Visibility of variables not declared in the header file (GNU C).

stf92 · 01-06-2014, 11:59 PM

Hi: I have three files: usbdrvasm.S, usbdrv.c and usbdrv.h. usbdrvasm.S uses the usbInputBuf variable. The only place where this variable is declared is usbdrv.c. Now, usbdrvasm.S includes the usbdrv.h:

Code:

#include "usbdrv.h"

But usbInputBuf is nowhere to be seen in usbdrv.h. How can usbdrvasm.S see it?

EDIT: maybe it is resolved at linking time? But there is no extern declaration in usbdrvasm.S.

NevemTeve · 01-07-2014, 03:09 AM

Now you do get a compilation error or you don't?

stf92 · 01-07-2014, 04:47 AM

No I don't.

NevemTeve · 01-07-2014, 05:01 AM

Then 'usbInputBuf' is somehow defined az 'extern' (or similiar) in usbdrvasm.S
Try this: 'nm usbdrvasm.o | grep usbInputBuf'

stf92 · 01-07-2014, 05:58 AM

Code:

semoi@server:~/takefuji_2013/02$ nm usbdrvasm.o | grep usbInputBuf 
         U usbInputBuf
semoi@server:~/takefuji_2013/02$

Magic! I tell you usbInputBuf is not defined as extern. The only place where usbInputBuf is defined is usbdrv.c.

gengisdave · 01-07-2014, 07:44 AM

can you post (segment of) sources and compile command?

stf92 · 01-07-2014, 08:17 AM

usbdrv.c:

Code:

#include <avr/io.h>
#include <avr/pgmspace.h>
#include "usbdrv.h"

/*
General Description:
This module implements the C-part of the USB driver. See usbdrv.h for a
documentation of the entire driver.
*/

/* ------------------------------------------------------------------------- */

/* raw USB registers / interface to assembler code: */
/* usbRxBuf MUST be in 1 byte addressable range (because usbInputBuf is only 1 byte) */
static char     usbRxBuf[2][USB_BUFSIZE];/* raw RX buffer: PID, 8 bytes data, 2 bytes CRC */
uchar           usbDeviceId;            /* assigned during enumeration, defaults to 0 */
uchar           usbInputBuf;            /* ptr to raw buffer used for receiving */
uchar           usbAppBuf;                      /* ptr to raw buffer passed to app for processing */

The only places in usbdrvasm.S where the variable is used are:

Code:

       lds             YL, usbInputBuf ;2 -> 8
       ..........................................
       lds             YL, usbInputBuf         ;2 reposition to buffer start
       ..........................................
       sts             usbInputBuf, x1         ;2 buffers now swapped

As to usbdrv.h, it does not mention the variable at all. The make file is:

Code:

semoi@server:~/takefuji_2013/02$ cat Makefile
TARGET = attiny85

COMPILE = avr-gcc -Wall -O2 -Iusbdrv -I. -mmcu=$(TARGET) # -DDEBUG_LEVEL=2

OBJECTS = usbdrv.o usbdrvasm.o da.o


all:	da.hex

.c.o:
	$(COMPILE) -c $< -o $@

.S.o:
	$(COMPILE) -x assembler-with-cpp -c $< -o $@

.c.s:
	$(COMPILE) -S $< -o $@

clean:
	rm -f da.hex da.lst da.obj da.cof da.list da.map da.eep.hex da.bin *.o da.s 

da.bin:	$(OBJECTS)
	$(COMPILE) -o da.bin $(OBJECTS) -Wl,-Map,da.map

da.hex:	da.bin
	rm -f da.hex da.eep.hex
	avr-objcopy -j .text -j .data -O ihex da.bin da.hex

cpp:
	$(COMPILE) -E da.c
semoi@server:~/takefuji_2013/02$

sundialsvcs · 01-07-2014, 08:32 AM

Then it would seem that the writers of that module did not intend to make that variable externally visible ... or, at least, they did not consider it "safe" to let other modules refer to it willy-nilly. They wanted it to be private, insofar as "C" supports such a notion.

There must have been a good reason . . .

ntubski · 01-07-2014, 09:54 AM

http://technofetish.net/repos/buffal...rv/usbdrvasm.S:

Code:

/* Some assembler dependent definitions and declarations: */

#ifdef __IAR_SYSTEMS_ASM__

#   define nop2     rjmp    $+2 /* jump to next instruction */
#   define XL       r26
#   define XH       r27
#   define YL       r28
#   define YH       r29
#   define ZL       r30
#   define ZH       r31
#   define lo8(x)   LOW(x)
#   define hi8(x)   ((x)>>8)    /* not HIGH to allow XLINK to make a proper range check */

    extern  usbRxBuf, usbDeviceAddr, usbNewDeviceAddr, usbInputBuf

Quote:

Originally Posted by sundialsvcs

Then it would seem that the writers of that module did not intend to make that variable externally visible

The comments indicate they intended the variable to be used by the assembly code:

Code:

/* raw USB registers / interface to assembler code: */

Since the assembler can't use C declarations there is no use putting it in the .h file...

stf92 · 01-07-2014, 04:50 PM

If the assembler can't use C declarations, how does it do to use usbInputBuf?

ntubski · 01-07-2014, 07:56 PM

Quote:

Originally Posted by stf92

If the assembler can't use C declarations, how does it do to use usbInputBuf?

By using assembler declarations (see highlights in post #9).

stf92 · 01-07-2014, 08:11 PM

I see. But that is not my original usbdrvasm.S. In mine there are not such declarations, notwithstanding the fact that every thing compiles well.

ntubski · 01-07-2014, 08:57 PM

Quote:

Originally Posted by stf92

But that is not my original usbdrvasm.S. In mine there are not such declarations, notwithstanding the fact that every thing compiles well.

Huh. That makes no sense. What's the output of

Code:

avr-gcc -E -I. -Iusbdrv usbdrvasm.S | grep -F usbInputBuf

stf92 · 01-07-2014, 09:58 PM

Code:

semoi@server:~/takefuji_2013/02$ avr-gcc -E -I. -Iusbdrv usbdrvasm.S -mmcu=attiny85| grep -F usbInputBuf
 lds r28, usbInputBuf ;2 -> 8
 lds r28, usbInputBuf ;2 reposition to buffer start
 sts usbInputBuf, r16 ;2 buffers now swapped
semoi@server:~/takefuji_2013/02$

ntubski · 01-08-2014, 03:20 PM

Apparently GNU assembler doesn't need extern, from https://sourceware.org/binutils/docs/as/Extern.html:

Quote:

.extern is accepted in the source program—for compatibility with other assemblers—but it is ignored. as treats all undefined symbols as external.