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| -rw-r--r-- | src/tools/gdb.md | 73 |
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diff --git a/src/tools/gdb.md b/src/tools/gdb.md index 40dea1b..d0191af 100644 --- a/src/tools/gdb.md +++ b/src/tools/gdb.md @@ -274,6 +274,79 @@ define hook-quit end ``` +## Watchpoint on struct / class member +A symbolic watchpoint defined on a member variable for debugging is only valid +as long as the expression is in scope. Once out of scope the watchpoint gets +deleted. + +When debugging some memory corruption we want to keep the watchpoint even the +expression goes out of scope to find the location that overrides the variable +and introduces the corruption. + +```markdown +(gdb) l +1 struct S { int v; }; +2 +3 void set(struct S* s, int v) { +4 s->v = v; +5 } +6 +7 int main() { +8 struct S s; +9 set(&s, 1); +10 set(&s, 2); +11 set(&s, 3); +... + +(gdb) s +set (s=0x7fffffffe594, v=1) at test.c:4 +4 s->v = v; + +# Define a new watchpoint on the member of the struct. The expression however +# is only valid in the current functions scope. + +(gdb) watch s->v +Hardware watchpoint 2: s->v + +(gdb) c +Hardware watchpoint 2: s->v +Old value = 0 +New value = 1 +set (s=0x7fffffffe594, v=1) at test.c:5 +5 } + +# The watchpoint gets deleted as soon as we leave the function scope. + +(gdb) c +Watchpoint 2 deleted because the program has left the block in +which its expression is valid. +main () at test.c:10 +10 set(&s, 2); + +(gdb) p &s->v +$1 = (int *) 0x7fffffffe594 + +# Define a watchpoint o the address of the member variable of the s instance. +# This of course only makes sense as long as the s instance is not moved in memory. + +(gdb) watch *0x7fffffffe594 +Hardware watchpoint 3: *0x7fffffffe594 + +(gdb) c +Hardware watchpoint 3: *0x7fffffffe594 +Old value = 1 +New value = 2 +set (s=0x7fffffffe594, v=2) at test.c:5 +5 } + +(gdb) c +Hardware watchpoint 3: *0x7fffffffe594 +Old value = 2 +New value = 3 +set (s=0x7fffffffe594, v=3) at test.c:5 +5 } +``` + # Know Bugs ## Workaround `command + finish` bug |