GPROF Tutorial – How to use Linux GNU GCC Profiling Tool

Profiling is an important aspect of software programming. Through profiling one can determine the parts in program code that are time consuming and need to be re-written. This helps make your program execution faster which is always desired.

In very large projects, profiling can save your day by not only determining the parts in your program which are slower in execution than expected but also can help you find many other statistics through which many potential bugs can be spotted and sorted out.

In this article, we will explore the GNU profiling tool ‘gprof’.

How to use gprof

Using the gprof tool is not at all complex. You just need to do the following on a high-level:

• Have profiling enabled while compiling the code
• Execute the program code to produce the profiling data
• Run the gprof tool on the profiling data file (generated in the step above).

The last step above produces an analysis file which is in human readable form. This file contains a couple of tables (flat profile and call graph) in addition to some other information. While flat profile gives an overview of the timing information of the functions like time consumption for the execution of a particular function, how many times it was called etc. On the other hand, call graph focuses on each function like the functions through which a particular function was called, what all functions were called from within this particular function etc So this way one can get idea of the execution time spent in the sub-routines too.

Lets try and understand the three steps listed above through a practical example. Following test code will be used throughout the article :

//test_gprof.c
#include<stdio.h>

void new_func1(void);

void func1(void)
{
    printf("\n Inside func1 \n");
    int i = 0;

    for(;i<0xffffffff;i++);
    new_func1();

    return;
}

static void func2(void)
{
    printf("\n Inside func2 \n");
    int i = 0;

    for(;i<0xffffffaa;i++);
    return;
}

int main(void)
{
    printf("\n Inside main()\n");
    int i = 0;

    for(;i<0xffffff;i++);
    func1();
    func2();

    return 0;
}

//test_gprof_new.c
#include<stdio.h>

void new_func1(void)
{
    printf("\n Inside new_func1()\n");
    int i = 0;

    for(;i<0xffffffee;i++);

    return;
}

Note that the ‘for’ loops inside the functions are there to consume some execution time.

Step-1 : Profiling enabled while compilation

In this first step, we need to make sure that the profiling is enabled when the compilation of the code is done. This is made possible by adding the ‘-pg’ option in the compilation step.

From the man page of gcc :

-pg : Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

So, lets compile our code with ‘-pg’ option :

$ gcc -Wall -pg test_gprof.c test_gprof_new.c -o test_gprof
$

Please note : The option ‘-pg’ can be used with the gcc command that compiles (-c option), gcc command that links(-o option on object files) and with gcc command that does the both(as in example above).

Step-2 : Execute the code

In the second step, the binary file produced as a result of step-1 (above) is executed so that profiling information can be generated.

$ ls
test_gprof  test_gprof.c  test_gprof_new.c

$ ./test_gprof 

 Inside main()

 Inside func1 

 Inside new_func1()

 Inside func2 

$ ls
gmon.out  test_gprof  test_gprof.c  test_gprof_new.c

$

So we see that when the binary was executed, a new file ‘gmon.out’ is generated in the current working directory.

Note that while execution if the program changes the current working directory (using chdir) then gmon.out will be produced in the new current working directory. Also, your program needs to have sufficient permissions for gmon.out to be created in current working directory.

Step-3 : Run the gprof tool

In this step, the gprof tool is run with the executable name and the above generated ‘gmon.out’ as argument. This produces an analysis file which contains all the desired profiling information.

$  gprof test_gprof gmon.out > analysis.txt

Note that one can explicitly specify the output file (like in example above) or the information is produced on stdout.

$ ls
analysis.txt  gmon.out  test_gprof  test_gprof.c  test_gprof_new.c

So we see that a file named ‘analysis.txt’ was generated.

On a related note, you should also understand how to debug your C program using gdb.

Comprehending the profiling information

As produced above, all the profiling information is now present in ‘analysis.txt’. Lets have a look at this text file :

Flat profile:

Each sample counts as 0.01 seconds.
%    cumulative self          self   total
time seconds    seconds calls s/call s/call name
33.86 15.52     15.52    1    15.52  15.52  func2
33.82 31.02     15.50    1    15.50  15.50  new_func1
33.29 46.27     15.26    1    15.26  30.75  func1
0.07  46.30     0.03                        main

% the percentage of the total running time of the
time program used by this function.

cumulative a running sum of the number of seconds accounted
seconds for by this function and those listed above it.

self the number of seconds accounted for by this
seconds function alone. This is the major sort for this
listing.

calls the number of times this function was invoked, if
this function is profiled, else blank.

self the average number of milliseconds spent in this
ms/call function per call, if this function is profiled,
else blank.

total the average number of milliseconds spent in this
ms/call function and its descendents per call, if this
function is profiled, else blank.

name the name of the function. This is the minor sort
for this listing. The index shows the location of
the function in the gprof listing. If the index is
in parenthesis it shows where it would appear in
the gprof listing if it were to be printed.

Call graph (explanation follows)

granularity: each sample hit covers 2 byte(s) for 0.02% of 46.30 seconds

index % time self children called name

[1]   100.0  0.03  46.27          main [1]
             15.26 15.50    1/1      func1 [2]
             15.52 0.00     1/1      func2 [3]
-----------------------------------------------
             15.26 15.50    1/1      main [1]
[2]   66.4   15.26 15.50    1     func1 [2]
             15.50 0.00     1/1      new_func1 [4]
-----------------------------------------------
             15.52 0.00     1/1      main [1]
[3]   33.5   15.52 0.00     1     func2 [3]
-----------------------------------------------
             15.50 0.00     1/1      func1 [2]
[4] 33.5     15.50 0.00     1     new_func1 [4]
-----------------------------------------------

This table describes the call tree of the program, and was sorted by
the total amount of time spent in each function and its children.

Each entry in this table consists of several lines. The line with the
index number at the left hand margin lists the current function.
The lines above it list the functions that called this function,
and the lines below it list the functions this one called.
This line lists:
index A unique number given to each element of the table.
Index numbers are sorted numerically.
The index number is printed next to every function name so
it is easier to look up where the function in the table.

% time This is the percentage of the `total' time that was spent
in this function and its children. Note that due to
different viewpoints, functions excluded by options, etc,
these numbers will NOT add up to 100%.

self This is the total amount of time spent in this function.

children This is the total amount of time propagated into this
function by its children.

called This is the number of times the function was called.
If the function called itself recursively, the number
only includes non-recursive calls, and is followed by
a `+' and the number of recursive calls.

name The name of the current function. The index number is
printed after it. If the function is a member of a
cycle, the cycle number is printed between the
function's name and the index number.

For the function's parents, the fields have the following meanings:

self This is the amount of time that was propagated directly
from the function into this parent.

children This is the amount of time that was propagated from
the function's children into this parent.

called This is the number of times this parent called the
function `/' the total number of times the function
was called. Recursive calls to the function are not
included in the number after the `/'.

name This is the name of the parent. The parent's index
number is printed after it. If the parent is a
member of a cycle, the cycle number is printed between
the name and the index number.

If the parents of the function cannot be determined, the word
`' is printed in the `name' field, and all the other
fields are blank.

For the function's children, the fields have the following meanings:

self This is the amount of time that was propagated directly
from the child into the function.

children This is the amount of time that was propagated from the
child's children to the function.

called This is the number of times the function called
this child `/' the total number of times the child
was called. Recursive calls by the child are not
listed in the number after the `/'.

name This is the name of the child. The child's index
number is printed after it. If the child is a
member of a cycle, the cycle number is printed
between the name and the index number.

If there are any cycles (circles) in the call graph, there is an
entry for the cycle-as-a-whole. This entry shows who called the
cycle (as parents) and the members of the cycle (as children.)
The `+' recursive calls entry shows the number of function calls that
were internal to the cycle, and the calls entry for each member shows,
for that member, how many times it was called from other members of
the cycle.

Index by function name

[2] func1 [1] main
[3] func2 [4] new_func1

So (as already discussed) we see that this file is broadly divided into two parts :

1. Flat profile
2. Call graph

The individual columns for the (flat profile as well as call graph) are very well explained in the output itself.

Customize gprof output using flags

There are various flags available to customize the output of the gprof tool. Some of them are discussed below:

1. Suppress the printing of statically(private) declared functions using -a

If there are some static functions whose profiling information you do not require then this can be achieved using -a option :

$ gprof -a test_gprof gmon.out > analysis.txt

Now if we see that analysis file :

Flat profile:

Each sample counts as 0.01 seconds.
%        cumulative self           self    total
time  seconds       seconds calls  s/call  s/call  name
67.15 30.77         30.77     2    15.39  23.14    func1
33.82 46.27         15.50     1    15.50  15.50    new_func1
0.07   46.30         0.03                          main

...
...
...

Call graph (explanation follows)

granularity: each sample hit covers 2 byte(s) for 0.02% of 46.30 seconds

index   %time        self  children  called  name

[1]     100.0        0.03   46.27             main [1]
                     30.77  15.50     2/2      func1 [2]
-----------------------------------------------------
                     30.77  15.50     2/2      main [1]
[2]     99.9         30.77  15.50     2      func1 [2]
                     15.50   0.00     1/1      new_func1 [3]
----------------------------------------------------
                     15.50   0.00     1/1      func1 [2]
[3]        33.5      15.50 0.00       1      new_func1 [3]
-----------------------------------------------

...
...
...

So we see that there is no information related to func2 (which is defined static)

2. Suppress verbose blurbs using -b

As you would have already seen that gprof produces output with lot of verbose information so in case this information is not required then this can be achieved using the -b flag.

$ gprof -b test_gprof gmon.out > analysis.txt

Now if we see the analysis file :

Flat profile:

Each sample counts as 0.01 seconds.
%       cumulative    self            self    total
time    seconds       seconds  calls  s/call  s/call   name
33.86 15.52            15.52      1    15.52  15.52    func2
33.82 31.02            15.50      1    15.50  15.50    new_func1
33.29 46.27            15.26      1    15.26  30.75    func1
0.07   46.30            0.03                           main

Call graph

granularity: each sample hit covers 2 byte(s) for 0.02% of 46.30 seconds
index % time self children called name

[1]   100.0  0.03  46.27          main [1]
             15.26 15.50    1/1      func1 [2]
             15.52 0.00     1/1      func2 [3]
-----------------------------------------------
             15.26 15.50    1/1      main [1]
[2]   66.4   15.26 15.50    1     func1 [2]
             15.50 0.00     1/1      new_func1 [4]
-----------------------------------------------
             15.52 0.00     1/1      main [1]
[3]   33.5   15.52 0.00     1     func2 [3]
-----------------------------------------------
             15.50 0.00     1/1      func1 [2]
[4] 33.5     15.50 0.00     1     new_func1 [4]
-----------------------------------------------
Index by function name

[2] func1 [1] main
[3] func2 [4] new_func1

So we see that all the verbose information is not present in the analysis file.

3. Print only flat profile using -p

In case only flat profile is required then :

$ gprof -p -b test_gprof gmon.out > analysis.txt

Note that I have used(and will be using) -b option so as to avoid extra information in analysis output.

Now if we see that analysis output:

Flat profile:

Each sample counts as 0.01 seconds.
%       cumulative    self            self   total
time    seconds       seconds  calls  s/call  s/call  name
33.86   15.52          15.52      1   15.52   15.52    func2
33.82   31.02          15.50      1   15.50   15.50    new_func1
33.29   46.27          15.26      1   15.26   30.75    func1
0.07    46.30          0.03                            main

So we see that only flat profile was there in the output.

4. Print information related to specific function in flat profile

This can be achieved by providing the function name along with the -p option:

$ gprof -pfunc1 -b test_gprof gmon.out > analysis.txt

Now if we see that analysis output :

Flat profile:

Each sample counts as 0.01 seconds.
%          cumulative     self            self     total
time       seconds        seconds  calls  s/call   s/call  name
103.20     15.26          15.26     1     15.26   15.26    func1

So we see that a flat profile containing information related to only function func1 is displayed.

5. Suppress flat profile in output using -P

If flat profile is not required then it can be suppressed using the -P option :

$ gprof -P -b test_gprof gmon.out > analysis.txt

Now if we see the analysis output :

Call graph

granularity: each sample hit covers 2 byte(s) for 0.02% of 46.30 seconds

index % time self children called name

[1]   100.0  0.03  46.27          main [1]
             15.26 15.50    1/1      func1 [2]
             15.52 0.00     1/1      func2 [3]
-----------------------------------------------
             15.26 15.50    1/1      main [1]
[2]   66.4   15.26 15.50    1     func1 [2]
             15.50 0.00     1/1      new_func1 [4]
-----------------------------------------------
             15.52 0.00     1/1      main [1]
[3]   33.5   15.52 0.00     1     func2 [3]
-----------------------------------------------
             15.50 0.00     1/1      func1 [2]
[4] 33.5     15.50 0.00     1     new_func1 [4]
-----------------------------------------------
Index by function name

[2] func1 [1] main
[3] func2 [4] new_func1

So we see that flat profile was suppressed and only call graph was displayed in output.

Also, if there is a requirement to print flat profile but excluding a particular function then this is also possible using -P flag by passing the function name (to exclude) along with it.

$ gprof -Pfunc1 -b test_gprof gmon.out > analysis.txt

In the above example, we tried to exclude ‘func1’ by passing it along with the -P option to gprof. Now lets see the analysis output:

Flat profile:

Each sample counts as 0.01 seconds.
%         cumulative      self              self    total
time      seconds         seconds   calls   s/call  s/call  name
50.76     15.52            15.52      1     15.52   15.52   func2
50.69     31.02            15.50      1     15.50   15.50   new_func1
0.10      31.05            0.03                             main

So we see that flat profile was displayed but information on func1 was suppressed.

6. Print only call graph information using -q

gprof -q -b test_gprof gmon.out > analysis.txt

In the example above, the option -q was used. Lets see what effect it casts on analysis output:

Call graph

granularity: each sample hit covers 2 byte(s) for 0.02% of 46.30 seconds
index % time self children called name

[1]   100.0  0.03  46.27          main [1]
             15.26 15.50    1/1      func1 [2]
             15.52 0.00     1/1      func2 [3]
-----------------------------------------------
             15.26 15.50    1/1      main [1]
[2]   66.4   15.26 15.50    1     func1 [2]
             15.50 0.00     1/1      new_func1 [4]
-----------------------------------------------
             15.52 0.00     1/1      main [1]
[3]   33.5   15.52 0.00     1     func2 [3]
-----------------------------------------------
             15.50 0.00     1/1      func1 [2]
[4] 33.5     15.50 0.00     1     new_func1 [4]
-----------------------------------------------
Index by function name

[2] func1 [1] main
[3] func2 [4] new_func1

So we see that only call graph was printed in the output.

7. Print only specific function information in call graph.

This is possible by passing the function name along with the -q option.

$ gprof -qfunc1 -b test_gprof gmon.out > analysis.txt

Now if we see the analysis output:

Call graph

granularity: each sample hit covers 2 byte(s) for 0.02% of 46.30 seconds
index % time self children called name

             15.26 15.50    1/1      main [1]
[2]   66.4   15.26 15.50    1     func1 [2]
             15.50 0.00     1/1      new_func1 [4]
-----------------------------------------------
             15.50 0.00     1/1      func1 [2]
[4]   33.5   15.50 0.00     1     new_func1 [4]
-----------------------------------------------
Index by function name

[2] func1 (1) main
(3) func2 [4] new_func1

So we see that information related to only func1 was displayed in call graph.

8. Suppress call graph using -Q

If the call graph information is not required in the analysis output then -Q option can be used.

$ gprof -Q -b test_gprof gmon.out > analysis.txt

Now if we see the analysis output :

Flat profile:

Each sample counts as 0.01 seconds.
%       cumulative    self            self    total
time    seconds       seconds  calls  s/call  s/call   name
33.86 15.52            15.52      1   15.52   15.52    func2
33.82 31.02            15.50      1   15.50   15.50    new_func1
33.29 46.27            15.26      1   15.26   30.75    func1
0.07   46.30            0.03                           main

So we see that only flat profile is there in the output. The whole call graph got suppressed.

Also, if it is desired to suppress a specific function from call graph then this can be achieved by passing the desired function name along with the -Q option to the gprof tool.

$ gprof -Qfunc1 -b test_gprof gmon.out > analysis.txt

In the above example, the function name func1 is passed to the -Q option.

Now if we see the analysis output:

Call graph

granularity: each sample hit covers 2 byte(s) for 0.02% of 46.30 seconds
index % time self children called name

[1]   100.0  0.03  46.27          main [1]
             15.26 15.50    1/1      func1 [2]
             15.52 0.00     1/1      func2 [3]
-----------------------------------------------
             15.52 0.00     1/1      main [1]
[3]   33.5   15.52 0.00     1     func2 [3]
-----------------------------------------------
             15.50 0.00     1/1      func1 [2]
[4]   33.5   15.50 0.00     1     new_func1 [4]
-----------------------------------------------
Index by function name

(2) func1 [1] main
[3] func2 [4] new_func1

So we see that call graph information related to func1 was suppressed.