### 5.3 Line-by-line Profiling

`gprof`'s ‘`-l`’ option causes the program to perform line-by-line 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 `gcc` compiler. Newer versions of `gcc` use a different program - `gcov` - to display line-by-line profiling information.

With the older versions of `gcc` the program usually has to be compiled with a ‘`-g`’ option, in addition to ‘`-pg`’, in order to generate debugging symbols for tracking source code lines. Note, in much older versions of `gcc` the program had to be compiled with the ‘`-a`’ command line option as well.

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 `gprof` 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.

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

         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

Now let's look at some of `gprof`'s output from the same program run, this time with line-by-line profiling enabled. Note that `ct_init`'s four histogram hits are broken down into four lines of source code—one hit occurred on each of lines 349, 351, 382 and 385. In the call graph, note how `ct_init`'s 13327 calls to `init_block` are broken down into one call from line 396, 3071 calls from line 384, 3730 calls from line 385, and 6525 calls from 387.

         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)