
136       parent to inherit register values from the child. This can create prob‐
137       lems  for certain optimizing compilers if <unistd.h> is not included in
138       the source calling vfork() or if <sys/fork.h> is not  included  in  the
139       source calling vforkx().
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143SunOS 5.11                        13 Dec 2006                         vfork(2)
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