difference between main() and alt_main() ?

In short the difference is that main() does initialization for you then calls alt_main() (if you have the compiler spit out an object dump file you'll see both if you use main).

So if you're confident enought to do any initialization yourself then you can speed up your time to live of the NIOS II by using alt_main(). I think alt_main() is a unix thing if I'm not mistaken (I'm not used to them cause I'm just a windows programmer).

Cheers

I haven't looked at those two sample codes, but the led one probably uses alt_main since it's pretty simple to write to leds, hello world would use main since it need to set up a lot more (Jtag UART).

If you have ever done assembly programming in NIOS I think of alt_main as the assembly version you would do without all the extra overhead involved, and the main version as what the C compiler would spit out (with all kinds of overhead). If you need a fast start out of reset and want to save memory then alt_main is the way to go (if you know what you are doing with the setup of other devices attached to the NIOS II)

Please read the manual to clear up the confusion.

Below is a paste from the manual which may clear this up.

"The ANSI C standard defines a hosted application as one that calls

main() to begin execution. At the start of main(), a hosted application

presumes the runtime environment and all system services are initialized

and ready to use. This presumption is the case with the HAL system

library. In fact, the hosted environment is one of the HAL’s greatest

benefits to new Nios II programmers, because you don’t have to consider

what devices exist in the system or how to initialize each one; the HAL

automatically initializes the whole system.

The ANSI C standard also provides for an alternate entry point that

avoids automatic initialization, and assumes that the Nios II programmer

manually initializes any hardware that is used. The alt_main()

function provides a free-standing environment, giving you complete

control over the initialization of the system. The free-standing

environment places upon the programmer the burden of manually

initializing any system feature used in the program. For example, calls to

printf() do not function correctly in the free-standing environment,

unless alt_main() first instantiates a character-mode device driver, and

redirects stdout to the device."

In summary if you don't understand what alt_main is you probably should not be using it.