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# Using the GNU tools

This is a short summary of the AVR-specific aspects of using the GNU tools. Normally, the generic documentation of these tools is fairly large and maintained in `texinfo` files. Command-line options are explained in detail in the manual page.

## Options for the C compiler avr-gcc

### Machine-specific options for the AVR

The following machine-specific options are recognized by the C compiler frontend. In addition to the preprocessor macros indicated in the tables below, the preprocessor will define the macros __AVR and __AVR__ (to the value 1) when compiling for an AVR target. The macro AVR will be defined as well when using the standard levels gnu89 (default) and gnu99 but not with c89 and c99.

- `-mmcu=`architecture

Compile code for architecture. Currently known architectures are

Architecture | Macros | Description
avr1 | __AVR_ARCH__=1 __AVR_ASM_ONLY__ __AVR_2_BYTE_PC__ [2] | Simple CPU core, only assembler support
avr2 | __AVR_ARCH__=2 __AVR_2_BYTE_PC__ [2] | "Classic" CPU core, up to 8 KB of ROM
avr25 [1] | __AVR_ARCH__=25 __AVR_HAVE_MOVW__ [1] __AVR_HAVE_LPMX__ [1] __AVR_2_BYTE_PC__ [2] | "Classic" CPU core with 'MOVW' and 'LPM Rx, Z[+]' instruction, up to 8 KB of ROM
avr3 | __AVR_ARCH__=3 __AVR_MEGA__ [5] __AVR_HAVE_JMP_CALL__ [4] __AVR_2_BYTE_PC__ [2] | "Classic" CPU core, 16 KB to 64 KB of ROM
avr31 | __AVR_ARCH__=31 __AVR_MEGA__ __AVR_HAVE_RAMPZ__[4] __AVR_HAVE_ELPM__[4] __AVR_2_BYTE_PC__ [2] | "Classic" CPU core, 128 KB of ROM
avr35 [3] | __AVR_ARCH__=35 __AVR_MEGA__ [5] __AVR_HAVE_JMP_CALL__ [4] __AVR_HAVE_MOVW__ [1] __AVR_HAVE_LPMX__ [1] __AVR_2_BYTE_PC__ [2] | "Classic" CPU core with 'MOVW' and 'LPM Rx, Z[+]' instruction, 16 KB to 64 KB of ROM
avr4 | __AVR_ARCH__=4 __AVR_ENHANCED__ [5] __AVR_HAVE_MOVW__ [1] __AVR_HAVE_LPMX__ [1] __AVR_HAVE_MUL__ [1] __AVR_2_BYTE_PC__ [2] | "Enhanced" CPU core, up to 8 KB of ROM
avr5 | __AVR_ARCH__=5 __AVR_MEGA__ [5] __AVR_ENHANCED__ [5] __AVR_HAVE_JMP_CALL__ [4] __AVR_HAVE_MOVW__ [1] __AVR_HAVE_LPMX__ [1] __AVR_HAVE_MUL__ [1] __AVR_2_BYTE_PC__ [2] | "Enhanced" CPU core, 16 KB to 64 KB of ROM
avr51 | __AVR_ARCH__=51 __AVR_MEGA__ __AVR_ENHANCED__ __AVR_HAVE_MOVW__ [1] __AVR_HAVE_LPMX__ [1] __AVR_HAVE_MUL__ [1] __AVR_HAVE_RAMPZ__[4] __AVR_HAVE_ELPM__[4] __AVR_HAVE_ELPMX__[4] __AVR_2_BYTE_PC__ [2] | "Enhanced" CPU core, 128 KB of ROM
avr6 [2] | __AVR_ARCH__=6 __AVR_MEGA__ [5] __AVR_ENHANCED__ [5] __AVR_HAVE_JMP_CALL__ [4] __AVR_HAVE_MOVW__ [1] __AVR_HAVE_LPMX__ [1] __AVR_HAVE_MUL__ [1] __AVR_HAVE_RAMPZ__[4] __AVR_HAVE_ELPM__[4] __AVR_HAVE_ELPMX__[4] __AVR_3_BYTE_PC__ [2] | "Enhanced" CPU core, 256 KB of ROM

[1] New in GCC 4.2
[2] Unofficial patch for GCC 4.1
[3] New in GCC 4.2.3
[4] New in GCC 4.3
[5] Obsolete.

By default, code is generated for the avr2 architecture.

Note that when only using `-mmcu=`architecture but no `-mmcu=`MCU type, including the file `<avr/io.h>` cannot work since it cannot decide which device's definitions to select.

- `-mmcu=`MCU type

The following MCU types are currently understood by avr-gcc. The table matches them against the corresponding avr-gcc architecture name, and shows the preprocessor symbol declared by the `-mmcu` option.

Architecture | MCU name | Macro
avr1 | at90s1200 | __AVR_AT90S1200__
avr1 | attiny11 | __AVR_ATtiny11__
avr1 | attiny12 | __AVR_ATtiny12__
avr1 | attiny15 | __AVR_ATtiny15__
avr1 | attiny28 | __AVR_ATtiny28__
avr2 | at90s2313 | __AVR_AT90S2313__
avr2 | at90s2323 | __AVR_AT90S2323__
avr2 | at90s2333 | __AVR_AT90S2333__
avr2 | at90s2343 | __AVR_AT90S2343__
avr2 | attiny22 | __AVR_ATtiny22__
avr2 | attiny26 | __AVR_ATtiny26__
avr2 | at90s4414 | __AVR_AT90S4414__
avr2 | at90s4433 | __AVR_AT90S4433__
avr2 | at90s4434 | __AVR_AT90S4434__
avr2 | at90s8515 | __AVR_AT90S8515__
avr2 | at90c8534 | __AVR_AT90C8534__
avr2 | at90s8535 | __AVR_AT90S8535__
avr2/avr25 [1] | at86rf401 | __AVR_AT86RF401__
avr2/avr25 [1] | ata6289 | __AVR_ATA6289__
avr2/avr25 [1] | attiny13 | __AVR_ATtiny13__
avr2/avr25 [1] | attiny13a | __AVR_ATtiny13A__
avr2/avr25 [1] | attiny2313 | __AVR_ATtiny2313__
avr2/avr25 [1] | attiny2313a | __AVR_ATtiny2313A__
avr2/avr25 [1] | attiny24 | __AVR_ATtiny24__
avr2/avr25 [1] | attiny24a | __AVR_ATtiny24A__
avr2/avr25 [1] | attiny25 | __AVR_ATtiny25__
avr2/avr25 [1] | attiny261 | __AVR_ATtiny261__
avr2/avr25 [1] | attiny261a | __AVR_ATtiny261A__
avr2/avr25 [1] | attiny4313 | __AVR_ATtiny4313__
avr2/avr25 [1] | attiny43u | __AVR_ATtiny43U__
avr2/avr25 [1] | attiny44 | __AVR_ATtiny44__
avr2/avr25 [1] | attiny44a | __AVR_ATtiny44A__
avr2/avr25 [1] | attiny45 | __AVR_ATtiny45__
avr2/avr25 [1] | attiny461 | __AVR_ATtiny461__
avr2/avr25 [1] | attiny461a | __AVR_ATtiny461A__
avr2/avr25 [1] | attiny48 | __AVR_ATtiny48__
avr2/avr25 [1] | attiny84 | __AVR_ATtiny84__
avr2/avr25 [1] | attiny85 | __AVR_ATtiny85__
avr2/avr25 [1] | attiny861 | __AVR_ATtiny861__
avr2/avr25 [1] | attiny861a | __AVR_ATtiny861A__
avr2/avr25 [1] | attiny87 | __AVR_ATtiny87__
avr2/avr25 [1] | attiny88 | __AVR_ATtiny88__
avr3 | atmega603 | __AVR_ATmega603__
avr3 | at43usb355 | __AVR_AT43USB355__
avr3/avr31 [3] | atmega103 | __AVR_ATmega103__
avr3/avr31 [3] | at43usb320 | __AVR_AT43USB320__
avr3/avr35 [2] | at90usb82 | __AVR_AT90USB82__
avr3/avr35 [2] | at90usb162 | __AVR_AT90USB162__
avr3/avr35 [2] | atmega8u2 | __AVR_ATmega8U2__
avr3/avr35 [2] | atmega16u2 | __AVR_ATmega16U2__
avr3/avr35 [2] | atmega32u2 | __AVR_ATmega32U2__
avr3/avr35 [2] | attiny167 | __AVR_ATtiny167__
avr3 | at76c711 | __AVR_AT76C711__
avr4 | atmega48 | __AVR_ATmega48__
avr4 | atmega48a | __AVR_ATmega48A__
avr4 | atmega48p | __AVR_ATmega48P__
avr4 | atmega8 | __AVR_ATmega8__
avr4 | atmega8515 | __AVR_ATmega8515__
avr4 | atmega8535 | __AVR_ATmega8535__
avr4 | atmega88 | __AVR_ATmega88__
avr4 | atmega88a | __AVR_ATmega88A__
avr4 | atmega88p | __AVR_ATmega88P__
avr4 | atmega88pa | __AVR_ATmega88PA__
avr4 | atmega8hva | __AVR_ATmega8HVA__
avr4 | at90pwm1 | __AVR_AT90PWM1__
avr4 | at90pwm2 | __AVR_AT90PWM2__
avr4 | at90pwm2b | __AVR_AT90PWM2B__
avr4 | at90pwm3 | __AVR_AT90PWM3__
avr4 | at90pwm3b | __AVR_AT90PWM3B__
avr4 | at90pwm81 | __AVR_AT90PWM81__
avr5 | at90can32 | __AVR_AT90CAN32__
avr5 | at90can64 | __AVR_AT90CAN64__
avr5 | at90pwm216 | __AVR_AT90PWM216__
avr5 | at90pwm316 | __AVR_AT90PWM316__
avr5 | at90scr100 | __AVR_AT90SCR100__
avr5 | at90usb646 | __AVR_AT90USB646__
avr5 | at90usb647 | __AVR_AT90USB647__
avr5 | at94k | __AVR_AT94K__
avr5 | atmega16 | __AVR_ATmega16__
avr5 | atmega161 | __AVR_ATmega161__
avr5 | atmega162 | __AVR_ATmega162__
avr5 | atmega163 | __AVR_ATmega163__
avr5 | atmega164a | __AVR_ATmega164A__
avr5 | atmega164p | __AVR_ATmega164P__
avr5 | atmega165 | __AVR_ATmega165__
avr5 | atmega165a | __AVR_ATmega165A__
avr5 | atmega165p | __AVR_ATmega165P__
avr5 | atmega168 | __AVR_ATmega168__
avr5 | atmega168a | __AVR_ATmega168A__
avr5 | atmega168p | __AVR_ATmega168P__
avr5 | atmega169 | __AVR_ATmega169__
avr5 | atmega169a | __AVR_ATmega169A__
avr5 | atmega169p | __AVR_ATmega169P__
avr5 | atmega169pa | __AVR_ATmega169PA__
avr5 | atmega16a | __AVR_ATmega16A__
avr5 | atmega16hva | __AVR_ATmega16HVA__
avr5 | atmega16hva2 | __AVR_ATmega16HVA2__
avr5 | atmega16hvb | __AVR_ATmega16HVB__
avr5 | atmega16m1 | __AVR_ATmega16M1__
avr5 | atmega16u4 | __AVR_ATmega16U4__
avr5 | atmega32 | __AVR_ATmega32__
avr5 | atmega323 | __AVR_ATmega323__
avr5 | atmega324a | __AVR_ATmega324A__
avr5 | atmega324p | __AVR_ATmega324P__
avr5 | atmega324pa | __AVR_ATmega324PA__
avr5 | atmega325 | __AVR_ATmega325__
avr5 | atmega325p | __AVR_ATmega325P__
avr5 | atmega3250 | __AVR_ATmega3250__
avr5 | atmega3250p | __AVR_ATmega3250P__
avr5 | atmega328 | __AVR_ATmega328__
avr5 | atmega328p | __AVR_ATmega328P__
avr5 | atmega329 | __AVR_ATmega329__
avr5 | atmega329p | __AVR_ATmega329P__
avr5 | atmega329pa | __AVR_ATmega329PA__
avr5 | atmega3290 | __AVR_ATmega3290__
avr5 | atmega3290p | __AVR_ATmega3290P__
avr5 | atmega32c1 | __AVR_ATmega32C1__
avr5 | atmega32hvb | __AVR_ATmega32HVB__
avr5 | atmega32m1 | __AVR_ATmega32M1__
avr5 | atmega32u4 | __AVR_ATmega32U4__
avr5 | atmega32u6 | __AVR_ATmega32U6__
avr5 | atmega406 | __AVR_ATmega406__
avr5 | atmega64 | __AVR_ATmega64__
avr5 | atmega640 | __AVR_ATmega640__
avr5 | atmega644 | __AVR_ATmega644__
avr5 | atmega644a | __AVR_ATmega644A__
avr5 | atmega644p | __AVR_ATmega644P__
avr5 | atmega644pa | __AVR_ATmega644PA__
avr5 | atmega645 | __AVR_ATmega645__
avr5 | atmega645a | __AVR_ATmega645A__
avr5 | atmega645p | __AVR_ATmega645P__
avr5 | atmega6450 | __AVR_ATmega6450__
avr5 | atmega6450a | __AVR_ATmega6450A__
avr5 | atmega6450p | __AVR_ATmega6450P__
avr5 | atmega649 | __AVR_ATmega649__
avr5 | atmega649a | __AVR_ATmega649A__
avr5 | atmega6490 | __AVR_ATmega6490__
avr5 | atmega6490a | __AVR_ATmega6490A__
avr5 | atmega6490p | __AVR_ATmega6490P__
avr5 | atmega649p | __AVR_ATmega649P__
avr5 | atmega64c1 | __AVR_ATmega64C1__
avr5 | atmega64hve | __AVR_ATmega64HVE__
avr5 | atmega64m1 | __AVR_ATmega64M1__
avr5/avr51 [3] | at90can128 | __AVR_AT90CAN128__
avr5/avr51 [3] | at90usb1286 | __AVR_AT90USB1286__
avr5/avr51 [3] | at90usb1287 | __AVR_AT90USB1287__
avr5/avr51 [3] | atmega128 | __AVR_ATmega128__
avr5/avr51 [3] | atmega1280 | __AVR_ATmega1280__
avr5/avr51 [3] | atmega1281 | __AVR_ATmega1281__
avr5/avr51 [3] | atmega1284p | __AVR_ATmega1284P__
avr6 | atmega2560 | __AVR_ATmega2560__
avr6 | atmega2561 | __AVR_ATmega2561__
avrxmega2 | atxmega16a4 | __AVR_ATxmega16A4__
avrxmega2 | atxmega16d4 | __AVR_ATxmega16D4__
avrxmega2 | atxmega32d4 | __AVR_ATxmega32D4__
avrxmega3 | atxmega32a4 | __AVR_ATxmega32A4__
avrxmega4 | atxmega64a3 | __AVR_ATxmega64A3__
avrxmega4 | atxmega64d3 | __AVR_ATxmega64D3__
avrxmega5 | atxmega64a1 | __AVR_ATxmega64A1__
avrxmega6 | atxmega128a3 | __AVR_ATxmega128A3__
avrxmega6 | atxmega128d3 | __AVR_ATxmega128D3__
avrxmega6 | atxmega192a3 | __AVR_ATxmega192A3__
avrxmega6 | atxmega192d3 | __AVR_ATxmega192D3__
avrxmega6 | atxmega256a3 | __AVR_ATxmega256A3__
avrxmega6 | atxmega256a3b | __AVR_ATxmega256A3B__
avrxmega6 | atxmega256d3 | __AVR_ATxmega256D3__
avrxmega7 | atxmega128a1 | __AVR_ATxmega128A1__

[1] 'avr25' architecture is new in GCC 4.2
[2] 'avr35' architecture is new in GCC 4.2.3

- `-morder1`
- `-morder2`

Change the order of register assignment. The default is

r24, r25, r18, r19, r20, r21, r22, r23, r30, r31, r26, r27, r28, r29, r17, r16, r15, r14, r13, r12, r11, r10, r9, r8, r7, r6, r5, r4, r3, r2, r0, r1

Order 1 uses

r18, r19, r20, r21, r22, r23, r24, r25, r30, r31, r26, r27, r28, r29, r17, r16, r15, r14, r13, r12, r11, r10, r9, r8, r7, r6, r5, r4, r3, r2, r0, r1

Order 2 uses

r25, r24, r23, r22, r21, r20, r19, r18, r30, r31, r26, r27, r28, r29, r17, r16, r15, r14, r13, r12, r11, r10, r9, r8, r7, r6, r5, r4, r3, r2, r1, r0

- `-mint8`

Assume `int` to be an 8-bit integer. Note that this is not really supported by `avr-libc`, so it should normally not be used. The default is to use 16-bit integers.

- `-mno-interrupts`

Generates code that changes the stack pointer without disabling interrupts. Normally, the state of the status register `SREG` is saved in a temporary register, interrupts are disabled while changing the stack pointer, and `SREG` is restored.

Specifying this option will define the preprocessor macro `__NO_INTERRUPTS__` to the value 1.

- `-mcall-prologues`

Use subroutines for function prologue/epilogue. For complex functions that use many registers (that needs to be saved/restored on function entry/exit), this saves some space at the cost of a slightly increased execution time.

- `-mtiny-stack`

Change only the low 8 bits of the stack pointer.

- `-mno-tablejump`

Deprecated, use `-fno-jump-tables` instead.

- `-mshort-calls`

Use `rjmp/rcall` (limited range) on >8K devices. On `avr2` and `avr4` architectures (less than 8 KB or flash memory), this is always the case. On `avr3` and `avr5` architectures, calls and jumps to targets outside the current function will by default use `jmp/call` instructions that can cover the entire address range, but that require more flash ROM and execution time.

- `-mrtl`

Dump the internal compilation result called "RTL" into comments in the generated assembler code. Used for debugging avr-gcc.

- `-msize`

Dump the address, size, and relative cost of each statement into comments in the generated assembler code. Used for debugging avr-gcc.

- `-mdeb`

Generate lots of debugging information to `stderr`.

### Selected general compiler options

The following general gcc options might be of some interest to AVR users.

- `-O` n

Optimization level n. Increasing n is meant to optimize more, an optimization level of 0 means no optimization at all, which is the default if no `-O` option is present. The special option `-Os` is meant to turn on all `-O2` optimizations that are not expected to increase code size.

Note that at `-O3`, gcc attempts to inline all "simple" functions. For the AVR target, this will normally constitute a large pessimization due to the code increasement. The only other optimization turned on with `-O3` is `-frename-registers`, which could rather be enabled manually instead.

A simple `-O` option is equivalent to `-O1`.

Note also that turning off all optimizations will prevent some warnings from being issued since the generation of those warnings depends on code analysis steps that are only performed when optimizing (unreachable code, unused variables).

See also the appropriate FAQ entry for issues regarding debugging optimized code.

- `-Wa,`assembler-options
- `-Wl,`linker-options

Pass the listed options to the assembler, or linker, respectively.

- `-g`

Generate debugging information that can be used by avr-gdb.

- `-ffreestanding`

Assume a "freestanding" environment as per the C standard. This turns off automatic builtin functions (though they can still be reached by prepending `__builtin_` to the actual function name). It also makes the compiler not complain when `main()` is declared with a `void` return type which makes some sense in a microcontroller environment where the application cannot meaningfully provide a return value to its environment (in most cases, `main()` won't even return anyway). However, this also turns off all optimizations normally done by the compiler which assume that functions known by a certain name behave as described by the standard. E. g., applying the function strlen() to a literal string will normally cause the compiler to immediately replace that call by the actual length of the string, while with `-ffreestanding`, it will always call strlen() at run-time.

- `-funsigned-char`

Make any unqualfied `char` type an unsigned char. Without this option, they default to a signed char.

- `-funsigned-bitfields`

Make any unqualified bitfield type unsigned. By default, they are signed.

- `-fshort-enums`

Allocate to an `enum` type only as many bytes as it needs for the declared range of possible values. Specifically, the enum type will be equivalent to the smallest integer type which has enough room.

- `-fpack-struct`

Pack all structure members together without holes.

- `-fno-jump-tables`

Do not generate tablejump instructions. By default, jump tables can be used to optimize `switch` statements. When turned off, sequences of compare statements are used instead. Jump tables are usually faster to execute on average, but in particular for `switch` statements, where most of the jumps would go to the default label, they might waste a bit of flash memory.

NOTE: The tablejump instructions use the LPM assembler instruction for access to jump tables. Always use `-fno-jump-tables` switch, if compiling a bootloader for devices with more than 64 KB of code memory.

## Options for the assembler avr-as

### Machine-specific assembler options

- `-mmcu=`architecture
- `-mmcu=`MCU name

avr-as understands the same `-mmcu=` options as avr-gcc. By default, avr2 is assumed, but this can be altered by using the appropriate ``.arch pseudo-instruction inside the assembler source file.

- `-mall-opcodes`

Turns off opcode checking for the actual MCU type, and allows any possible AVR opcode to be assembled.

- `-mno-skip-bug`

Don't emit a warning when trying to skip a 2-word instruction with a `CPSE/SBIC/SBIS/SBRC/SBRS` instruction. Early AVR devices suffered from a hardware bug where these instructions could not be properly skipped.

- `-mno-wrap`

For `RJMP/RCALL` instructions, don't allow the target address to wrap around for devices that have more than 8 KB of memory.

- `--gstabs`

Generate ``.stabs debugging symbols for assembler source lines. This enables avr-gdb to trace through assembler source files. This option must not be used when assembling sources that have been generated by the C compiler; these files already contain the appropriate line number information from the C source files.

- `-a[cdhlmns=`file`]`

Turn on the assembler listing. The sub-options are:

- `c` omit false conditionals
- `d` omit debugging directives
- `h` include high-level source
- `l` include assembly
- `m` include macro expansions
- `n` omit forms processing
- `s` include symbols
- `=`file set the name of the listing file

The various sub-options can be combined into a single `-a` option list; =file must be the last one in that case.

### Examples for assembler options passed through the C compiler

Remember that assembler options can be passed from the C compiler frontend using `-Wa` (see above), so in order to include the C source code into the assembler listing in file `foo.lst`, when compiling `foo.c`, the following compiler command-line can be used:

    	$ avr-gcc -c -O foo.c -o foo.o -Wa,-ahls=foo.lst

In order to pass an assembler file through the C preprocessor first, and have the assembler generate line number debugging information for it, the following command can be used:

    	$ avr-gcc -c -x assembler-with-cpp -o foo.o foo.S -Wa,--gstabs

Note that on Unix systems that have case-distinguishing file systems, specifying a file name with the suffix ``.S (upper-case letter S) will make the compiler automatically assume `-x assembler-with-cpp`, while using ``.s would pass the file directly to the assembler (no preprocessing done).

## Controlling the linker avr-ld

### Selected linker options

While there are no machine-specific options for avr-ld, a number of the standard options might be of interest to AVR users.

- `-l` name

Locate the archive library named `lib` name`.a`, and use it to resolve currently unresolved symbols from it. The library is searched along a path that consists of builtin pathname entries that have been specified at compile time (e. g. `/usr/local/avr/lib` on Unix systems), possibly extended by pathname entries as specified by `-L` options (that must precede the `-l` options on the command-line).

- `-L` path

Additional location to look for archive libraries requested by `-l` options.

- `--defsym` symbol=expr

Define a global symbol symbol using expr as the value.

- `-M`

Print a linker map to `stdout`.

- `-Map` mapfile

Print a linker map to mapfile.

- `--cref`

Output a cross reference table to the map file (in case `-Map` is also present), or to `stdout`.

- `--section-start` sectionname=org

Start section sectionname at absolute address org.

- `-Tbss` org
- `-Tdata` org
- `-Ttext` org

Start the `bss`, `data`, or `text` section at org, respectively.

- `-T` scriptfile

Use scriptfile as the linker script, replacing the default linker script. Default linker scripts are stored in a system-specific location (e. g. under `/usr/local/avr/lib/ldscripts` on Unix systems), and consist of the AVR architecture name (avr2 through avr5) with the suffix ``.x appended. They describe how the various memory sections will be linked together.

### Passing linker options from the C compiler

By default, all unknown non-option arguments on the avr-gcc command-line (i. e., all filename arguments that don't have a suffix that is handled by avr-gcc) are passed straight to the linker. Thus, all files ending in ``.o (object files) and ``.a (object libraries) are provided to the linker.

System libraries are usually not passed by their explicit filename but rather using the `-l` option which uses an abbreviated form of the archive filename (see above). avr-libc ships two system libraries, `libc.a`, and `libm.a`. While the standard library `libc.a` will always be searched for unresolved references when the linker is started using the C compiler frontend (i. e., there's always at least one implied `-lc` option), the mathematics library `libm.a` needs to be explicitly requested using `-lm`. See also the entry in the FAQ explaining this.

Conventionally, Makefiles use the `make` macro `LDLIBS` to keep track of `-l` (and possibly `-L`) options that should only be appended to the C compiler command-line when linking the final binary. In contrast, the macro `LDFLAGS` is used to store other command-line options to the C compiler that should be passed as options during the linking stage. The difference is that options are placed early on the command-line, while libraries are put at the end since they are to be used to resolve global symbols that are still unresolved at this point.

Specific linker flags can be passed from the C compiler command-line using the `-Wl` compiler option, see above. This option requires that there be no spaces in the appended linker option, while some of the linker options above (like `-Map` or `--defsym`) would require a space. In these situations, the space can be replaced by an equal sign as well. For example, the following command-line can be used to compile `foo.c` into an executable, and also produce a link map that contains a cross-reference list in the file `foo.map:`

    	$ avr-gcc -O -o foo.out -Wl,-Map=foo.map -Wl,--cref foo.c

Alternatively, a comma as a placeholder will be replaced by a space before passing the option to the linker. So for a device with external SRAM, the following command-line would cause the linker to place the data segment at address 0x2000 in the SRAM:

    	$ avr-gcc -mmcu=atmega128 -o foo.out -Wl,-Tdata,0x802000

See the explanation of the data section for why 0x800000 needs to be added to the actual value. Note that the stack will still remain in internal RAM, through the symbol `__stack` that is provided by the run-time startup code. This is probably a good idea anyway (since internal RAM access is faster), and even required for some early devices that had hardware bugs preventing them from using a stack in external RAM. Note also that the heap for `malloc()` will still be placed after all the variables in the data section, so in this situation, no stack/heap collision can occur.

In order to relocate the stack from its default location at the top of interns RAM, the value of the symbol `__stack` can be changed on the linker command-line. As the linker is typically called from the compiler frontend, this can be achieved using a compiler option like

    -Wl,--defsym=__stack=0x8003ff

The above will make the code use stack space from RAM address 0x3ff downwards. The amount of stack space available then depends on the bottom address of internal RAM for a particular device. It is the responsibility of the application to ensure the stack does not grow out of bounds, as well as to arrange for the stack to not collide with variable allocations made by the compiler (sections .data and .bss).

Automatically generated by Doxygen 1.5.6 on 6 Jan 2010.