复旦大学软件学院计算机系统课件 Link.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * ELF header Program header table (required for executables) .text section .data section .bss section .symtab .rel.txt .rel.data .debug Section header table (required for relocatables) .line .strtab EFI object file format * Symbols Three kinds of symbols Defined global symbols Referenced global symbols Local symbols * Symbols Defined global symbols Defined by module m and can be referenced by other modules Nonstatic C functions Global variables that are defined without the C static attribute * Symbols Referenced global symbols Referenced by module m but defined by some other module C functions and variables that are defined in other modules Local symbols Defined and referenced exclusively by module m. C functions and global variables with static attribute * Symbol Tables Each relocatable object module has a symbol table A symbol table contains information about the symbols that are defined and referenced by the module * Symbol Tables Local nonstatic program variables does not contain in the symbol table in .symbol Local static procedure variables Are not managed on the stack Be allocated in .data or .bss * Examples int f() { static int x=1 ; return x; } int g() { static int x = 1; return x ; } x.1 and x.2 are allocated in .data * Symbol Tables Compiler exports symbols in .s file Assembler builds symbol tables using exported symbols An ELF symbol table is contained in .symtab section Symbol table contains an array of entries * extern int buf[]; int *bufp0 = buf[0]; static int *bufp1; static void incr() { static int count=0; count++; } Example (Symbol Table) (Hw 7.6) * void swap() { int temp; incr(); bufp1 = buf[1]; temp = *bufp0; *bufp0 = *bufp1; *bufp1 = temp; } Example (Symbol Table) (Hw 7.6) * Example (Symbol Table) (Hw 7.6) Symbol symtab entry? Symbol type Module where defined Section buf bufp0 bufp1 swap temp incr count * the module that defines the symbol swap.o main