A practical example

We can walk through the steps taken to build a simple application step by step.

Note that when you type gcc that actually runs a driver program that hides most of the steps from you. Under normal circumstances this is exactly what you want, because the exact commands and options to get a real life working executable on a real system can be quite complicated and architecture specific.

We will show the compliation process with the two following examples. Both are C source files, one defined the main() function for the inital program entry point, and another declares a helper type function. There is one global variable too, just for illustration.

Example 7-4. Hello World

    #include <stdio.h>
    
    /* We need a prototype so the compiler knows what types function() takes */
    int function(char *input);
  5 
    /* Since this is static, we can define it in both hello.c and function.c */
    static int i = 100;
    
    /* This is a global variable */
 10 int global = 10;
    
    int main(void)
    {
    	/* function() should return the value of global */
 15 	int ret = function("Hello, World!");
    	exit(ret);
    };

Example 7-5. Function Example

    #include <stdio.h>
    
    static int i = 100;
    
  5 /* Declard as extern since defined in hello.c */
    extern int global;
    
    int function(char *input)
    {
 10 	printf("%s\n", input);
    	return global;
    };

Compiling

All compilers have an option to only execute the first step of compilation. Usually this is something like -S and the output will generally be put into a file with the same name as the input file but with a .s extension.

Thus we can show the first step with gcc -S as illustrated in the example below.

Example 7-6. Compilation Example

    ianw@lime:~/programs/csbu/wk7/code$ gcc -S hello.c
    ianw@lime:~/programs/csbu/wk7/code$ gcc -S function.c
    ianw@lime:~/programs/csbu/wk7/code$ cat function.s
            .file   "function.c"
  5         .pred.safe_across_calls p1-p5,p16-p63
            .section        .sdata,"aw",@progbits
            .align 4
            .type   i#, @object
            .size   i#, 4
 10 i:
            data4   100
            .section        .rodata
            .align 8
    .LC0:
 15         stringz "%s\n"
            .text
            .align 16
            .global function#
            .proc function#
 20 function:
            .prologue 14, 33
            .save ar.pfs, r34
            alloc r34 = ar.pfs, 1, 4, 2, 0
            .vframe r35
 25         mov r35 = r12
            adds r12 = -16, r12
            mov r36 = r1
            .save rp, r33
            mov r33 = b0
 30         .body
            ;;
            st8 [r35] = r32
            addl r14 = @ltoffx(.LC0), r1
            ;;
 35         ld8.mov r37 = [r14], .LC0
            ld8 r38 = [r35]
            br.call.sptk.many b0 = printf#
            mov r1 = r36
            ;;
 40         addl r15 = @ltoffx(global#), r1
            ;;
            ld8.mov r14 = [r15], global#
            ;;
            ld4 r14 = [r14]
 45         ;;
            mov r8 = r14
            mov ar.pfs = r34
            mov b0 = r33
            .restore sp
 50         mov r12 = r35
            br.ret.sptk.many b0
            ;;
            .endp function#
            .ident  "GCC: (GNU) 3.3.5 (Debian 1:3.3.5-11)";

The assembly is a little to complex to fully describe, but you should be able to see where i is defined as a data4 (i.e. 4 bytes or 32 bits, the size of an int), where function is defined (function:) and a call to printf().

We now have two assembly files ready to be assembled into machine code!

Assembly

Assembly is a fairly straight forward process. The assembler is usually called as and takes arguments in a similar fasion to gcc

Example 7-7. Assembly Example

    ianw@lime:~/programs/csbu/wk7/code$ as -o function.o function.s
    ianw@lime:~/programs/csbu/wk7/code$ as -o hello.o hello.s
    ianw@lime:~/programs/csbu/wk7/code$ ls
    function.c  function.o  function.s  hello.c  hello.o  hello.s;

After assembling we have object code, which is ready to be linked together into the final executable. You can usually skip having to use the assembler by hand by calling the compiler with -c, which will directly convert the input file to object code, putting it in a file with the same prefix but .o as an extension.

We can't inspect the object code directly, as it is in a binary format (in future weeks we will learn about this binary format). However we can use some tools to inspect the object files, for example readelf --symbols will show us symbols in the object file.

Example 7-8. Readelf Example

    ianw@lime:~/programs/csbu/wk7/code$ readelf --symbols ./hello.o
    
    Symbol table '.symtab' contains 15 entries:
       Num:    Value          Size Type    Bind   Vis      Ndx Name
  5      0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
         1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS hello.c
         2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1
         3: 0000000000000000     0 SECTION LOCAL  DEFAULT    3
         4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4
 10      5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5
         6: 0000000000000000     4 OBJECT  LOCAL  DEFAULT    5 i
         7: 0000000000000000     0 SECTION LOCAL  DEFAULT    6
         8: 0000000000000000     0 SECTION LOCAL  DEFAULT    7
         9: 0000000000000000     0 SECTION LOCAL  DEFAULT    8
 15     10: 0000000000000000     0 SECTION LOCAL  DEFAULT   10
        11: 0000000000000004     4 OBJECT  GLOBAL DEFAULT    5 global
        12: 0000000000000000    96 FUNC    GLOBAL DEFAULT    1 main
        13: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND function
        14: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND exit
 20 
    ianw@lime:~/programs/csbu/wk7/code$ readelf --symbols ./function.o
    
    Symbol table '.symtab' contains 14 entries:
       Num:    Value          Size Type    Bind   Vis      Ndx Name
 25      0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
         1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS function.c
         2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1
         3: 0000000000000000     0 SECTION LOCAL  DEFAULT    3
         4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4
 30      5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5
         6: 0000000000000000     4 OBJECT  LOCAL  DEFAULT    5 i
         7: 0000000000000000     0 SECTION LOCAL  DEFAULT    6
         8: 0000000000000000     0 SECTION LOCAL  DEFAULT    7
         9: 0000000000000000     0 SECTION LOCAL  DEFAULT    8
 35     10: 0000000000000000     0 SECTION LOCAL  DEFAULT   10
        11: 0000000000000000   128 FUNC    GLOBAL DEFAULT    1 function
        12: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND printf
        13: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND global;

Although the output is quite complicated (again!) you should be able to understand much of it. For example

Linking

Actually invoking the linker, called ld, is a very complicated process on a real system (are you sick of hearing this yet?). This is why we leave the linking process up to gcc.

But of course we can spy on what gcc is doing under the hood with the -v (verbose) flag.

Example 7-9. Linking Example

     /usr/lib/gcc-lib/ia64-linux/3.3.5/collect2 -static 
    /usr/lib/gcc-lib/ia64-linux/3.3.5/../../../crt1.o 
    /usr/lib/gcc-lib/ia64-linux/3.3.5/../../../crti.o 
    /usr/lib/gcc-lib/ia64-linux/3.3.5/crtbegin.o 
  5 -L/usr/lib/gcc-lib/ia64-linux/3.3.5 
    -L/usr/lib/gcc-lib/ia64-linux/3.3.5/../../.. 
    hello.o 
    function.o 
    --start-group 
 10 -lgcc 
    -lgcc_eh 
    -lunwind 
    -lc 
    --end-group 
 15 /usr/lib/gcc-lib/ia64-linux/3.3.5/crtend.o 
    /usr/lib/gcc-lib/ia64-linux/3.3.5/../../../crtn.o;

The first thing you notice is that a program called collect2 is being called. This is a simple wrapper around ld that is used internally by gcc.

The next thing you notice is object files starting with crt being specified to the linker. These functions are provided by gcc and the system libraries and contain code required to start the program. In actuality, the main() function is not the first one called when a program runs, but a function called _start which is in the crt object files. This function does some generic setup which application programmers do not need to worry about.

The path heirarchy is quite complicated, but in essence we can see that the final step is to link in some extra object files, namely

We discuss how these are used to start the program a little later.

Next you can see that we link in our two object files, hello.o and function.o. After that we specify some extra libraries with -l flags. These libraries are system specific and required for every program. The major one is -lc which brings in the C library, which has all common functions like printf().

After that we again link in some more system object files which do some cleanup after programs exit.

Although the details are complicated, the concept is straight forward. All the object files will be linked together into a single executable file, ready to run!

The Executable

We will go into more details about the executable in the short future, but we can do some inspection in a similar fashion to the object files to see what has happened.

Example 7-10. Executable Example

    ianw@lime:~/programs/csbu/wk7/code$ gcc -o program hello.c function.c
    ianw@lime:~/programs/csbu/wk7/code$ readelf --symbols ./program
    
    Symbol table '.dynsym' contains 11 entries:
  5    Num:    Value          Size Type    Bind   Vis      Ndx Name
         0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
         1: 6000000000000de0     0 OBJECT  GLOBAL DEFAULT  ABS _DYNAMIC
         2: 0000000000000000   176 FUNC    GLOBAL DEFAULT  UND printf@GLIBC_2.2 (2)
         3: 600000000000109c     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start
 10      4: 0000000000000000   704 FUNC    GLOBAL DEFAULT  UND exit@GLIBC_2.2 (2)
         5: 600000000000109c     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
         6: 6000000000000fe8     0 OBJECT  GLOBAL DEFAULT  ABS _GLOBAL_OFFSET_TABLE_     7: 60000000000010b0     0 NOTYPE  GLOBAL DEFAULT  ABS _end
         8: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
         9: 0000000000000000   544 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.2 (2)
 15     10: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
    
    Symbol table '.symtab' contains 127 entries:
       Num:    Value          Size Type    Bind   Vis      Ndx Name
         0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
 20      1: 40000000000001c8     0 SECTION LOCAL  DEFAULT    1
         2: 40000000000001e0     0 SECTION LOCAL  DEFAULT    2
         3: 4000000000000200     0 SECTION LOCAL  DEFAULT    3
         4: 4000000000000240     0 SECTION LOCAL  DEFAULT    4
         5: 4000000000000348     0 SECTION LOCAL  DEFAULT    5
 25      6: 40000000000003d8     0 SECTION LOCAL  DEFAULT    6
         7: 40000000000003f0     0 SECTION LOCAL  DEFAULT    7
         8: 4000000000000410     0 SECTION LOCAL  DEFAULT    8
         9: 4000000000000440     0 SECTION LOCAL  DEFAULT    9
        10: 40000000000004a0     0 SECTION LOCAL  DEFAULT   10
 30     11: 40000000000004e0     0 SECTION LOCAL  DEFAULT   11
        12: 40000000000005e0     0 SECTION LOCAL  DEFAULT   12
        13: 4000000000000b00     0 SECTION LOCAL  DEFAULT   13
        14: 4000000000000b40     0 SECTION LOCAL  DEFAULT   14
        15: 4000000000000b60     0 SECTION LOCAL  DEFAULT   15
 35     16: 4000000000000bd0     0 SECTION LOCAL  DEFAULT   16
        17: 4000000000000ce0     0 SECTION LOCAL  DEFAULT   17
        18: 6000000000000db8     0 SECTION LOCAL  DEFAULT   18
        19: 6000000000000dd0     0 SECTION LOCAL  DEFAULT   19
        20: 6000000000000dd8     0 SECTION LOCAL  DEFAULT   20
 40     21: 6000000000000de0     0 SECTION LOCAL  DEFAULT   21
        22: 6000000000000fc0     0 SECTION LOCAL  DEFAULT   22
        23: 6000000000000fd0     0 SECTION LOCAL  DEFAULT   23
        24: 6000000000000fe0     0 SECTION LOCAL  DEFAULT   24
        25: 6000000000000fe8     0 SECTION LOCAL  DEFAULT   25
 45     26: 6000000000001040     0 SECTION LOCAL  DEFAULT   26
        27: 6000000000001080     0 SECTION LOCAL  DEFAULT   27
        28: 60000000000010a0     0 SECTION LOCAL  DEFAULT   28
        29: 60000000000010a8     0 SECTION LOCAL  DEFAULT   29
        30: 0000000000000000     0 SECTION LOCAL  DEFAULT   30
 50     31: 0000000000000000     0 SECTION LOCAL  DEFAULT   31
        32: 0000000000000000     0 SECTION LOCAL  DEFAULT   32
        33: 0000000000000000     0 SECTION LOCAL  DEFAULT   33
        34: 0000000000000000     0 SECTION LOCAL  DEFAULT   34
        35: 0000000000000000     0 SECTION LOCAL  DEFAULT   35
 55     36: 0000000000000000     0 SECTION LOCAL  DEFAULT   36
        37: 0000000000000000     0 SECTION LOCAL  DEFAULT   37
        38: 0000000000000000     0 SECTION LOCAL  DEFAULT   38
        39: 0000000000000000     0 SECTION LOCAL  DEFAULT   39
        40: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
 60     41: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        42: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        43: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        44: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        45: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
 65     46: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <command line>
        47: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        48: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <command line>
        49: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <built-in>
        50: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS abi-note.S
 70     51: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        52: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS abi-note.S
        53: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        54: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS abi-note.S
        55: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <command line>
 75     56: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        57: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <command line>
        58: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <built-in>
        59: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS abi-note.S
        60: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS init.c
 80     61: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        62: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        63: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS initfini.c
        64: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        65: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <command line>
 85     66: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        67: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <command line>
        68: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <built-in>
        69: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        70: 4000000000000670   128 FUNC    LOCAL  DEFAULT   12 gmon_initializer
 90     71: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        72: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        73: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS initfini.c
        74: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        75: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <command line>
 95     76: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        77: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <command line>
        78: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <built-in>
        79: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.2
        80: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS auto-host.h
100     81: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <command line>
        82: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <built-in>
        83: 6000000000000fc0     0 NOTYPE  LOCAL  DEFAULT   22 __CTOR_LIST__
        84: 6000000000000fd0     0 NOTYPE  LOCAL  DEFAULT   23 __DTOR_LIST__
        85: 6000000000000fe0     0 NOTYPE  LOCAL  DEFAULT   24 __JCR_LIST__
105     86: 6000000000001088     8 OBJECT  LOCAL  DEFAULT   27 dtor_ptr
        87: 40000000000006f0   128 FUNC    LOCAL  DEFAULT   12 __do_global_dtors_aux    
        88: 4000000000000770   128 FUNC    LOCAL  DEFAULT   12 __do_jv_register_classes
        89: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS hello.c
        90: 6000000000001090     4 OBJECT  LOCAL  DEFAULT   27 i
110     91: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS function.c
        92: 6000000000001098     4 OBJECT  LOCAL  DEFAULT   27 i
        93: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS auto-host.h
        94: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <command line>
        95: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS <built-in>
115     96: 6000000000000fc8     0 NOTYPE  LOCAL  DEFAULT   22 __CTOR_END__
        97: 6000000000000fd8     0 NOTYPE  LOCAL  DEFAULT   23 __DTOR_END__
        98: 6000000000000fe0     0 NOTYPE  LOCAL  DEFAULT   24 __JCR_END__
        99: 6000000000000de0     0 OBJECT  GLOBAL DEFAULT  ABS _DYNAMIC
       100: 4000000000000a70   144 FUNC    GLOBAL HIDDEN   12 __do_global_ctors_aux
120    101: 6000000000000dd8     0 NOTYPE  GLOBAL DEFAULT  ABS __fini_array_end
       102: 60000000000010a8     8 OBJECT  GLOBAL HIDDEN   29 __dso_handle
       103: 40000000000009a0   208 FUNC    GLOBAL DEFAULT   12 __libc_csu_fini
       104: 0000000000000000   176 FUNC    GLOBAL DEFAULT  UND printf@@GLIBC_2.2
       105: 40000000000004a0    32 FUNC    GLOBAL DEFAULT   10 _init
125    106: 4000000000000850   128 FUNC    GLOBAL DEFAULT   12 function
       107: 40000000000005e0   144 FUNC    GLOBAL DEFAULT   12 _start
       108: 6000000000001094     4 OBJECT  GLOBAL DEFAULT   27 global
       109: 6000000000000dd0     0 NOTYPE  GLOBAL DEFAULT  ABS __fini_array_start
       110: 40000000000008d0   208 FUNC    GLOBAL DEFAULT   12 __libc_csu_init
130    111: 600000000000109c     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start
       112: 40000000000007f0    96 FUNC    GLOBAL DEFAULT   12 main
       113: 6000000000000dd0     0 NOTYPE  GLOBAL DEFAULT  ABS __init_array_end
       114: 6000000000000dd8     0 NOTYPE  WEAK   DEFAULT   20 data_start
       115: 4000000000000b00    32 FUNC    GLOBAL DEFAULT   13 _fini
135    116: 0000000000000000   704 FUNC    GLOBAL DEFAULT  UND exit@@GLIBC_2.2
       117: 600000000000109c     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
       118: 6000000000000fe8     0 OBJECT  GLOBAL DEFAULT  ABS _GLOBAL_OFFSET_TABLE_   
       119: 60000000000010b0     0 NOTYPE  GLOBAL DEFAULT  ABS _end
       120: 6000000000000db8     0 NOTYPE  GLOBAL DEFAULT  ABS __init_array_start
140    121: 6000000000001080     4 OBJECT  GLOBAL DEFAULT   27 _IO_stdin_used
       122: 60000000000010a0     8 OBJECT  GLOBAL DEFAULT   28 __libc_ia64_register_back
       123: 6000000000000dd8     0 NOTYPE  GLOBAL DEFAULT   20 __data_start
       124: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
       125: 0000000000000000   544 FUNC    GLOBAL DEFAULT  UND __libc_start_main@@GLIBC_
145    126: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__;

Some things to note