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<h3 class="section">5.3 Line-by-line Profiling</h3>

<p><code>gprof</code>'s &lsquo;<samp><span class="samp">-l</span></samp>&rsquo; option causes the program to perform
<dfn>line-by-line</dfn> profiling.  In this mode, histogram
samples are assigned not to functions, but to individual
lines of source code.  This only works with programs compiled with
older versions of the <code>gcc</code> compiler.  Newer versions of <code>gcc</code>
use a different program - <code>gcov</code> - to display line-by-line
profiling information.

   <p>With the older versions of <code>gcc</code> the program usually has to be
compiled with a &lsquo;<samp><span class="samp">-g</span></samp>&rsquo; option, in addition to &lsquo;<samp><span class="samp">-pg</span></samp>&rsquo;, in order
to generate debugging symbols for tracking source code lines. 
Note, in much older versions of <code>gcc</code> the program had to be
compiled with the &lsquo;<samp><span class="samp">-a</span></samp>&rsquo; command line option as well.

   <p>The flat profile is the most useful output table
in line-by-line mode. 
The call graph isn't as useful as normal, since
the current version of <code>gprof</code> does not propagate
call graph arcs from source code lines to the enclosing function. 
The call graph does, however, show each line of code
that called each function, along with a count.

   <p>Here is a section of <code>gprof</code>'s output, without line-by-line profiling. 
Note that <code>ct_init</code> accounted for four histogram hits, and
13327 calls to <code>init_block</code>.

<pre class="smallexample">     Flat profile:
     
     Each sample counts as 0.01 seconds.
       %   cumulative   self              self     total
      time   seconds   seconds    calls  us/call  us/call  name
      30.77      0.13     0.04     6335     6.31     6.31  ct_init
     
     
     		     Call graph (explanation follows)
     
     
     granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds
     
     index % time    self  children    called     name
     
                     0.00    0.00       1/13496       name_too_long
                     0.00    0.00      40/13496       deflate
                     0.00    0.00     128/13496       deflate_fast
                     0.00    0.00   13327/13496       ct_init
     [7]      0.0    0.00    0.00   13496         init_block
     
</pre>
   <p>Now let's look at some of <code>gprof</code>'s output from the same program run,
this time with line-by-line profiling enabled.  Note that <code>ct_init</code>'s
four histogram hits are broken down into four lines of source code&mdash;one hit
occurred on each of lines 349, 351, 382 and 385.  In the call graph,
note how
<code>ct_init</code>'s 13327 calls to <code>init_block</code> are broken down
into one call from line 396, 3071 calls from line 384, 3730 calls
from line 385, and 6525 calls from 387.

<pre class="smallexample">     Flat profile:
     
     Each sample counts as 0.01 seconds.
       %   cumulative   self
      time   seconds   seconds    calls  name
       7.69      0.10     0.01           ct_init (trees.c:349)
       7.69      0.11     0.01           ct_init (trees.c:351)
       7.69      0.12     0.01           ct_init (trees.c:382)
       7.69      0.13     0.01           ct_init (trees.c:385)
     
     
     		     Call graph (explanation follows)
     
     
     granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds
     
       % time    self  children    called     name
     
                 0.00    0.00       1/13496       name_too_long (gzip.c:1440)
                 0.00    0.00       1/13496       deflate (deflate.c:763)
                 0.00    0.00       1/13496       ct_init (trees.c:396)
                 0.00    0.00       2/13496       deflate (deflate.c:727)
                 0.00    0.00       4/13496       deflate (deflate.c:686)
                 0.00    0.00       5/13496       deflate (deflate.c:675)
                 0.00    0.00      12/13496       deflate (deflate.c:679)
                 0.00    0.00      16/13496       deflate (deflate.c:730)
                 0.00    0.00     128/13496       deflate_fast (deflate.c:654)
                 0.00    0.00    3071/13496       ct_init (trees.c:384)
                 0.00    0.00    3730/13496       ct_init (trees.c:385)
                 0.00    0.00    6525/13496       ct_init (trees.c:387)
     [6]  0.0    0.00    0.00   13496         init_block (trees.c:408)
     
</pre>
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