GNU cflow: 3 Two Types of Flow Graphs.

In the previous chapter we have discussed direct graphs, displaying caller—callee dependencies. Another type of cflow output, called reverse graph, charts callee—caller dependencies. To produce a reverse graph, run cflow with ‘--reverse’ (‘-r’) command line option. For example, using a sample ‘whoami.c’:

$ cflow --reverse whoami.c
fprintf():
    who_am_i() <int who_am_i (void) at whoami.c:8>:
        main() <int main (int argc,char **argv) at whoami.c:26>
    main() <int main (int argc,char **argv) at whoami.c:26>
getenv():
    who_am_i() <int who_am_i (void) at whoami.c:8>:
        main() <int main (int argc,char **argv) at whoami.c:26>
geteuid():
    who_am_i() <int who_am_i (void) at whoami.c:8>:
        main() <int main (int argc,char **argv) at whoami.c:26>
getpwuid():
    who_am_i() <int who_am_i (void) at whoami.c:8>:
        main() <int main (int argc,char **argv) at whoami.c:26>
main() <int main (int argc,char **argv) at whoami.c:26>
printf():
    who_am_i() <int who_am_i (void) at whoami.c:8>:
        main() <int main (int argc,char **argv) at whoami.c:26>
who_am_i() <int who_am_i (void) at whoami.c:8>:
    main() <int main (int argc,char **argv) at whoami.c:26>

This output consists of several subgraphs, each describing callers for a particular function. Thus, the first subgraph tells that the function fprintf is called from two functions: who_am_i and main. First of them is, in turn, also called directly by main.

The first thing that draws attention in the above output is that the subgraph starting with who_am_i function is repeated several times. This is a verbose output. To make it brief, use ‘--brief’ (‘-b’) command line option. For example:

$ cflow --brief --reverse whoami.c
fprintf():
    who_am_i() <int who_am_i (void) at whoami.c:8>:
        main() <int main (int argc,char **argv) at whoami.c:26>
    main() <int main (int argc,char **argv) at whoami.c:26> [see 3]
getenv():
    who_am_i() <int who_am_i (void) at whoami.c:8>: [see 2]
geteuid():
    who_am_i() <int who_am_i (void) at whoami.c:8>: [see 2]
getpwuid():
    who_am_i() <int who_am_i (void) at whoami.c:8>: [see 2]
main() <int main (int argc,char **argv) at whoami.c:26> [see 3]
printf():
    who_am_i() <int who_am_i (void) at whoami.c:8>: [see 2]
who_am_i() <int who_am_i (void) at whoami.c:8>: [see 2]

In brief output, once a subgraph for a given function is written, subsequent instances of calls to that function contain only its definition and the reference to the output line where the expanded subgraph can be found.

If the output graph is large, it can be tedious to find out the required line number (unless you use Emacs cflow-mode, see section Using cflow with GNU Emacs.). For such cases a special option ‘--number’ (‘-n’) is provided, which makes cflow begin each line of the output with a reference number, that is the ordinal number of this line in the output. With this option, the above output will look like:

$ cflow --number --brief --reverse whoami.c

    1 fprintf():
    2     who_am_i() <int who_am_i (void) at whoami.c:8>:
    3         main() <int main (int argc,char **argv) at whoami.c:26>
    4     main() <int main (int argc,char **argv) at whoami.c:26> [see 3]
    5 getenv():
    6     who_am_i() <int who_am_i (void) at whoami.c:8>: [see 2]
    7 geteuid():
    8     who_am_i() <int who_am_i (void) at whoami.c:8>: [see 2]
    9 getpwuid():
   10     who_am_i() <int who_am_i (void) at whoami.c:8>: [see 2]
   11 main() <int main (int argc,char **argv) at whoami.c:26> [see 3]
   12 printf():
   13     who_am_i() <int who_am_i (void) at whoami.c:8>: [see 2]
   14 who_am_i() <int who_am_i (void) at whoami.c:8>: [see 2]

Of course, ‘--brief’ and ‘--number’ options take effect for both direct and reverse flow graphs.