From 2c660ca7455e0bca382da32f0084e4e7545423c7 Mon Sep 17 00:00:00 2001
From: Perry Kivolowitz <perry@kivolowitz.com>
Date: Sun, 25 Dec 2022 15:37:29 -0600
Subject: [PATCH] lots of progress on file_ops - tired now

---
 section_1/funcs/file_ops.s | 131 +++++++++++++++++++++++++++----------
 1 file changed, 96 insertions(+), 35 deletions(-)

diff --git a/section_1/funcs/file_ops.s b/section_1/funcs/file_ops.s
index 2851c8d..280deda 100644
--- a/section_1/funcs/file_ops.s
+++ b/section_1/funcs/file_ops.s
@@ -19,71 +19,132 @@ fd 		.req	w28
 main:   stp     x29, x30, [sp, -16]!
         stp     x27, x28, [sp, -16]!
         bl      open_file
-		// w0 will contain either the file descriptor of the new
-		// file or -1 for a failure. Note that the value in w0
-		// has also been copied to "fd" - a register alias.
-		cmp 	w0, wzr
-		bge 	1f
+        // w0 will contain either the file descriptor of the new
+        // file or -1 for a failure. Note that the value in w0
+        // has also been copied to "fd" - a register alias.
+        cmp 	w0, wzr
+        bge 	1f
         // If we get here, the open has failed. Use perror() to
         // print a meaningful error and branch to exit. The return
-		// code of the program will be set to non-zero inside open_fail.
-        bl      open_fail
+        // code of the program will be set to non-zero inside fail.
+        ldr     x0, =fname
+        bl      fail
         b       99f
 
 1:		// When we get here, the file is open. Write some data to it.
-		// If write_file returns non-zero, it signifies an error. If
-		// so, branch to the file closing code since the file is open.
-		// The error message will be printed in write_data.
-		bl		write_data
-		cbnz	w0, 10f
+        // If write_file returns non-zero, it signifies an error. If
+        // so, branch to the file closing code since the file is open
+        // after printing an error message.
+        bl		write_data
+        cbz	    w0, 10f
+        // If we get here, there was an error in write_data. Print
+        // a reasonable error message then branch to the clean usleep
+        // code.
+        ldr     x0, =wf     // load legend
+        bl      fail        // print error
+        b       50f         // branch to clean up.
 
-		// When we get here, we are done. Close the file.
-10:		mov		w0, fd
-		bl 		close
-		mov 	retval, wzr
+        // Seek back to position zero preparing to read the file back.
+        // The return value in x0 (off_t) is the return value of
+        // lseek(). 
+10:     bl      seek_zero
+        cbz     x0, 20f
+        // If we get here, the seek failed. Cause a reasonable
+        // message to be printed then branch to the clean up code.
+        ldr     x0, =sf
+        bl      fail
+        b       50f
+20:
+        // When we get here, we are done. Close the file.
+50:		mov		w0, fd
+        bl 		close
+        mov 	retval, wzr
 
 99:     ldp     x27, x28, [sp], 16
         ldp     x29, x30, [sp], 16
         mov     w0, retval
         ret
 
-/*	This function attempts to open a file for both reading and
-	writing. Return values will be checked to ensure the file is
-	opened. If successful, 0 is returned (and the file descriptor
-	is squirreled away in register "fd".
+/*	open_file()
+    This function attempts to open a file for both reading and
+    writing. Return values will be checked to ensure the file is
+    opened. If successful, the fd is returned (and is squirreled
+    away in register "fd"). If unsuccessful, the -1 returned by
+    open() is passed back to the caller.
+
+    Explanation of the magic numbers:
+
+    int open(const char *pathname, int flags, mode_t mode);
+    octal 102 for flags is O_RDRW | O_CREAT
+    octal 600 for mode is rw------- i.e. read and write for
+        the owner but no permissions for anyone else.
 */
 
+        .equ    O_FLAGS, 0102
+        .equ    O_MODE, 0600
+
 open_file:
-       stp      x29, x30, [sp, -16]!
-	   ldr		x0, =fname
-	   mov		w1, 0102
-	   mov		w2, 0600
-	   bl		open
-	   mov		fd, w0
-	   ldp		x29, x30, [sp], 16
-       ret
+        stp      x29, x30, [sp, -16]!
+        ldr      x0, =fname
+        mov      w1, O_FLAGS
+        mov      w2, O_MODE
+        bl       open
+        mov      fd, w0
+        ldp      x29, x30, [sp], 16
+        ret
 
 
 /*  This function uses perror to print a meaningful error
-    message in the event of an open failure.
+    message in the event of a failure. The string value
+    passed to perror arrives to us as a pointer in x0.
 */
 
-open_fail:
+fail:
         stp     x29, x30, [sp, -16]!
-		ldr		x0, =fname
-		bl 		perror
-		mov		retval, 1
+        bl 		perror
+        mov		retval, 1
         ldp     x29, x30, [sp], 16
         ret
 
+/*  ssize_t write(int fd, const void *buf, size_t count);
+*/
+
 write_data:
         stp     x29, x30, [sp, -16]!
+        str     x20, [sp, -16]!
+        mov     w0, fd              // file descriptor
+        ldr     x1, =txt            // address to print from
+        ldr     x2, =txt_s          // load pointer to size
+        ldr     x2, [x2]            // dereference the pointer
+        mov     w20, w2             // need this value for error check.
+        bl      write
+        cmp     x0, x20             // Did we write the expected amount?
+        bne     90f
+        // successful write - return 0
+        mov     x0, xzr
+        b       99f
+90:     // failure - ensure we return non-zero!
+        mov     x0, 1
+99:     ldr     x20, [sp], 16
         ldp     x29, x30, [sp], 16
         ret
 
-		.data
+/*  off_t lseek(int fd, off_t offset, int whence);
+*/
+seek_zero:
+        stp     x29, x30, [sp, -16]!
+        mov     w0, fd          // file descriptor
+        mov     x1, xzr         // beginning of file
+        mov     w2, wzr         // SEEK_SET - absolute offset
+        bl      lseek
+        ldp     x29, x30, [sp], 16
+        ret
+
+        .data
 prog:	.asciz	"file_ops"
+wf:     .asciz  "write failed"
+sf:     .asciz  "lseek failed"
 fname:	.asciz	"test.txt"
 txt:	.asciz	"some data\n"
 txt_s:	.word	txt_s - txt - 1		// strlen(txt)
-		.end
+        .end