251 lines
11 KiB
XML
251 lines
11 KiB
XML
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<?xml version="1.0" encoding='UTF-8'?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<sect1 id="gprof"><title>Profiling Cygwin Programs</title>
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<sect2 id="gprof-intro"><title>Introduction</title>
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<para>Profiling is a way to analyze your program to find out where it is
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spending its time. You might need to do this if it seems your program is
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taking more time to do its job than you think it should. It is always
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preferable to profile your program than to just guess where the time is
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being spent; even expert programmers are known to guess badly at this.
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</para>
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<para>In Cygwin, you enable profiling with a compiler flag and you display
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the resulting profiling data with gprof. Read on to find out how.
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</para>
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<para>To enable profiling of your program, first compile it with an
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additional gcc flag: <userinput>-pg</userinput>. That flag should be used
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when compiling every source file of the program. If your program has a
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Makefile, you would add the flag to all gcc compilation commands or to the
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CFLAGS= setting. A manual compilation that enables profiling looks like this:
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</para>
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<screen>
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<prompt>bash$</prompt> <userinput>gcc -pg -g -o myapp myapp.c</userinput>
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</screen>
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<para>The <userinput>-pg</userinput> flag causes gcc to do two additional
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things as it compiles your program. First, a small bit of code is added to
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the beginning of each function that records its address and the address it
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was called from at run time. gprof uses this data to generate a call graph.
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Second, gcc arranges to have a special "front end" added to the beginning
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of your program. The front end starts a recurring timer and every time the
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timer fires, 100 times per second, the currently executing address is saved.
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gprof uses this data to generate a "flat profile" showing where your
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program is spending its time.
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</para>
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<para>After compiling your program (and linking it, if you do that as a
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separate step), you are ready to profile it. Just run it as you normally
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would. If there are specific code paths you want to profile, take the
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actions that would exercise those code paths. When your program exits,
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you will have an additional file in the current directory: gmon.out.
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That file contains the profiling data gprof processes and displays.
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</para>
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<para>gprof has many flags to control its operation. The
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<citation>gprof man page</citation> details everything gprof can do. We
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will only use a few of gprof's flags here. You launch gprof as follows:
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<screen>
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<prompt>bash$</prompt> <userinput>gprof [flags] appname [datafile]...</userinput>
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</screen>
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If you don't specify any flags, gprof operates as if you gave it flags
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<userinput>-p -q</userinput> which means: generate a flat profile with
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descriptive text and generate a call graph with more descriptive text. In
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the examples below we will give specific flags to gprof to demonstrate
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specific displays. We'll also use flag <userinput>-b</userinput> which
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means: be brief, i.e. don't display the descriptive text. You can also
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specify a trailing list of one or more profiling data files. If you don't,
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gprof assumes gmon.out is the only file to process and display.
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</para>
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</sect2>
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<sect2 id="gprof-ex"><title>Examples</title>
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<example id="gprof-flat"><title>Flat profile</title>
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<screen>
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<prompt>bash$</prompt> <userinput>gprof -b -p myapp</userinput>
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<literal>Flat profile:
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Each sample counts as 0.01 seconds.
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% cumulative self self total
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time seconds seconds calls s/call s/call name
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25.11 13.34 13.34 1 13.34 13.34 func0
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25.00 26.62 13.28 1 13.28 13.28 func1
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25.00 39.90 13.28 1 13.28 13.28 func3
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24.89 53.12 13.22 1 13.22 13.22 func2
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</literal> </screen> </example>
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<example id="gprof-cg"><title>Call graph</title>
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<screen>
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<prompt>bash$</prompt> <userinput>gprof -b -q myapp</userinput>
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<literal> Call graph
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granularity: each sample hit covers 4 byte(s) for 0.02% of 53.12 seconds
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index % time self children called name
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<spontaneous>
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[1] 100.0 0.00 53.12 main [1]
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13.34 0.00 1/1 func0 [2]
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13.28 0.00 1/1 func1 [3]
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13.28 0.00 1/1 func3 [4]
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13.22 0.00 1/1 func2 [5]
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-----------------------------------------------
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13.34 0.00 1/1 main [1]
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[2] 25.1 13.34 0.00 1 func0 [2]
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-----------------------------------------------
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13.28 0.00 1/1 main [1]
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[3] 25.0 13.28 0.00 1 func1 [3]
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-----------------------------------------------
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13.28 0.00 1/1 main [1]
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[4] 25.0 13.28 0.00 1 func3 [4]
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-----------------------------------------------
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13.22 0.00 1/1 main [1]
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[5] 24.9 13.22 0.00 1 func2 [5]
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-----------------------------------------------
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Index by function name
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[2] func0 [5] func2
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[3] func1 [4] func3
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</literal> </screen> </example>
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<example id="gprof-line"><title>Source line profile</title>
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<screen>
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<prompt>bash$</prompt> <userinput>gprof -b -l myapp</userinput>
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<literal>Flat profile:
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Each sample counts as 0.01 seconds.
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% cumulative self self total
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time seconds seconds calls s/call s/call name
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25.11 13.34 13.34 1 13.34 13.34 func0 (myapp.c:9 @ 1004010e0)
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25.00 26.62 13.28 1 13.28 13.28 func1 (myapp.c:10 @ 10040111a)
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25.00 39.90 13.28 1 13.28 13.28 func3 (myapp.c:12 @ 10040118e)
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24.89 53.12 13.22 1 13.22 13.22 func2 (myapp.c:11 @ 100401154)
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Call graph
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granularity: each sample hit covers 4 byte(s) for 0.02% of 53.12 seconds
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index % time self children called name
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13.34 0.00 1/1 main (myapp.c:26 @ 10040123c) [1]
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[2] 25.1 13.34 0.00 1 func0 (myapp.c:9 @ 1004010e0) [2]
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-----------------------------------------------
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13.28 0.00 1/1 main (myapp.c:26 @ 10040123c) [1]
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[3] 25.0 13.28 0.00 1 func1 (myapp.c:10 @ 10040111a) [3]
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-----------------------------------------------
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13.28 0.00 1/1 main (myapp.c:28 @ 100401246) [5]
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[4] 25.0 13.28 0.00 1 func3 (myapp.c:12 @ 10040118e) [4]
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-----------------------------------------------
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13.22 0.00 1/1 main (myapp.c:27 @ 100401241) [7]
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[6] 24.9 13.22 0.00 1 func2 (myapp.c:11 @ 100401154) [6]
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-----------------------------------------------
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Index by function name
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[2] func0 (myapp.c:9 @ 1004010e0) [6] func2 (myapp.c:11 @ 100401154)
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[3] func1 (myapp.c:10 @ 10040111a) [4] func3 (myapp.c:12 @ 10040118e)
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</literal> </screen> </example>
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</sect2>
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<sect2 id="gprof-ss"><title>Special situations</title>
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<sect3 id="gprof-mt"><title>Profiling multi-threaded programs</title>
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<para>Multi-threaded programs are profiled just like single-threaded programs.
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There is no mechanism to turn profiling on or off for specific threads.
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gprof combines the data for all threads when generating its displays.
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</para>
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</sect3>
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<sect3 id="gprof-fork"><title>Profiling programs that fork</title>
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<para>Programs that fork, i.e., use the fork() system call with or without
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using exec() afterwards, require special care. Since there is only one
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gmon.out file, profiling data from the parent process might get overwritten
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by the child process, or vice-versa, after a fork(). You can avoid this by
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setting the environment variable GMON_OUT_PREFIX before running your
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program. If the variable is non-empty, its contents will be used as a
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prefix to name the profiling data files. Here's an example:
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</para>
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<example id="gprof-prefix">
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<screen>
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<prompt>bash$</prompt> <userinput>export GMON_OUT_PREFIX=myapp.out</userinput>
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<prompt>bash$</prompt> <userinput>./myapp -fork</userinput>
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<prompt>bash$</prompt> <userinput>ls myapp.out*</userinput>
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<literal>myapp.out.2728 myapp.out.3224
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</literal>
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<prompt>bash$</prompt> <userinput>gprof -bp myapp myapp.out.2728</userinput>
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<literal>Flat profile:
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Each sample counts as 0.01 seconds.
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% cumulative self self total
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time seconds seconds calls s/call s/call name
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50.25 30.28 30.28 2 15.14 15.14 func3
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24.99 45.34 15.06 1 15.06 15.06 func1
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24.76 60.26 14.92 1 14.92 14.92 func2
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</literal>
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<prompt>bash$</prompt> <userinput>gprof -bp myapp myapp.out.3224</userinput>
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<literal>Flat profile:
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Each sample counts as 0.01 seconds.
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% cumulative self self total
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time seconds seconds calls s/call s/call name
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49.25 29.36 29.36 2 14.68 14.68 func3
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25.43 44.52 15.16 1 15.16 15.16 func1
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25.33 59.62 15.10 1 15.10 15.10 func2
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</literal>
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<prompt>bash$</prompt> <userinput>gprof -bp myapp myapp.out*</userinput>
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<literal>Flat profile:
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Each sample counts as 0.01 seconds.
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% cumulative self self total
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time seconds seconds calls s/call s/call name
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49.75 59.64 59.64 4 14.91 14.91 func3
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25.21 89.86 30.22 2 15.11 15.11 func1
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25.04 119.88 30.02 2 15.01 15.01 func2
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</literal> </screen> </example>
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<para>As the last gprof command above shows, gprof can combine the data
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from a selection of profiling data files to generate its displays. Just
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list the names of those files at the end of the gprof command; you can use
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a wildcard here. NOTE: If you update your program, remember to remove stale
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profiling data files before profiling your program again. If you aren't
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careful about this, gprof could combine data from your most recent version
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with stale data from prior versions, possibly giving misleading displays.
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</para>
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</sect3>
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<sect3 id="gprof-res"><title>Getting better profiling resolution</title>
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<para>To get better resolution (i.e., more data points) when profiling
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your program, try running it multiple times with the environment variable
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GMON_OUT_PREFIX set, as described in the previous situation. There will be
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multiple profiling data files generated and you can have gprof combine
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the data from all of them into one display.
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</para>
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</sect3>
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<sect3 id="gprof-lib"><title>Profiling programs with their libraries</title>
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<para>At the time of this writing Cygwin's profiling support only allows
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for one range of addresses per program. It is hard-wired to be the range
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covering the .text segment of your program, which is where your code resides.
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If you build your program with static libraries (e.g., libfoo.a), the code
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from those libraries is linked into your program's .text segment so will be
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included when profiling. But dynamic libraries (e.g., libfoo.dll) reside in
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other address ranges and code within them won't be included.
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</para>
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</sect3>
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<sect3 id="gprof-cyg"><title>Profiling Cygwin itself</title>
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<para>Due to the issue mentioned in the previous situation and other issues,
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at the time of this writing there is no support for profiling Cygwin itself.
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</para>
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</sect3>
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</sect2>
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</sect1>
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