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# ld(1)
```
ld [opts] files...
-T <script> use <script> as linker script
--trace report each file the linker touches
--start-group archives --end-group
search archives repearepeatedly until no new
undefined references are created
(eg helpfull with list of static libraries)
```
## Linker Script
`output` sections are defined as follows (full description at [output
section][ld-out] and [input section][ld-in]).
```
section_name [vaddr] : [AT(paddr)] {
file_pattern (section_pattern)
}
```
The following gives an example of an `output` section with two `input` section rules.
```
.foo : {
abc.o (.foo)
*.o (.foo.*)
}
```
### Example: virtual vs physical (load) address
Sometimes code is initially located at a different location as when being run.
For example in embedded cases, where code may initially resides in a _rom_ and
startup code will copy a section with writable _data_ into _ram_. Code accessing
the writable data accesses the data in the _ram_.
In this case we need different addresses for the same data.
- The `virtual` or _runtime_ address, this is the address used when the linker
resolves accesses to the data. Hence, this is the address the data will have
when the code is running.
- The `physical` or _load_ address, this is the address the data is stored at
initially. Startup code typically copies the initial values from the
`physical` to the `virtual` address.
The following shows an example linker script which uses _virtual_ and _physical_
addresses. The full source files can be found [here][src].
```
{{#include ld/link-nomem.ld}}
```
We can use the following assembly snippet to explore the linker script.
```x86asm
{{#include ld/data.S}}
```
> `gcc -c data.S && ld -o link-nomem -T link-nomem.ld data.o`
The elf load segments show the difference in _physical_ and _virtual_ address
for the segment containing the `.data` section.
```sh
> readelf -W -l link-nomem
# There are 4 program headers, starting at offset 64
#
# Program Headers:
# Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
# LOAD 0x001100 0x0000000000800100 0x0000000000100000 0x000020 0x000020 RW 0x1000
# LOAD 0x002000 0x0000000000800000 0x0000000000800000 0x000018 0x000018 R E 0x1000
# LOAD 0x003000 0x0000000000804000 0x0000000000804000 0x000004 0x000004 R 0x1000
# LOAD 0x000000 0x0000000000805000 0x0000000000805000 0x000000 0x001000 RW 0x1000
#
# Section to Segment mapping:
# Segment Sections...
# 00 .data
# 01 .text
# 02 .rodata
# 03 .stack
```
Startup code could copy data from `_data_paddr` to `_data_vaddr`.
```sh
> nm link-nomem
# 0000000000800100 d asm_array
# 0000000000804000 r asm_len
# 0000000000100000 a _data_paddr
# 0000000000800100 d _data_vaddr
# 0000000000800000 T _entry
# 0000000000806000 b _stack_top
```
The linker resolves symbols to their _virtual_ address, this can be seen by the
access to the `asm_array` variable.
```sh
> objdump -d link-nomem
# Disassembly of section .text:
#
# 0000000000800000 <_entry>:
# 800000: 48 c7 c4 00 60 80 00 mov $0x806000,%rsp
# 800007: 48 c7 c0 00 01 80 00 mov $0x800100,%rax ;; mov $asm_array, %rax
# 80000e: 8b 04 25 00 40 80 00 mov 0x804000,%eax
# 800015: f4 hlt
# 800016: eb e8 jmp 800000 <_entry>
```
The following linker script shows an example with the `MEMORY` command.
```
{{#include ld/link-mem.ld}}
```
## References
- [ld manual][ld]
- [ld script: input sections][ld-in]
- [ld script: output sections][ld-out]
- [notes/ld example files][src]
[ld]: https://sourceware.org/binutils/docs/ld/
[ld-in]: https://sourceware.org/binutils/docs/ld/Input-Section.html
[ld-out]: https://sourceware.org/binutils/docs/ld/Output-Section-Attributes.html
[src]: https://github.com/johannst/notes/tree/master/src/development/ld
|
