mån 2018-06-25 klockan 21:32 +0000 skrev Michael Witten:
> Anyway, putting aside the fact that this code example makes certain
> assumptions about the sizes of data types (including a `short', as
> not even POSIX constrains it to exactly 16 bits), there is still the
> issue of endianness. Namely, the following works only on machines of
> little-endian nature:
Good catch, let's fix that as well while we're at it. Updated patch
below.
One thing I didn't fix, but that you may want to do something about in
a wider overhaul of the page, is the fact that the example claims to
change the background colour at the cursor position but that is only
true on a VGA console; framebuffer consoles seem to have their
attribute bits arranged differently, and the example affects the
foreground colour instead. Is there a way for user code to find out how
the attribute bits are laid out for a specific device?
----------------
Fix broken example code in the vcs.4 man page
- use of wrong variable (attrib, which is uninitialised, instead of s)
- variable ch too narrow
- printing a font char index with %c, as if it were ASCII (it's not)
- removing the high font bit while changing the background colour
- unwarranted assumption of little-endian byte order
Also be friendly and use SEEK_* instead of numbers.
diff --git a/man4/vcs.4 b/man4/vcs.4
index aebe8cfda..f695b956c 100644
--- a/man4/vcs.4
+++ b/man4/vcs.4
@@ -140,7 +140,8 @@ main(void)
struct {unsigned char lines, cols, x, y;} scrn;
unsigned short s;
unsigned short mask;
- unsigned char ch, attrib;
+ unsigned char attrib;
+ int ch;
fd = open(console, O_RDWR);
if (fd < 0) {
@@ -158,16 +159,16 @@ main(void)
exit(EXIT_FAILURE);
}
(void) read(fd, &scrn, 4);
- (void) lseek(fd, 4 + 2*(scrn.y*scrn.cols + scrn.x), 0);
+ (void) lseek(fd, 4 + 2*(scrn.y*scrn.cols + scrn.x), SEEK_SET);
(void) read(fd, &s, 2);
ch = s & 0xff;
- if (attrib & mask)
+ if (s & mask)
ch |= 0x100;
attrib = ((s & ~mask) >> 8);
- printf("ch=\(aq%c\(aq attrib=0x%02x\\n", ch, attrib);
- attrib ^= 0x10;
- (void) lseek(fd, \-1, 1);
- (void) write(fd, &attrib, 1);
+ printf("ch=0x%03x attrib=0x%02x\\n", ch, attrib);
+ s ^= 0x1000;
+ (void) lseek(fd, \-2, SEEK_CUR);
+ (void) write(fd, &s, 2);
exit(EXIT_SUCCESS);
}
.EE
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