Electronic – avr-gcc _delay_ms timings are off; ISP to blame

A little explanation of what's really going on:

The -l argument order does not normally matter on computers since most libraries are dynamically loaded (.dll, .dylib or .so) and are truly linked only at program startup time.

OTOH on the AVR all libraries are static. When linking with static libraries the linker adds to the program only the functions he currently needs from the library. If you pass -lm on the command line before a file that needs a particular function you will get unresolved symbols errors.

I suspect the problem is not with your C code but with your Makefile.

The following lines in your Makefile produce an example.o object file.

main:
    avr-gcc -g -Os -Wall -mmcu=atmega328 -c ../src/example.c

The created .o file only contains the symbols and code from example.c, not the additional source required to actually make it run on a target system such as interrupt vector jump tables and code to initialise the BSS RAM segment to zeros, and load your initialised data sections.

You'll need to add an additional line something like this to run the linker and produce an output object suitable for download to the AVR part. Alternatively, use avr-ld, but you'll have to work out all the required linker options.

main.elf: example.o
    avr-gcc example.o -o main.elf

You can use avr-objdump --disassemble-all <filename> on both example.o and main.elf yourself to verify the different content of each file.

It's always a good idea to try to reduce your problem in steps to the most simple example possible. In this case, it would probably mean dropping into the AVR Studio software and creating a project running on the simulator using their managed build process. From there, you could them export the Makefile in use by their build process by using the 'Export Makefile' menu option. The generated makefile could then be compared with your version.

Actually, it's probably a good idea to use a Makefile similar to the one generated by AVR Studio because it has the correct rules already defined, you just have to set up some variables with regard to which objects need to be generated and the final target file name.