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For your eyes only: object code....

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• Notable fact: a .o file contains binary machine code !
  • Fact:

    A object file (.o)) contains machine instructions in binay code It's not for human consumption !

  • You can disassemble the binary instructions back into assembler code (you cannot get back C code....) using this command:

    objdump -d prog.o

    Example:

    int f( int x ) { return ( x*x ); } Disassemble code: 0000000000000000 <f>: 0: 55 push %rbp 1: 48 89 e5 mov %rsp,%rbp 4: 89 7d fc mov %edi,-0x4(%rbp) 7: 8b 45 fc mov -0x4(%rbp),%eax // get x in reg. eax a: 0f af 45 fc imul -0x4(%rbp),%eax // (x*x) e: c9 leaveq f: c3 retq

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• Linking: assembling object codes into an executable program file
  • Terminology:

    Linking = constructing an executable program from one or more object (compiled) code files. Linker = a computer program that perform the linking process. I.e., a computer program that takes one or more object code files (generated by a compiler) and combines them into a single executable program (Wikipedia: click here )

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  • Some technical details on linking:

    Facts: A function defined inside a file file1 can use variables and functions that are defined inside another file file2. During the separate compilation process, the compiler cannot resolve (= find the location) variables and functions that are defined inside another program file: Because during separate compilation, the compiler is only using one program file, so it can never find the variables and functions defined inside another program file. The compiler will enter extra information into the object file (while it is compiling the source code) on these unresolved symbols (= identifiers used for the variables and functions) The linker will combine ("link") the information stored in the different object files The resulting program file will only contain numerical addresses (= address to computer memory) so that it can be executed by the computer

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  • Command to link a number of object code file into a executable program file:

    gcc [-o outputFileName] file1.o file2.o ....

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• A peek inside the relocation mechanism
  • This is not a compiler course, but I do want to show you how relocation is done.

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  • Take a look at this reloc.c file:

    int x; void f() { x = 4; x = 7; }

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  • Do this:

    Compile: gcc -c reloc.c Dump the relocation table: objdump -r reloc.o Disassembler: objdump -d reloc.o RELOCATION RECORDS FOR [.text]: OFFSET TYPE VALUE 0000000000000006 R_X86_64_PC32 x-0x0000000000000008 0000000000000010 R_X86_64_PC32 x-0x0000000000000008 The relocable code contains information on the use of the variable x !!! How is it use ? Look at the machine code: 0000000000000000 <f>: 0: 55 push %rbp 1: 48 89 e5 mov %rsp,%rbp 4: c7 05 00 00 00 00 04 movl $0x4,0x0(%rip) # e b: 00 00 00 e: c7 05 00 00 00 00 07 movl $0x7,0x0(%rip) # 18 15: 00 00 00 18: c9 leaveq 19: c3 retq ^ ^ | | Machine code Disassembled assembler instruction

    How is the relocation info used:

    The linker will allocate memory space for every variable So: the variable x will be stored somewhere. Depending on where the variable x is stored, the linker will patch the relocation location with the allocated location