Bitfields

[Leutwein Tobias (BEG-PG/EES)]

Hello,

I wrote a program that uses the gdb to get the address and type of global values. I use the gdb therefore because so I mustn't care about the debug format. This works quiet well, accept for single bits and bitfields. Single bits aren't standard at C/C++, so I tried to solve the issue with bitfields first.

If I want to determine the address an type from the global values
flags.uc and flags.s9 from the program build from ./gdb/testsuite/gdb.base/bitfields.c.

gcc -g ./gdb/testsuite/gdb.base/bitfields.c -o ./zzz
gdb zzz

First I check if the variable flags exists through
(gdb) ptype flags
type = struct fields {
    unsigned char uc;
    int s1 : 1;
    unsigned int u1 : 1;
    int s2 : 2;
    unsigned int u2 : 2;
    int s3 : 3;
    unsigned int u3 : 3;
    int s9 : 9;
    unsigned int u9 : 9;
    signed char sc;
}

Next determine the type of flags.uc
(gdb) ptype flags.uc
type = unsigned char

Then the address of the value
(gdb) interpreter-exec mi2 "-data-evaluate-expression &flags.uc"
^done,value="0x402040 \"\""

The same for flags.s9:
(gdb) ptype flags.s9
type = int
-> No difference to a normal integer value. Also
(gdb) interpreter-exec mi2 "-data-evaluate-expression &flags.s9"
^done,value="0x402042"
shows no difference to a normal value.


At attachment ptype.diff is a source code patch for gdb7.0.1, which shows the bitpos and bitsize of values if they are not equal to 0. This patch changes only the behaviour at c/c++ language.

tar -xf gdb-7.0.1.tar.bz2
cd gdb-7.0.1
patch -u  -i ../gdb.diff -p1
./configure
make

gcc -g ./gdb/testsuite/gdb.base/bitfields.c -o ./zzz

gdb/gdb zzz

(gdb) ptype flags
type = struct fields {
    unsigned char uc;
    int s1 : 1;
    unsigned int u1 : 1;
    int s2 : 2;
    unsigned int u2 : 2;
    int s3 : 3;
    unsigned int u3 : 3;
    int s9 : 9;
    unsigned int u9 : 9;
    signed char sc;
}

(gdb) ptype flags.s9
type = int, bitpos = 20, bitsize = 9
(gdb) interpreter-exec mi2 "-data-evaluate-expression &flags.s9"
^done,value="0x402040"

(gdb) ptype flags.u9
type = unsigned int, bitsize = 9
(gdb) interpreter-exec mi2 "-data-evaluate-expression &flags.u9"
^done,value="0x402044"

(gdb) ptype flags.uc
type = unsigned char
(gdb) interpreter-exec mi2 "-data-evaluate-expression &flags.uc"
^done,value="0x402040 \"\""


I use the ptype command, because I didn't find a gdb/mi command through which I can get such informative results.

Would it be possible to extend the ptype command like I did?
Would it be better to make a new command?
Or is there a other way to get this information's.

Tobias


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