CODEWITHSUNDEEP
cdebugginggdb
Nicholas Smith
Nicholas Smith

Reputation: 11754

gdb not setting breakpoint on correct line

I'm working through a bug in Qt/zlib at the moment, and I'm trying to check whether a variable has become corrupt prior to the line it's crashing on, so I've used break 1245 to set the break on the previous line, however:

(gdb) info break
Num     Type           Disp Enb Address    What
6       breakpoint     keep y   0x02cb0e1e in inflateEnd at inflate.c:1245
breakpoint already hit 6 times
(gdb) c
Continuing.
[Thread 0xb103db70 (LWP 26146) exited]
[Thread 0xb5a74b70 (LWP 26143) exited]

Breakpoint 6, inflateEnd (strm=0x86ccdc0) at inflate.c:1246
1246        if (state->window != Z_NULL) ZFREE(strm, state->window);

Which is the line that it's SEGFAULTing on rather than the breakpoint I've set. Bug in gdb or some quirky behaviour?

EDIT: Adding the list of the area I'm working on:

(gdb) list
1241    {
1242        struct inflate_state FAR *state;
1243        if (strm == Z_NULL || strm->state == Z_NULL || strm->zfree ==         (free_func)0)
1244            return Z_STREAM_ERROR;
1245        state = (struct inflate_state FAR *)strm->state;
1246        if (state->window != Z_NULL) ZFREE(strm, state->window);
1247        ZFREE(strm, strm->state);
1248        strm->state = Z_NULL;
1249        Tracev((stderr, "inflate: end\n"));
1250        return Z_OK;
(gdb)

EDIT: Took the advice from the comments and rebuilt it using the source packages from ubuntu (apt-get source) and building with CFLAGS and SFLAGS forced to -O0, however it now doesn't return any line numbers within gdb for the segfault, so I think I've gone wrong somewhere.

Upvotes: 2

Views: 3279

Answers (1)

Employed Russian
Employed Russian

Reputation: 213877

Which is the line that it's SEGFAULTing on rather than the breakpoint I've set

This is expected when debugging optimized code (which inflate.c likely is). The compiler moves instructions around, making code execution "jump around" when you step through it.

Upvotes: 4

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