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| <h4 class="subsection">13.1.10 Tracepoint Restrictions</h4> |
| |
| <p><a name="index-tracepoint-restrictions-926"></a>There are a number of restrictions on the use of tracepoints. As |
| described above, tracepoint data gathering occurs on the target |
| without interaction from <span class="sc">gdb</span>. Thus the full capabilities of |
| the debugger are not available during data gathering, and then at data |
| examination time, you will be limited by only having what was |
| collected. The following items describe some common problems, but it |
| is not exhaustive, and you may run into additional difficulties not |
| mentioned here. |
| |
| <ul> |
| <li>Tracepoint expressions are intended to gather objects (lvalues). Thus |
| the full flexibility of GDB's expression evaluator is not available. |
| You cannot call functions, cast objects to aggregate types, access |
| convenience variables or modify values (except by assignment to trace |
| state variables). Some language features may implicitly call |
| functions (for instance Objective-C fields with accessors), and therefore |
| cannot be collected either. |
| |
| <li>Collection of local variables, either individually or in bulk with |
| <code>$locals</code> or <code>$args</code>, during <code>while-stepping</code> may |
| behave erratically. The stepping action may enter a new scope (for |
| instance by stepping into a function), or the location of the variable |
| may change (for instance it is loaded into a register). The |
| tracepoint data recorded uses the location information for the |
| variables that is correct for the tracepoint location. When the |
| tracepoint is created, it is not possible, in general, to determine |
| where the steps of a <code>while-stepping</code> sequence will advance the |
| program—particularly if a conditional branch is stepped. |
| |
| <li>Collection of an incompletely-initialized or partially-destroyed object |
| may result in something that <span class="sc">gdb</span> cannot display, or displays |
| in a misleading way. |
| |
| <li>When <span class="sc">gdb</span> displays a pointer to character it automatically |
| dereferences the pointer to also display characters of the string |
| being pointed to. However, collecting the pointer during tracing does |
| not automatically collect the string. You need to explicitly |
| dereference the pointer and provide size information if you want to |
| collect not only the pointer, but the memory pointed to. For example, |
| <code>*ptr@50</code> can be used to collect the 50 element array pointed to |
| by <code>ptr</code>. |
| |
| <li>It is not possible to collect a complete stack backtrace at a |
| tracepoint. Instead, you may collect the registers and a few hundred |
| bytes from the stack pointer with something like <code>*(unsigned char *)$esp@300</code> |
| (adjust to use the name of the actual stack pointer register on your |
| target architecture, and the amount of stack you wish to capture). |
| Then the <code>backtrace</code> command will show a partial backtrace when |
| using a trace frame. The number of stack frames that can be examined |
| depends on the sizes of the frames in the collected stack. Note that |
| if you ask for a block so large that it goes past the bottom of the |
| stack, the target agent may report an error trying to read from an |
| invalid address. |
| |
| <li>If you do not collect registers at a tracepoint, <span class="sc">gdb</span> can |
| infer that the value of <code>$pc</code> must be the same as the address of |
| the tracepoint and use that when you are looking at a trace frame |
| for that tracepoint. However, this cannot work if the tracepoint has |
| multiple locations (for instance if it was set in a function that was |
| inlined), or if it has a <code>while-stepping</code> loop. In those cases |
| <span class="sc">gdb</span> will warn you that it can't infer <code>$pc</code>, and default |
| it to zero. |
| |
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