How do you get assembler output from C C source in gcc

Question

How does one do this   If I want to analyze how something is getting compiled  how would I get the emitted assembly code

User · Answer

The following command line is from Christian Garbin's blog

g++ -g -O -Wa,-aslh horton_ex2_05.cpp >list.txt

I ran G++ from a DOS window on Win-XP, against a routine that contains an implicit cast

c:\gpp_code>g++ -g -O -Wa,-aslh horton_ex2_05.cpp >list.txt
horton_ex2_05.cpp: In function `int main()':
horton_ex2_05.cpp:92: warning: assignment to `int' from `double'

The output is asssembled generated code iterspersed with the original C++ code (the C++ code is shown as comments in the generated asm stream)

  16:horton_ex2_05.cpp **** using std::setw;
  17:horton_ex2_05.cpp ****
  18:horton_ex2_05.cpp **** void disp_Time_Line (void);
  19:horton_ex2_05.cpp ****
  20:horton_ex2_05.cpp **** int main(void)
  21:horton_ex2_05.cpp **** {
 164                    %ebp
 165                            subl $128,%esp
?GAS LISTING C:\DOCUME~1\CRAIGM~1\LOCALS~1\Temp\ccx52rCc.s
166 0128 55                    call ___main
167 0129 89E5          .stabn 68,0,21,LM2-_main
168 012b 81EC8000      LM2:
168      0000
169 0131 E8000000      LBB2:
169      00
170                    .stabn 68,0,25,LM3-_main
171                    LM3:
172                            movl $0,-16(%ebp)

User · Answer

As mentioned before  look at the -S flag   It s also worth looking at the  -fdump-tree  family of flags  in particular  -fdump-tree-all   which lets you see some of gcc s intermediate forms  These can often be more readable than assembler  at least to me   and let you see how optimisation passes perform

User · Answer

Well  as everyone said  use -S option  If you use -save-temps option  you can also get preprocessed file  i   assembly file  s  and object file   o    get each of them by using -E  -S  and -c

User · Answer

Here are the steps to see print the assembly code of any C program on your Windows   console  terminal  command prompt     Write a C program in a C code editor like codeblocks and save it with an extention  c Compile and run it  Once run successfully  go to the folder where you have installed your gcc compiler and give the   following command to get a    s   file of the    c  file  C   gcc  gcc -S complete path of the C file  ENTER  An example command   as in my case   C  gcc  gcc  -S D  Aa C Certified alternate letters c    This outputs a    s  file of the original     c  file   4   After this   type the following command  C  gcc  cpp filename s ENTER  Example command   as in my case   C  gcc  cpp alternate letters s    This will print output the entire Assembly language code of your C program

User · Answer

As everyone has pointed out  use the -S option to GCC   I would also like to add that the results may vary  wildly   depending on whether or not you add optimization options  -O0 for none  -O2 for agressive optimization    On RISC architectures in particular  the compiler will often transform the code almost beyond recognition in doing optimization   It s impressive and fascinating to look at the results

User · Answer

recently i wanted to know the assembly of each functions in a program this is how i did it     gcc main c                         main c source file   gdb a exe                          gdb a out in linux    gdb  disass main                  note here main is a function                                      similary it can be done for other functions

User · Answer

Use the -S option   gcc -S program c

User · Answer

This will generate assembly code with the C code   line numbers interweaved  to more easily see which lines generate what code    create assembler code  g   -S -fverbose-asm -g -O2 test cc -o test s   create asm interlaced with source lines  as -alhnd test s  gt  test lst  Found in Algorithms for programmers  page 3  which is the overall 15th page of the PDF

User · Answer

Use the -S option to gcc  or g      gcc -S helloworld c   This will run the preprocessor  cpp  over helloworld c  perform the initial compilation and then stop before the assembler is run   By default this will output a file helloworld s  The output file can be still be set by using the -o option   gcc -S -o my asm output s helloworld c   Of course this only works if you have the original source  An alternative if you only have the resultant object file is to use objdump  by setting the --disassemble option  or -d for the abbreviated form    objdump -S --disassemble helloworld  gt  helloworld dump   This option works best if debugging option is enabled for the object file  -g at compilation time  and the file hasn t been stripped   Running file helloworld will give you some indication as to the level of detail that you will get by using objdump

User · Answer

Use  -S  as an option  It displays the assembly output in the terminal

User · Answer

I don t see this possibility among answers  probably because the question is from 2008  but in 2018 you can use Matt Goldbolt s online website https   godbolt org  You can also locally git clone and run his project https   github com mattgodbolt compiler-explorer

User · Answer

-save-temps This was mentioned at https   stackoverflow com a 17083009 895245 but let me further exemplify it  The big advantage of this option over -S is that it is very easy to add it to any build script  without interfering much in the build itself  When you do  gcc -save-temps -c -o main o main c  main c  define INC 1  int myfunc int i        return i   INC     and now  besides the normal output main o  the current working directory also contains the following files   main i is a bonus and contains the preprossessed file    1  quot main c quot    1  quot  lt built-in gt  quot    1  quot  lt command-line gt  quot    31  quot  lt command-line gt  quot    1  quot  usr include stdc-predef h quot  1 3 4   32  quot  lt command-line gt  quot  2   1  quot main c quot    int myfunc int i        return i   1      main s contains the desired generated assembly       file    quot main c quot       text      globl  myfunc      type   myfunc   function myfunc   LFB0       cfi startproc     pushq    rbp      cfi def cfa offset 16      cfi offset 6  -16     movq     rsp   rbp      cfi def cfa register 6     movl     edi  -4  rbp      movl    -4  rbp    eax     addl     1   eax     popq     rbp      cfi def cfa 7  8     ret      cfi endproc  LFE0       size   myfunc   -myfunc      ident   quot GCC   Ubuntu 8 3 0-6ubuntu1  8 3 0 quot       section     note GNU-stack  quot  quot   progbits    If you want to do it for a large number of files  consider using instead   -save-temps obj  which saves the intermediate files to the same directory as the -o object output instead of the current working directory  thus avoiding potential basename conflicts  Another cool thing about this option is if you add -v  gcc -save-temps -c -o main o -v main c  it actually shows the explicit files being used instead of ugly temporaries under  tmp  so it is easy to know exactly what is going on  which includes the preprocessing   compilation   assembly steps   usr lib gcc x86 64-linux-gnu 8 cc1 -E -quiet -v -imultiarch x86 64-linux-gnu main c -mtune generic -march x86-64 -fpch-preprocess -fstack-protector-strong -Wformat -Wformat-security -o main i  usr lib gcc x86 64-linux-gnu 8 cc1 -fpreprocessed main i -quiet -dumpbase main c -mtune generic -march x86-64 -auxbase-strip main o -version -fstack-protector-strong -Wformat -Wformat-security -o main s as -v --64 -o main o main s  Tested in Ubuntu 19 04 amd64  GCC 8 3 0  CMake predefined targets CMake automatically provides a targets for the preprocessed file  make help  shows us that we can do  make main s  and that target runs  Compiling C source to assembly CMakeFiles main dir main c s  usr bin cc    -S  home ciro hello main c -o CMakeFiles main dir main c s  so the file can be seen at CMakeFiles main dir main c s Tested on cmake 3 16 1

User · Answer

If you re looking for LLVM assembly   llvm-gcc -emit-llvm -S hello c

User · Answer

Here is a solution for C using gcc    gcc -S program c  amp  amp  gcc program c -o output    Here the first part stores the assembly output of the program in the same file name as Program but with a changed  s extension  you can open it as any normal text file  The second part here compiles your program for actual usage and generates an executable for your Program with a specified file name    The program c used above is the name of your program and output is the name of the executable you want to generate   BTW It s my First post on StackOverFlow  -

User · Answer

As mentioned before  look at the -S flag   It s also worth looking at the  -fdump-tree  family of flags  in particular  -fdump-tree-all   which lets you see some of gcc s intermediate forms  These can often be more readable than assembler  at least to me   and let you see how optimisation passes perform

User · Answer

Use the -S switch   g   -S main cpp   or also with gcc  gcc -S main c   Also see this

User · Answer

If what you want to see depends on the linking of the output  then objdump on the output object file executable may also be useful in addition to the aforementioned gcc -S   Here s a very useful script by Loren Merritt that converts the default objdump syntax into the more readable nasm syntax      usr bin perl -w  ptr   BYTE WORD DWORD QWORD XMMWORD  PTR     reg      er      abcd x  sd i  sb p   abcd  hl  r1  0-589  dwb   mm 0-7  xmm1  0-9     open FH   -      usr bin objdump    -w    -M    intel    ARGV or die   prev       while  lt FH gt        if   ptr o            s  ptr               reg   2 o or         s   reg   ptr                1 3 o or         s  ptr lc  1 oe            if  prev      t repz   ret   and                tnop   txchg  ax ax               drop this line       else          print  prev          prev               print  prev  close FH    I suspect this can also be used on the output of gcc -S

User · Answer

From  http   www delorie com djgpp v2faq faq8 20 html  gcc -c -g -Wa -a -ad  other GCC options  foo c   foo lst  in alternative to PhirePhly s answer Or just use -S as everyone said

[c++] How do you get assembler output from C/C++ source in gcc?

Examples related to c++

Examples related to c

Examples related to debugging

Examples related to gcc

Examples related to assembly