使用profile来得到程序运行信息
profile程序可以用来监测程序不同部分（主要是各个函数）的使用时间和调用次数。
因此可以使用这个程序来鉴别程序中那个函数是整个程序的瓶颈，从而可以通过优化这个
函数来提高程序的性能。
Unix/Linux系统提供了GPROF这个profile程序。该程序提供两种信息：
1、程序中每个函数的CPU使用时间。
2、每个函数的调用次数。并提供简单调用关系图。
使用GPROF步骤如下：
1、用gcc或g++编译程序时，使用-pg参数，如：g++ -pg -o test.out test.cpp
2、执行编译得到的运行程序。如：./test.out
该步骤运行程序的时间会稍慢于正常编译出来的程序执行时间。而且会产生一个
gmon.out文件。
3、使用gprof命令。如：gprof test.out
于是可以在显示器上看到上述信息。
/////////////////////////////////////////////////////////////////////////////
ps1：test.cpp程序使用gprof得到的结果
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls ms/call ms/call name
60.00 0.09 0.09 50 1.80 1.80 test2(int)
40.00 0.15 0.06 50 1.20 1.20 test1(int)
% 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 4 byte(s) for 6.67% of 0.15 seconds
index % time self children called name
<spontaneous>
[1] 100.0 0.00 0.15 main [1]
0.09 0.00 50/50 test2(int) [2]
0.06 0.00 50/50 test1(int) [3]
-----------------------------------------------
0.09 0.00 50/50 main [1]
[2] 60.0 0.09 0.00 50 test2(int) [2]
-----------------------------------------------
0.06 0.00 50/50 main [1]
[3] 40.0 0.06 0.00 50 test1(int) [3]
-----------------------------------------------
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
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