1VFORK(2) Linux Programmer's Manual VFORK(2)
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6 vfork - create a child process and block parent
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9 #include <sys/types.h>
10 #include <unistd.h>
11 pid_t vfork(void);
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15 (From SUSv2 / POSIX draft.) The vfork() function has the same effect
16 as fork(), except that the behaviour is undefined if the process cre‐
17 ated by vfork() either modifies any data other than a variable of type
18 pid_t used to store the return value from vfork(), or returns from the
19 function in which vfork() was called, or calls any other function
20 before successfully calling _exit() or one of the exec() family of
21 functions.
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24 vfork(), just like fork(2), creates a child process of the calling
25 process. For details and return value and errors, see fork(2).
26 vfork() is a special case of clone(2). It is used to create new pro‐
28 cesses without copying the page tables of the parent process. It may
29 be useful in performance sensitive applications where a child will be
30 created which then immediately issues an execve().
31 vfork() differs from fork() in that the parent is suspended until the
33 child makes a call to execve(2) or _exit(2). The child shares all mem‐
34 ory with its parent, including the stack, until execve() is issued by
35 the child. The child must not return from the current function or call
36 exit(), but may call _exit().
37 Signal handlers are inherited, but not shared. Signals to the parent
39 arrive after the child releases the parent's memory.
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42 Under Linux, fork() is implemented using copy-on-write pages, so the
43 only penalty incurred by fork() is the time and memory required to
44 duplicate the parent's page tables, and to create a unique task struc‐
45 ture for the child. However, in the bad old days a fork() would
46 require making a complete copy of the caller's data space, often need‐
47 lessly, since usually immediately afterwards an exec() is done. Thus,
48 for greater efficiency, BSD introduced the vfork() system call, that
49 did not fully copy the address space of the parent process, but bor‐
50 rowed the parent's memory and thread of control until a call to
51 execve() or an exit occurred. The parent process was suspended while
52 the child was using its resources. The use of vfork() was tricky: for
53 example, not modifying data in the parent process depended on knowing
54 which variables are held in a register.
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57 It is rather unfortunate that Linux revived this spectre from the past.
58 The BSD manpage states: "This system call will be eliminated when
59 proper system sharing mechanisms are implemented. Users should not
60 depend on the memory sharing semantics of vfork() as it will, in that
61 case, be made synonymous to fork()."
62 Details of the signal handl