GCC Tips

What's Here, Why You Should Care
A collection of tips useful to those doing embedded development. Accumulated over several years of doing project work, helping other engineers, untangling projects for customers and feedback from several CELF presentations related to this topic.

View Compilation Plan
gcc -### 

The GCC you run is a driver program for a bunch of other programs. With this parameter, gcc will produce (but not actually execute) the commands it would have used to accomplish the task you asked it to do. This way, you can see the gory details of what's going on behind the scenes. What library is being used? What is -mcpu set to? It's all there.

You can pipe this output to a file and execute that to compile a program, making it easy to experiment with tweaks to the linker or assembler.

 Reading specs from /opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/specs Configured with: ../configure --prefix=/opt/timesys/toolchains/ppc7xx-linux --mandir=/opt/timesys/toolchains/ppc7xx-linux/share/man --infodir=/opt/timesys/toolchains/ppc7xx-linux/share/info --enable-shared --enable-threads=posix --disable-checking --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-languages=c,c++ --with-sysroot=/here/workdir/i386-x-ppc7xx/deleteme --disable-libgcj --build=i686-timesys-linux --host=i686-timesys-linux --target=powerpc-linux --program-prefix=ppc7xx-linux- Thread model: posix gcc version 3.4.1 20040714 (TimeSys 3.4.1-7) /opt/timesys/toolchains/ppc7xx-linux/libexec/gcc/powerpc-linux/3.4.1/cc1 -quiet -v -D__unix__ -D__gnu_linux__ -D__linux__ -Dunix -D__unix -Dlinux -D__linux -Asystem=linux -Asystem=unix -Asystem=posix -I/opt/timesys/toolchains/ppc7xx-linux/powerpc-linux/include/nptl file.c -quiet -dumpbase file.c -auxbase file -version -o /tmp/ccShiHn4.s ignoring nonexistent directory "/here/workdir/i386-x-ppc7xx/deleteme/usr/local/include" ignoring nonexistent directory "/here/workdir/i386-x-ppc7xx/deleteme/usr/include" /opt/timesys/toolchains/ppc7xx-linux/powerpc-linux/include/nptl /opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/include /opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/../../../../powerpc-linux/include End of search list. GNU C version 3.4.1 20040714 (TimeSys 3.4.1-7) (powerpc-linux) compiled by GNU C version 3.2.2 20030222 (Red Hat Linux 3.2.2-5). GGC heuristics: --param ggc-min-expand=47 --param ggc-min-heapsize=32138 /opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/../../../../powerpc-linux/bin/as -mppc -many -V -Qy -o /tmp/ccWeV3a3.o /tmp/ccShiHn4.s GNU assembler version 2.15.90.0.3 (powerpc-linux) using BFD version 2.15.90.0.3 20040415 /opt/timesys/toolchains/ppc7xx-linux/libexec/gcc/powerpc-linux/3.4.1/collect2 --eh-frame-hdr -V -Qy -L/opt/timesys/toolchains/ppc7xx-linux/powerpc-linux/lib/nptl --rpath-link /opt/timesys/toolchains/ppc7xx-linux/powerpc-linux/lib/tls -m elf32ppclinux -dynamic-linker /lib/ld.so.1 -o file /opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/../../../../powerpc-linux/lib/crt1.o /opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/../../../../powerpc-linux/lib/crti.o /opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/crtbegin.o -L/opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1 -L/opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/../../../../powerpc-linux/lib /tmp/ccWeV3a3.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/crtsavres.o /opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/crtend.o /opt/timesys/toolchains/ppc7xx-linux/lib/gcc/powerpc-linux/3.4.1/../../../../powerpc-linux/lib/crtn.o GNU ld version 2.15.90.0.3 20040415 Supported emulations: elf32ppclinux elf32ppc elf32ppcsim
 * 1) include "..." search starts here:
 * 2) include <...> search starts here:

Note: on my (Tim Bird's) Ubuntu 12.04 system, using gcc 4.6.3, it shows quite different output. Specifically, it is missing anything about the include directories.

See default include search paths
You can get a report about the include search paths with the following command:

$ gcc -xc -E -v - Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.3-1ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) COLLECT_GCC_OPTIONS='-E' '-v' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/4.6/cc1 -E -quiet -v -imultilib. -imultiarch x86_64-linux-gnu - -mtune=generic -march=x86-64 -fstack-protector ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../x86_64-linux-gnu/include" /usr/lib/gcc/x86_64-linux-gnu/4.6/include /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/4.6/include-fixed /usr/include/x86_64-linux-gnu /usr/include End of search list.
 * 1) include "..." search starts here:
 * 2) include <...> search starts here:

Timing sub-commands
You can get a report for the amount of time taken for each sub-command that gcc uses, using the '-time' command.

Example: $ gcc -time -static hello.c -o hello $
 * 1) cc1 0.01 0.01
 * 2) as 0.00 0.00
 * 3) collect2 0.03 0.02

This was on a fast machine, with a very small sample program. It shows that the sub-programs 'cc1', 'as', and 'collect2' (the compiler, the assembler, and the linker (collect2 is a front-end for 'ld'), respectively), took less than a few hundredths of a second each to run.

Pre-Process, Retain Comments
gcc -C -E  -o file

Some engineers love to do coding in macros. The rest of us would like to break their fingers. This command will run the file through the pre-processor, expanding all macros, but retaining all comments. Stick a comment like "LOOK HERE" and search for that so you reduce the amount of time you spend looking for the offending code.

Pre-Process, Retain Defines/Macros
gcc -dD -E  -o file

Just like the previous example, but perhaps you're trying to trace the macro define mess.

See what Files the Linker is Using
gcc -Wl,-t

Displays what files the linker opens in what order. When looking in archive files, the archive file is displayed in para theses, followed by the file in the archive. Very handy when working through a legacy project that depends on files linking in a certain order that suddenly breaks because of a small (probably viewed as not noteworthy) change in a makefile somewhere.

/usr/bin/ld: mode elf_i386 /usr/lib/gcc-lib/i386-redhat-linux/3.3.3/../../../crt1.o /usr/lib/gcc-lib/i386-redhat-linux/3.3.3/../../../crti.o /usr/lib/gcc-lib/i386-redhat-linux/3.3.3/crtbegin.o /tmp/cc37FxnS.o -lgcc_s (/usr/lib/gcc-lib/i386-redhat-linux/3.3.3/libgcc_s.so) /lib/libc.so.6 (/usr/lib/libc_nonshared.a)elf-init.oS -lgcc_s (/usr/lib/gcc-lib/i386-redhat-linux/3.3.3/libgcc_s.so) /usr/lib/gcc-lib/i386-redhat-linux/3.3.3/crtend.o /usr/lib/gcc-lib/i386-redhat-linux/3.3.3/../../../crtn.o

Print Pre-defined Macros
gcc -E -dM - < /dev/null | cut -c 9- | sort

Very handy when porting code. Lets you know if your target processor has some missing defines or if something is different (like __INT_MAX__) that can have interesting effects on your project. Diff the output from the old to the new compiler so you can easily see the differences, makes it easy to spot problems before getting started.

Sample output, from a compiler targeting an ARM processor.

__APCS_32__ 1 __arm__ 1 __ARM_ARCH_4T__ 1 __ARMEL__ 1 __CHAR_BIT__ 8 __CHAR_UNSIGNED__ 1 __DBL_DENORM_MIN__ 4.9406564584124654e-324 __DBL_DIG__ 15 __DBL_EPSILON__ 2.2204460492503131e-16 __DBL_HAS_DENORM__ 1 __DBL_HAS_INFINITY__ 1 __DBL_HAS_QUIET_NAN__ 1 __DBL_MANT_DIG__ 53 __DBL_MAX_10_EXP__ 308 __DBL_MAX__ 1.7976931348623157e+308 __DBL_MAX_EXP__ 1024

Mixed Assembler and Source Output
gcc -g somefile.c -o somefile objdump -S somefile

Prints out each line in the program and the corresponding assembly code. Very handy when you're trying to see that the processor is generating the correct code, with the instructions you're expecting. You can also see the effects of optimization, but would recommend doing this for a small amount of code because when the optimization level is high, there's a much lower relationship between line of code and generated assembler.

Here's an example of what objdump produces for a few lines of code:

gpvSharedMemory = shmat(hSharedMemory, NULL, 0); 10000958:      80 7f 00 10     lwz     r3,16(r31) 1000095c:      38 80 00 00     li      r4,0 10000960:      38 a0 00 00     li      r5,0 10000964:      48 01 09 31     bl      10011294  10000968:      7c 60 1b 78     mr      r0,r3 1000096c:      3d 20 10 01     lis     r9,4097 10000970:      90 09 11 d0     stw     r0,4560(r9) if (errno != 0) { 10000974:      48 01 08 d1     bl      10011244 <__errno_location@plt> 10000978:      7c 60 1b 78     mr      r0,r3 1000097c:      7c 09 03 78     mr      r9,r0 10000980:      80 09 00 00     lwz     r0,0(r9) 10000984:      2f 80 00 00     cmpwi   cr7,r0,0 10000988:      41 9e 00 50     beq-    cr7,100009d8 

Specify Language
gcc -x c a-c-source-file.with-a-non-standard-extension -o test.out

Great for legacy projects where where the file extensions don't match with GCC's expectations, while less of a problem since many projects got their start with GCC, this still is an issue with long-running projects that years back, used some other compiler. This stays in effect for the following file on the command line.

List Include File Dependencies
There's a whole family of things around -M. These produce a rule that could be used in a make file, with the included files as dependencies.

gcc -M

This shows you all includes, even those on the system path. Useful if you're doing porting work or validating if your compiler is working as expected and getting the files from the right place. You'll see something like this for a basic hello world program

hello.o: hello.c /usr/include/stdio.h /usr/include/features.h \ /usr/include/sys/cdefs.h /usr/include/gnu/stubs.h \ /usr/lib/gcc-lib/i386-redhat-linux/3.3.3/include/stddef.h \ /usr/include/bits/types.h /usr/include/bits/wordsize.h \ /usr/include/bits/typesizes.h /usr/include/libio.h \ /usr/include/_G_config.h /usr/include/wchar.h /usr/include/bits/wchar.h \ /usr/include/gconv.h \ /usr/lib/gcc-lib/i386-redhat-linux/3.3.3/include/stdarg.h \ /usr/include/bits/stdio_lim.h /usr/include/bits/sys_errlist.h

gcc -MM

Like -M, but no system files. Great to see if your project is configured and working as expected.

gcc -M -MG

The prior -M and -MM commands will stop if header files can be located. The parameter -MG will just produce the dependency list with the missing file. Engineers that have projects that generate header files as part of the build find -MM very handy.

gcc -M -MT ' '

By default, the target will be the .o This command will make the default the value of.

If the command was

gcc -M -MT '$(target)' hello.c

You would see

$(target): hello.c

Symbol Trace
gcc -Wl,-y,printf hello.c

This is very handy when you want to understand where the linker is finding a definition of a symbol. Some projects have name collisions or link order dependencies. This lets you see precisely what the linker is doing.

Given a hello world program, you would see output like

/tmp/ccwZx5UV.o: reference to printf /lib/libc.so.6: definition of printf

The reference is in a temporary file created during the compilation process. If you were linking several object files together explicitly, you would see the name of the object file where printf was referenced.

Saving temporary files
gcc -save-temps hello.c

The compilers temporary files are saved. This is often invaluable dealing with complicated makefiles to peek at the preprocessed output without having to figure out how to do a -E option by hand.