Explain the semantics of exec in more detail.

[pintos-anon] / doc / userprog.texi

diff --git a/doc/userprog.texi b/doc/userprog.texi
index 11d9875ce7beabe78e2ebb8e9079d7402bb31113..60a53ee09cc6d876cb562d64be4544689e4716fc 100644 (file)
--- a/doc/userprog.texi
+++ b/doc/userprog.texi
@@ -301,7 +301,7 @@ regardless of what user process or kernel thread is running.  In
 Pintos, kernel virtual memory is mapped one-to-one to physical
 memory, starting at @code{PHYS_BASE}.  That is, virtual address
 @code{PHYS_BASE} accesses physical
-address 0, virtual address @code{PHYS_BASE} + @t{0x1234} access
+address 0, virtual address @code{PHYS_BASE} + @t{0x1234} accesses
 physical address @t{0x1234}, and so on up to the size of the machine's
 physical memory.
 
@@ -616,13 +616,17 @@ Runs the executable whose name is given in @var{cmd_line}, passing any
 given arguments, and returns the new process's program id (pid).  Must
 return pid -1, which otherwise should not be a valid pid, if
 the program cannot load or run for any reason.
+Thus, the parent process cannot return from the @code{exec} until it
+knows whether the child process successfully loaded its executable.
+You must use appropriate synchronization to ensure this.
 @end deftypefn
 
 @deftypefn {System Call} int wait (pid_t @var{pid})
-Waits for process @var{pid} to die and returns the status it passed to
-@code{exit}.  Returns -1 if @var{pid}
+If process @var{pid} is still alive, waits until it dies.
+Then, returns the status that @var{pid} passed to @code{exit},
+or -1 if @var{pid}
 was terminated by the kernel (e.g.@: killed due to an exception).  If
-@var{pid} is does not refer to a child of the
+@var{pid} does not refer to a child of the
 calling thread, or if @code{wait} has already been successfully
 called for the given @var{pid}, returns -1 immediately, without
 waiting.
@@ -654,11 +658,15 @@ of the rest.
 @deftypefn {System Call} bool create (const char *@var{file}, unsigned @var{initial_size})
 Creates a new file called @var{file} initially @var{initial_size} bytes
 in size.  Returns true if successful, false otherwise.
+Opening the new file is a separate operation using the @code{open}
+system call.
 @end deftypefn
 
 @deftypefn {System Call} bool remove (const char *@var{file})
 Deletes the file called @var{file}.  Returns true if successful, false
 otherwise.
+A file may be removed regardless of whether it is open or closed
+(@pxref{Removing an Open File}, for more information).
 @end deftypefn
 
 @deftypefn {System Call} int open (const char *@var{file})
@@ -674,6 +682,12 @@ as explicitly described below.
 
 Each process has an independent set of file descriptors.  File
 descriptors are not inherited by child processes.
+
+When a single file is opened more than once, whether by a single
+process or different processes, each @code{open} returns a new file
+descriptor.  Different file descriptors for a single file are closed
+independently in separate calls to @code{close} and they do not share
+a file position.
 @end deftypefn
 
 @deftypefn {System Call} int filesize (int @var{fd})
@@ -964,6 +978,18 @@ or the machine shuts down.
 You may modify the stack setup code to allocate more than one page of
 stack space for each process.  In the next project, you will implement a
 better solution.
+
+@item What should happen if an @code{exec} fails midway through loading?
+
+@code{exec} should return -1 if the child process fails to load for
+any reason.  This includes the case where the load fails part of the
+way through the process (e.g.@: where it runs out of memory in the
+@code{multi-oom} test).  Therefore, the parent process cannot return
+from the @code{exec} system call until it is established whether the
+load was successful or not.  The child must communicate this
+information to its parent using appropriate synchronization, such as a
+semaphore (@pxref{Semaphores}), to ensure that the information is
+communicated without race conditions.
 @end table
 
 @node 80x86 Calling Convention