On Mon, 2025-02-03 at 08:59 +0100, Alejandro Colomar wrote: > [CC += linux-man@] > > Hi Chen, > > On Mon, Feb 03, 2025 at 02:25:14PM +0800, Chen Linxuan wrote: > > Alejandro Colomar <alx@xxxxxxxxxx> ???2025???2???3????????? 00:56????????? > > > On Tue, Jan 21, 2025 at 11:13:51AM +0800, Chen Linxuan wrote: > > > > While reading the help manual for clone.2, I notice that the parent > > > > process in the example code does not release the stack of the child > > > > process. > > > > > > > > This is not a problem for the example program, but it is somewhat > > > > misleading. > > > > > > > > Signed-off-by: Chen Linxuan <chenlinxuan@xxxxxxxxxxxxx> > > > > --- > > > > man/man2/clone.2 | 2 ++ > > > > 1 file changed, 2 insertions(+) > > > > > > > > diff --git a/man/man2/clone.2 b/man/man2/clone.2 > > > > index 3ffe8e7b8..5e6b2ef1f 100644 > > > > --- a/man/man2/clone.2 > > > > +++ b/man/man2/clone.2 > > > > @@ -1910,6 +1910,8 @@ main(int argc, char *argv[]) > > > > child commences execution in childFunc(). */ > > > > \& > > > > pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]); > > > > + if (munmap(stack, STACK_SIZE)) > > > > + err(EXIT_FAILURE, "munmap"); > > > > if (pid == \-1) > > > > > > Would you mind clarifying why this munmap(2) call goes before the error > > > handling of clone(2)? I'm not very familiar with clone(2). > > > > The memory we mmap here is used as the stack of child process we are > > going to create. > > Once child process is created, I mean clone(2) return without error, > > it's OK to munmap(2) memory, > > as the child process has its own memory space, and that memory is not > > used in parent process at all. > > So, whether the clone(2) is success or not, we both need to call > > munmap(2) to release the memory. > > In case of a clone(2) error, exit(3) (called within err(3)) will release > the memory, right? Why do we need an explicit munmap(2) call? > > > > > If we call munmap(2) after the error handling of clone(2), we will > > print a error message and then exit > > before we free those memory. > > exit(3) releases all memory, doesn't it? Why would we want to > explicitly munmap(2) it before printing the error message? > In the code snippet provided as an example for clone(2), I believe we should strive to write example code in a way that avoids potential misunderstandings. From the perspective of whether memory is eventually released, the original program does not technically cause any memory leak since the parent process exits immediately after completing clone(2). However, I think it's necessary to explicitly perform munmap(2) in cases where clone(2) fails. The reason is: for readers studying this documentation, the error handling approach in their actual code might not be as simple as directly exiting the process. In real-world scenarios, if the error handling here involves returning error codes instead of terminating immediately, handling clone(2) error before calling munmap(2) could become misleading. By explicitly freeing resources before error handling, we ensure the example demonstrates robust practices that remain valid across different error-handling architectures. > Have a lovely day! > Alex >
