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| <h5 class="subsubsection">3.2.3.4 Interpreting the traces</h5> |
| |
| <p>If you take a look at the output it will look similar to this: |
| |
| <pre class="example"> = Start |
| [0x8048209] - 0x8064cc8 |
| [0x8048209] - 0x8064ce0 |
| [0x8048209] - 0x8064cf8 |
| [0x80481eb] + 0x8064c48 0x14 |
| [0x80481eb] + 0x8064c60 0x14 |
| [0x80481eb] + 0x8064c78 0x14 |
| [0x80481eb] + 0x8064c90 0x14 |
| = End |
| </pre> |
| <p>What this all means is not really important since the trace file is not |
| meant to be read by a human. Therefore no attention is given to |
| readability. Instead there is a program which comes with the GNU C |
| library which interprets the traces and outputs a summary in an |
| user-friendly way. The program is called <code>mtrace</code> (it is in fact a |
| Perl script) and it takes one or two arguments. In any case the name of |
| the file with the trace output must be specified. If an optional |
| argument precedes the name of the trace file this must be the name of |
| the program which generated the trace. |
| |
| <pre class="example"> drepper$ mtrace tst-mtrace log |
| No memory leaks. |
| </pre> |
| <p>In this case the program <code>tst-mtrace</code> was run and it produced a |
| trace file <samp><span class="file">log</span></samp>. The message printed by <code>mtrace</code> shows there |
| are no problems with the code, all allocated memory was freed |
| afterwards. |
| |
| <p>If we call <code>mtrace</code> on the example trace given above we would get a |
| different outout: |
| |
| <pre class="example"> drepper$ mtrace errlog |
| - 0x08064cc8 Free 2 was never alloc'd 0x8048209 |
| - 0x08064ce0 Free 3 was never alloc'd 0x8048209 |
| - 0x08064cf8 Free 4 was never alloc'd 0x8048209 |
| |
| Memory not freed: |
| ----------------- |
| Address Size Caller |
| 0x08064c48 0x14 at 0x80481eb |
| 0x08064c60 0x14 at 0x80481eb |
| 0x08064c78 0x14 at 0x80481eb |
| 0x08064c90 0x14 at 0x80481eb |
| </pre> |
| <p>We have called <code>mtrace</code> with only one argument and so the script |
| has no chance to find out what is meant with the addresses given in the |
| trace. We can do better: |
| |
| <pre class="example"> drepper$ mtrace tst errlog |
| - 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39 |
| - 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39 |
| - 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39 |
| |
| Memory not freed: |
| ----------------- |
| Address Size Caller |
| 0x08064c48 0x14 at /home/drepper/tst.c:33 |
| 0x08064c60 0x14 at /home/drepper/tst.c:33 |
| 0x08064c78 0x14 at /home/drepper/tst.c:33 |
| 0x08064c90 0x14 at /home/drepper/tst.c:33 |
| </pre> |
| <p>Suddenly the output makes much more sense and the user can see |
| immediately where the function calls causing the trouble can be found. |
| |
| <p>Interpreting this output is not complicated. There are at most two |
| different situations being detected. First, <code>free</code> was called for |
| pointers which were never returned by one of the allocation functions. |
| This is usually a very bad problem and what this looks like is shown in |
| the first three lines of the output. Situations like this are quite |
| rare and if they appear they show up very drastically: the program |
| normally crashes. |
| |
| <p>The other situation which is much harder to detect are memory leaks. As |
| you can see in the output the <code>mtrace</code> function collects all this |
| information and so can say that the program calls an allocation function |
| from line 33 in the source file <samp><span class="file">/home/drepper/tst-mtrace.c</span></samp> four |
| times without freeing this memory before the program terminates. |
| Whether this is a real problem remains to be investigated. |
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