From d32895b78e609bc5f4cc39c656d74af8d3f4d6dd Mon Sep 17 00:00:00 2001
From: Joshua Saxby <joshua.a.saxby@gmail.com>
Date: Wed, 4 Dec 2019 16:48:27 +0000
Subject: [PATCH] Implement tell()
---
Makefile.build | 1 +
userprog/syscall_tell.c | 19 +++++++++++++++++++
userprog/system_calls.h | 6 ++++++
3 files changed, 26 insertions(+)
create mode 100644 userprog/syscall_tell.c
diff --git a/Makefile.build b/Makefile.build
index 39f59e1..06c2627 100644
--- a/Makefile.build
+++ b/Makefile.build
@@ -73,6 +73,7 @@ userprog_SRC += userprog/syscall_write.c
userprog_SRC += userprog/file_descriptors_map.c
userprog_SRC += userprog/syscall_read.c
userprog_SRC += userprog/syscall_filesize.c
+userprog_SRC += userprog/syscall_tell.c
# No virtual memory code yet.
#vm_SRC = vm/file.c # Some file.
diff --git a/userprog/syscall_tell.c b/userprog/syscall_tell.c
new file mode 100644
index 0000000..4fa049a
--- /dev/null
+++ b/userprog/syscall_tell.c
@@ -0,0 +1,19 @@
+/*
+ * Returns the current offset into the file that the given file descriptor is at
+ *
+ * Authored by Joshua Saxby
+ */
+#include <stddef.h>
+#include "system_calls.h"
+#include "filesys/file.h"
+
+void syscall_tell(struct intr_frame *f) {
+ // pop off first int argument from interrupt frame
+ int file_descriptor = *((int*)f->esp + 1);
+ struct file *file = get_associated_file_pointer(file_descriptor);
+ /*
+ * tell() return type is unsigned so we can't return a special error code if
+ * file descriptor passed is invalid. Instead, we return 0 silently.
+ */
+ f->eax = (file != NULL) ? file_tell(file) : 0;
+}
diff --git a/userprog/system_calls.h b/userprog/system_calls.h
index b228670..ebeed4c 100644
--- a/userprog/system_calls.h
+++ b/userprog/system_calls.h
@@ -69,6 +69,12 @@ void syscall_filesize(struct intr_frame *f);
*/
void syscall_remove(struct intr_frame *f);
+/*
+ * Returns the position of the next byte to be read or written in open file fd,
+ * expressed in bytes from the beginning of the file.
+ */
+void syscall_tell(struct intr_frame *f);
+
/*
* special additional stuff for handling file descriptors because they're annoying
*/
--
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