@ -492,25 +492,29 @@ With the linker language we can control:
Commands written in the linker control language are usually placed in a file called linker script. We can pass it to `ld` with the `-T` command line option. The main command in a linker script is the `SECTIONS` command. Each linker script must contain this command and it determines the `map` of the output file. The special variable `.` contains current position of the output. Let's write a simple assembly program and we will look at how we can use a linker script to control linking of this program. We will take a hello world program for this example:
```assembly
section .data
msg db "hello, world!",`\n`
section .text
global _start
.data
msg: .ascii "hello, world!\n"
.text
.global _start
_start:
mov rax, 1
mov rdi, 1
mov rsi, msg
mov rdx, 14
syscall
mov rax, 60
mov rdi, 0
syscall
mov $1,%rax
mov $1,%rdi
mov $msg,%rsi
mov $14,%rdx
syscall
mov $60,%rax
mov $0,%rdi
syscall
```
We can compile and link it with the following commands:
```
$ nasm -f elf64 -o hello.o hello.asm
$ as -o hello.o hello.asm
$ ld -o hello hello.o
```
@ -540,14 +544,14 @@ SECTIONS
On the first three lines you can see a comment written in `C` style. After it the `OUTPUT` and the `OUTPUT_FORMAT` commands specify the name of our executable file and its format. The next command, `INPUT`, specifies the input file to the `ld` linker. Then, we can see the main `SECTIONS` command, which, as I already wrote, must be present in every linker script. The `SECTIONS` command represents the set and order of the sections which will be in the output file. At the beginning of the `SECTIONS` command we can see following line `. = 0x200000`. I already wrote above that `.` command points to the current position of the output. This line says that the code should be loaded at address `0x200000` and the line `. = 0x400000` says that data section should be loaded at address `0x400000`. The second line after the `. = 0x200000` defines `.text` as an output section. We can see `*(.text)` expression inside it. The `*` symbol is wildcard that matches any file name. In other words, the `*(.text)` expression says all `.text` input sections in all input files. We can rewrite it as `hello.o(.text)` for our example. After the following location counter `. = 0x400000`, we can see definition of the data section.
We can compile and link it with the:
We can compile and link it with the following command:
If we will look inside it with the `objdump` util, we can see that `.text` section starts from the address `0x200000` and the `.data` sections starts from the address `0x400000`:
If we look inside it with the `objdump` util, we can see that `.text` section starts from the address `0x200000` and the `.data` sections starts from the address `0x400000`:
mudongliang
8 years ago