gdb unable to print fn argument correctly

Manoj Iyer
2005-07-21T20:06:48Z

I have an test program which has a function of the nature 'void fn(double
c){}', which is compiled as a PwerPC 64bit application. When I set a
breakpoint and stop on this function, GDB is not printing the arguments
passed to this function correctly.

I did some initial debugging, the dwarf information and dissassembly of
the function looks good to me. But I am not able to pin point who is
corrupting the stack. I believe gdb is just printing what is avaiable in
the stack to output.

Any pointers to help me identify the culprit deeply appreciated.

============= test.c ========================

#include <stdio.h>

void fn1(int a) {}
void fn2(long b){}
void fn3(double c){}

int
main()
{
fn1(1234);
fn2(123456789L);
fn3(1234.4567);
return 0;
}
============= end test.c ======================

============= GDB output ======================
(gdb) br main
Breakpoint 1 at 0x10000574: file /tmp/test1.c, line 10.
(gdb) br fn3
Breakpoint 2 at 0x10000544: file /tmp/test1.c, line 5.
(gdb) r
Starting program: /tmp/test.gdb

Breakpoint 1, main () at /tmp/test1.c:10
10 fn1(1234);
(gdb) c
Continuing.

Breakpoint 2, 0x0000000010000544 in fn3 (c=6.0995758190771502e-316)
at /tmp/test1.c:5
5 void fn3(double c){}
(gdb)

================= end gdb output ==========================

When I change this testcase, change function fn3 to have something
meaningful in the body, gdb prints the arguments correctly.

=================== modified test.c ======================
#include <stdio.h>

void fn1(int a) {}
void fn2(long b){}
void fn3(double c){printf("hello world %lf\n", c);}

int
main()
{
fn1(1234);
fn2(123456789L);
fn3(1234.4567);
return 0;
}

================= end of test.c ==========================

================= output from gdb =======================
(gdb) br main
Breakpoint 1 at 0x100004c0: file test.c, line 9.
(gdb) br fn3
Breakpoint 2 at 0x10000488: file test.c, line 5.
(gdb) r
Starting program: /tmp/test

Breakpoint 1, main () at test.c:9
warning: Source file is more recent than executable.

9 fn1(1234);
(gdb) c
Continuing.

Breakpoint 2, fn3 (c=1234.4567) at test.c:5
5 void fn3(double c){printf("hello world %lf\n", c);}
(gdb)

================== end of output from gdb ===================

=================== DWARF info for fn3() ===================
<1><2bf>: Abbrev Number: 4 (DW_TAG_subprogram)
DW_AT_sibling : <2e3>
DW_AT_external : 1
DW_AT_name : fn3
DW_AT_decl_file : 1
DW_AT_decl_line : 5
DW_AT_prototyped : 1
DW_AT_low_pc : 0x10000448 268436552
DW_AT_high_pc : 0x10000468 268436584
DW_AT_frame_base : 1 byte block: 6f (DW_OP_reg31)
<2><2d6>: Abbrev Number: 5 (DW_TAG_formal_parameter)
DW_AT_name : c
DW_AT_decl_file : 1
DW_AT_decl_line : 5
DW_AT_type : <2e3>
DW_AT_location : 2 byte block: 91 8 (DW_OP_fbreg: 8)
<1><2e3>: Abbrev Number: 3 (DW_TAG_base_type)
DW_AT_name : double
DW_AT_byte_size : 8
DW_AT_encoding : 4 (float)
=================================================================

==== Disassembly of void fn3(double c){} =======================
fn3()
[5] 10000448: 94 21 ff e0 stwu r1, -32(r1)
1000044c: 93 e1 00 1c stw r31, 28(r1)
10000450: 7c 3f 0b 78 mr r31, r1
10000454: d8 3f 00 08 stfd fr1, 8(r31)
10000458: 81 61 00 00 lwz r11, 0(r1)
1000045c: 83 eb ff fc lwz r31, -4(r11)
10000460: 7d 61 5b 78 mr r1, r11
10000464: 4e 80 00 20 blr
==================================================================

Thanks
-----
manjo
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+ Cogito ergo sum +
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