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

#include <stdio.h>

/* We need a prototype so the compiler knows what types function() takes */
int function(char *input);

/* Since this is static, we can define it in both hello.c and function.c */
static int i = 100;

/* This is a global variable */
int global = 10;

int main(void)
{
	/* function() should return the value of global */
	int ret = function("Hello, World!");
	exit(ret);
}

Example 6.1 Hello World

#include <stdio.h>

static int i = 100;

/* Declard as extern since defined in hello.c */
extern int global;

int function(char *input)
{
	printf("%s\n", input);
	return global;
}

Example 6.2 Function Example

6.1 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.

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

Example 6.1.1 Compilation Example

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!

6.2 Assembly

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

$ as -o function.o function.s
$ as -o hello.o hello.s
$ ls
function.c  function.o  function.s  hello.c  hello.o  hello.s

Example 6.2.1 Assembly Example

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.

$ readelf --symbols ./hello.o

Symbol table '.symtab' contains 15 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     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
     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
    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

$ readelf --symbols ./function.o

Symbol table '.symtab' contains 14 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     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
     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
    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

Example 6.2.2 Readelf Example

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

In the output of hello.o have a look at the symbol with name i. Notice how it says it is LOCAL? That is because we declared it static and as such it has been flagged as being local to this object file.
In the same output, notice that the global variable is defined as a GLOBAL, meaning that it is visible outside this file. Similarly the main() function is externally visible.
Notice that the function symbol (for the call to function() is left has UND or undefined. This means that it has been left for the linker to find the address of the function.
Have a look at the symbols in the function.c file and how they fit into the output.

6.3 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.

 /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 
-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 
-lgcc 
-lgcc_eh 
-lunwind 
-lc 
--end-group 
/usr/lib/gcc-lib/ia64-linux/3.3.5/crtend.o 
/usr/lib/gcc-lib/ia64-linux/3.3.5/../../../crtn.o

Example 6.3.1 Linking Example

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

crt1.o : provided by the system libraries (libc) this object file contains the _start function which is actually the first thing called within the program.
crti.o : provided by the system libraries
crtbegin.o
crtsaveres.o
crtend.o
crtn.o

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!

6.4 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.

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:
   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
     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)
    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
     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
     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
    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
    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
    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
    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
    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
    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
    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
    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
    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>
    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
    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>
    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
    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>
    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
    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__
    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
    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>
    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
   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
   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
   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
   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
   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_
   126: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__

Example 6.4.1 Executable Example

Some things to note

Note I built the executable the "easy" way!
See there are two symbol tables; the dynsym and symtab ones. We explain how the dynsym symbols work soon, but notice that some of them are versioned with an @ symbol.
Note the many symbols that have been included from the extra object files. Many of them start with __ to avoid clashing with any names the programmer might choose. Read through and pick out the symbols we mentioned before from the object files and see if they have changed in any way.

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