CompileHello

1. Add hello apps to uClinux-dist (recommended)

Please follow the doc to add your user apps,

nios2-linux/uClinux-dist/Documentation/Adding-User-Apps-HOWTO

But change foo to hello.

cd nios2-linux/uClinux-dist
mkdir user/hello 

Edit user/Makefile,

add a line in the sorting order.

   dir_$(CONFIG_USER_HELLO_HELLO)            += hello

Edit user/Kconfig

after menu "Miscellaneous Applications".

config USER_HELLO_HELLO
    bool "hello"
    help
      This program prints hello world.

Create user/hello/Makefile

EXEC = hello
OBJS = hello.o

all: $(EXEC)

$(EXEC): $(OBJS)
     $(CC) $(LDFLAGS) -o $@ $(OBJS) $(LDLIBS)

romfs:
     $(ROMFSINST)    /bin/$(EXEC)

clean:
     -rm -f $(EXEC) *.elf *.gdb *.o

Create user/hello/hello.c.

#include <stdio.h>
int main(void)
{
  printf("hello world\n");
}

Then make menuconfig and select [*] hello. Now you can make. Boot nios2 and run "hello".

 2. Compile without adding to uClinux-dist (not recommended)

To compile a simple program for uClinux, just add -elf2flt to link flag

Create a file, hello.c

nios2-linux-uclibc-gcc hello.c -o hello -elf2flt

The compiled object format is FLAT.
You may check it with,

nios2-linux-uclibc-flthdr hello

hello
Magic: bFLT
Rev: 4
Build Date: Mon Jun 5 21:49:44 2006
Entry: 0x40
Data Start: 0x4a8c
Data End: 0x5c48
BSS End: 0x7ca8
Stack Size: 0x1000
Reloc Start: 0x5c48
Reloc Count: 0x11e
Flags: 0x1 ( Load-to-Ram )

Then copy hello to the romfs/bin dir. Rebuild the kernel image for initramfs.
cp hello ~/nios2-linux/uClinux-dist/romfs/bin
cd ~/nios2-linux/uClinux-dist
make

Boot nios2 uClinux, and run
hello

The default stack size of application is 4KB, you can change it with -elf2flt="-s <new stack size>" option, eg ,
nios2-linux-uclibc-gcc hello.c -o hello -elf2flt="-s 16000"
will have stack size as 16KiB. 

If using pthreads, only the main thread will have this 16KiB size.   Subsequent threads will have their stack at default size, and you will want to change that size at thread creation.  Otherwise, you're in for some stack-blowing fun, which hard to debug.

The default include dir search path is /opt/nios2/include (or staging_dir/include if you use buildroot)
The default library search path is /opt/nios2/lib (or staging_dir/lib if you use buildroot)
The default apps library is uClibc's libc, so you don't need -lc .
If you use math, you need -lm .
If you use pthread, you need -lpthread .
If you use crypt, you need -lcrypt .

example apps to use button pio.