SUMMARY: oddity with debugger and sprintf( )

From: Eiler, James A. (James.Eiler_at_alcoa.com)
Date: 09/10/04

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    Date: Fri, 10 Sep 2004 16:06:14 -0400
To: tru64-unix-managers@ornl.gov

    Original posting at the end...

    Thanks to James Sainsbury and Bob Harris for their replies.

    Both suggested line ordering as the root "problem" and James gave a very detailed response:

    It's probably not obvious but the compiler can and does rearrange
    code sequences.
    ie
    the code generated for source line `n+k' can precede that for line `n'
    cc -S and dis are useful here.

    The problem seems to be that you have set a breakpoint at line 12
    and branched to line 14 (jumping over line 13 - which in source is empty).
    The sequence
            stop at 12
            run
            next
    doesn't produce an exception.

    Looking at the .o file - stopping before line 13 and starting after line 13
    (ie line 15) would probably explain the observed problem

    dis jaetest.o
            main:

      [jaetest.c: 5] 0x0: 27bb0001 ldah gp, 1(t12)
      [jaetest.c: 5] 0x4: 23bd80c0 lda gp, -32576(gp)
      [jaetest.c: 5] 0x8: 23defec0 lda sp, -320(sp)
      [jaetest.c: 5] 0xc: b75e0018 stq ra, 24(sp)
      [jaetest.c: 11] 0x10: 221e0028 lda a0, 40(sp)
      [jaetest.c: 11] 0x14: a63d8028 ldq a1, -32728(gp)
      [jaetest.c: 11] 0x18: 22310000 lda a1, 0(a1)
      [jaetest.c: 11] 0x1c: a77d8020 ldq t12, -32736(gp)
      [jaetest.c: 11] 0x20: 6b5b4000 jsr ra, (t12), strcpy
      [jaetest.c: 11] 0x24: 27ba0001 ldah gp, 1(ra)
      [jaetest.c: 11] 0x28: 23bd809c lda gp, -32612(gp)
    # Break point here
      [jaetest.c: 13] 0x2c: 47e03400 bis zero, 0x1, v0
      [jaetest.c: 13] 0x30: b01e0020 stl v0, 32(sp)
      [jaetest.c: 16] 0x34: 203e0028 lda t0, 40(sp)
      [jaetest.c: 16] 0x38: 205e0028 lda t1, 40(sp)
      [jaetest.c: 16] 0x3c: 207e0028 lda t2, 40(sp)
    # Branched to here
      [jaetest.c: 15] 0x40: b43e0000 stq t0, 0(sp)
      [jaetest.c: 15] 0x44: b45e0008 stq t1, 8(sp)
      [jaetest.c: 15] 0x48: b47e0010 stq t2, 16(sp)
      [jaetest.c: 15] 0x4c: 221e0038 lda a0, 56(sp)
      [jaetest.c: 15] 0x50: a63d8018 ldq a1, -32744(gp)
      [jaetest.c: 15] 0x54: 22310000 lda a1, 0(a1)
      [jaetest.c: 15] 0x58: 225e0028 lda a2, 40(sp)
      [jaetest.c: 15] 0x5c: 227e0028 lda a3, 40(sp)
      [jaetest.c: 15] 0x60: 229e0028 lda a4, 40(sp)
      [jaetest.c: 15] 0x64: 22be0028 lda a5, 40(sp)
      [jaetest.c: 15] 0x68: a77d8010 ldq t12, -32752(gp)
      [jaetest.c: 15] 0x6c: 2ffe0000 ldq_u zero, 0(sp)
      [jaetest.c: 15] 0x70: 6b5b4000 jsr ra, (t12), sprintf
      [jaetest.c: 15] 0x74: 27ba0001 ldah gp, 1(ra)
      [jaetest.c: 15] 0x78: 23bd804c lda gp, -32692(gp)
      [jaetest.c: 18] 0x7c: 47ff0400 bis zero, zero, v0
      [jaetest.c: 18] 0x80: a75e0018 ldq ra, 24(sp)
      [jaetest.c: 18] 0x84: 23de0140 lda sp, 320(sp)
      [jaetest.c: 18] 0x88: 6bfa8001 ret zero, (ra), 1
      [jaetest.c: 18] 0x8c: 00000000 call_pal halt

    RGDS

    Original Posting:

    On Thu, 12 Aug 2004, Eiler, James A. wrote:

    > All,
    >
    > I've tried the following on several Alphas with different versions of T64 (4.0G, 5.1A, 5.1B) (and different versions of dbx and ladebug) and got the same result every time.
    >
    > My little C program is as follows:
    >
    > 1 #include <stdio.h>
    > 2 #include <string.h>
    > 3
    > 4 main( )
    > 5 {
    > 6 char sName [ 16 ];
    > 7 char sString[ 256 ];
    > 8 int iStatus;
    > 9
    > 10 strcpy( sName, "bar" );
    > 11
    > 12 iStatus = 1;
    > 13
    > 14 sprintf( sString, "%s %s %s %s %s %s %s",
    > 15 sName, sName, sName, sName, sName, sName, sName );
    > 16
    > 17 }
    >
    >
    > If I compile it and run it with the debugger (dbx or ladebug), set a breakpoint at line 12, then run it to the breakpoint, then goto line 14, then do a next, I get a segmentation violation:
    >
    > alpha1-mst> cc jaetest.c -g2 -o jaetest
    > alpha1-mst> dbx jaetest
    > dbx version 5.1
    > Type 'help' for help.
    >
    > main: 10 strcpy( sName, "bar" );
    > (dbx) b 12
    > [2] stop at "jaetest.c":12
    > (dbx) r
    > [2] stopped at [main:12 ,0x12000118c] iStatus = 1;
    > (dbx) goto 14
    > (dbx) n
    > signal Segmentation fault at >*[strlen, 0x3ff800d27e0] ldq_u t0, 0(a0)
    >
    > However, if I just run it (from the command prompt), it runs just fine.
    >
    > And if I run in from the debugger and just "next" through it, it also works just fine.
    >
    > Lastly, if I change the sprintf( ) to have less than 5 "%s" conversion strings, it also works just fine!
    >
    > I stumbled across this quite by accident while trying to debug one of my programs, and this is the essence of what I was running into.
    >
    > Has anyone seen anything like this before?
    >
    > Thanks!
    >
    > Jim
    >

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