Lightweight alternative to fork() in POSIX C?

Kyle

From the man pages I'm reading, it seems that popen, system, etc. tend to call fork(). In turn, fork() copies the entire memory state of the process. This does seem like a lot of work, especially in many cases where the child calling fork() barely has any memory allocated to the parent.

So my question is, is it possible to get fork()-like behavior without copying the entire memory state of the parent process? Or am I missing something, e.g. fork() is not as heavy as it seems (e.g. maybe inclined to optimize calls to avoid unnecessary memory duplication)?

Basil Starinkevich

From the point of view of a user-space application, fork(2), like all system calls , is a primitive operation (but some C libraries use clone(2) for this ). It's mostly a machine instruction, SYSCALLor a SYSENTERswitch from user mode to kernel mode, so the Linux kernel (latest versions) is doing quite a bit of processing.

In practice, it is very efficient (e.g. less than a millisecond, sometimes even less than a tenth) because the kernel uses the lazy copy-on-write technique extensively to share pages between parent and child processes. The actual copying will happen later due to a page fault when overwriting the shared page .

And forking has huge advantages, since the initiation of some other program is delegated to execve(2) : this is conceptually simple: the only difference between parent and child processes is the resultfork

BTW on POSIX systems like Linux, fork(2) or a suitable clone(2) is the only way to create a process (some weird exception should generally be ignored: the kernel is executing some process like that ...) because vfork (2) Obsolete./sbin/init

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