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| <h2 class="chapter">2 Compiling a Program for Profiling</h2> |
| |
| <p>The first step in generating profile information for your program is |
| to compile and link it with profiling enabled. |
| |
| <p>To compile a source file for profiling, specify the ‘<samp><span class="samp">-pg</span></samp>’ option when |
| you run the compiler. (This is in addition to the options you normally |
| use.) |
| |
| <p>To link the program for profiling, if you use a compiler such as <code>cc</code> |
| to do the linking, simply specify ‘<samp><span class="samp">-pg</span></samp>’ in addition to your usual |
| options. The same option, ‘<samp><span class="samp">-pg</span></samp>’, alters either compilation or linking |
| to do what is necessary for profiling. Here are examples: |
| |
| <pre class="example"> cc -g -c myprog.c utils.c -pg |
| cc -o myprog myprog.o utils.o -pg |
| </pre> |
| <p>The ‘<samp><span class="samp">-pg</span></samp>’ option also works with a command that both compiles and links: |
| |
| <pre class="example"> cc -o myprog myprog.c utils.c -g -pg |
| </pre> |
| <p>Note: The ‘<samp><span class="samp">-pg</span></samp>’ option must be part of your compilation options |
| as well as your link options. If it is not then no call-graph data |
| will be gathered and when you run <code>gprof</code> you will get an error |
| message like this: |
| |
| <pre class="example"> gprof: gmon.out file is missing call-graph data |
| </pre> |
| <p>If you add the ‘<samp><span class="samp">-Q</span></samp>’ switch to suppress the printing of the call |
| graph data you will still be able to see the time samples: |
| |
| <pre class="example"> Flat profile: |
| |
| Each sample counts as 0.01 seconds. |
| % cumulative self self total |
| time seconds seconds calls Ts/call Ts/call name |
| 44.12 0.07 0.07 zazLoop |
| 35.29 0.14 0.06 main |
| 20.59 0.17 0.04 bazMillion |
| </pre> |
| <p>If you run the linker <code>ld</code> directly instead of through a compiler |
| such as <code>cc</code>, you may have to specify a profiling startup file |
| <samp><span class="file">gcrt0.o</span></samp> as the first input file instead of the usual startup |
| file <samp><span class="file">crt0.o</span></samp>. In addition, you would probably want to |
| specify the profiling C library, <samp><span class="file">libc_p.a</span></samp>, by writing |
| ‘<samp><span class="samp">-lc_p</span></samp>’ instead of the usual ‘<samp><span class="samp">-lc</span></samp>’. This is not absolutely |
| necessary, but doing this gives you number-of-calls information for |
| standard library functions such as <code>read</code> and <code>open</code>. For |
| example: |
| |
| <pre class="example"> ld -o myprog /lib/gcrt0.o myprog.o utils.o -lc_p |
| </pre> |
| <p>If you are running the program on a system which supports shared |
| libraries you may run into problems with the profiling support code in |
| a shared library being called before that library has been fully |
| initialised. This is usually detected by the program encountering a |
| segmentation fault as soon as it is run. The solution is to link |
| against a static version of the library containing the profiling |
| support code, which for <code>gcc</code> users can be done via the |
| ‘<samp><span class="samp">-static</span></samp>’ or ‘<samp><span class="samp">-static-libgcc</span></samp>’ command line option. For |
| example: |
| |
| <pre class="example"> gcc -g -pg -static-libgcc myprog.c utils.c -o myprog |
| </pre> |
| <p>If you compile only some of the modules of the program with ‘<samp><span class="samp">-pg</span></samp>’, you |
| can still profile the program, but you won't get complete information about |
| the modules that were compiled without ‘<samp><span class="samp">-pg</span></samp>’. The only information |
| you get for the functions in those modules is the total time spent in them; |
| there is no record of how many times they were called, or from where. This |
| will not affect the flat profile (except that the <code>calls</code> field for |
| the functions will be blank), but will greatly reduce the usefulness of the |
| call graph. |
| |
| <p>If you wish to perform line-by-line profiling you should use the |
| <code>gcov</code> tool instead of <code>gprof</code>. See that tool's manual or |
| info pages for more details of how to do this. |
| |
| <p>Note, older versions of <code>gcc</code> produce line-by-line profiling |
| information that works with <code>gprof</code> rather than <code>gcov</code> so |
| there is still support for displaying this kind of information in |
| <code>gprof</code>. See <a href="Line_002dby_002dline.html#Line_002dby_002dline">Line-by-line Profiling</a>. |
| |
| <p>It also worth noting that <code>gcc</code> implements a |
| ‘<samp><span class="samp">-finstrument-functions</span></samp>’ command line option which will insert |
| calls to special user supplied instrumentation routines at the entry |
| and exit of every function in their program. This can be used to |
| implement an alternative profiling scheme. |
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