Disable iperf for darwin, update README for gcc

Makefile

@@ -100,12 +100,14 @@ endif
 
 iperf3:
 ifeq ($(arch), x86_64)
+ifneq ($(shell uname), Darwin)
 	@echo Building iperf3
 	@mkdir -p $(out_dir)/dev
 	@dd if=/dev/urandom of=$(out_dir)/dev/urandom count=512
 	@cd iperf3 && make arch=$(arch) all
 	@cp iperf3/build/$(arch)/iperf3 $(out_dir)
 endif
+endif
 
 $(alpine):
 	wget "http://dl-cdn.alpinelinux.org/alpine/v3.9/releases/$(arch)/$(alpine_file)" -O $(alpine)

README.md

@@ -2,15 +2,29 @@
 
 [![Build Status](https://travis-ci.org/rcore-os/rcore-user.svg?branch=master)](https://travis-ci.org/rcore-os/rcore-user)
 
-User programs for [rCore OS](https://github.com/rcore-os/rCore).
+<!--ts-->
+   * [rCore-user](#rcore-user)
+      * [Build](#build)
+      * [Support matrix](#support-matrix)
+      * [How to run real world programs](#how-to-run-real-world-programs)
+         * [How to use Redis](#how-to-use-redis)
+         * [How to use nginx](#how-to-use-nginx)
+         * [How to use gcc](#how-to-use-gcc)
+
+<!-- Added by: macbookpro, at: Fri Mar 29 14:02:36 CST 2019 -->
 
+<!--te-->
+
+User programs for [rCore OS](https://github.com/rcore-os/rCore).
 
 Now it has 4 parts:
 
 * `ucore`: C-lang, from the original [ucore_os_lab](https://github.com/chyyuu/ucore_os_plus)
 * `biscuit`: C/C++, from [Biscuit](https://github.com/mit-pdos/biscuit), based on a `musl` instead of original `litc`.
 * `rust`: Simple no_std Rust programs.
-* `nginx`, `redis`, `busybox`, `alpine`: Real world applications.
+* `nginx`, `redis`, `busybox`, `alpine`, `gcc`: Real world applications.
+
+
 
 ## Build
 
@@ -46,10 +60,11 @@ A rootfs is created at `build/$(arch)` and converted to `qcow2`.
 | redis (linux only) | ✅      | ✅       | ✅       | ❌       |
 | busybox            | ✅      | ✅       | ❌       | ✅       |
 | alpine rootfs      | ✅      | ✅       | ❌       | ❌       |
+| iperf3             | ✅      | ❌       | ❌       | ❌       |
 
+## How to run real world programs
 
-
-## How to use Redis
+### How to use Redis
 
 If redis is dynamically linked to musl (default if you use commands above), you might need to copy `ld-musl-$(arch).so.1` to rootfs `/lib` . Alpine rootfs includes one as well.
 
@@ -72,7 +87,7 @@ $ redis-cli -h 10.0.0.2 get abc
 
 Note: `redis-cli` in rCore is not working at the time.
 
-## How to use nginx
+### How to use nginx
 
 Nginx is statically linked to musl instead, so there is no need to copy its dynamic loader. A nginx.conf is provided in this repo and automatically copied to its destination. So, just start `nginx` directly:
 
@@ -120,4 +135,24 @@ $ curl http://10.0.0.2/
 
 It is generated by `autoindex`.
 
-Note: `nginx` might lead to an unresolved file system bug, so you might need to re-create sfs img after killing it.
+Note: `nginx` might lead to an unresolved file system bug, so you might need to re-create sfs img after killing it.
+
+### How to use gcc
+
+First, download prebuilt musl toolchain from [musl.cc](https://musl.cc), for example `x86_64-linux-musl-cross.tgz`. Untar it into `build/x86_64` directory. You are expected have a `build/x86_64/x86_64-linux-musl` directory now.
+
+To build a simple C program, `printf.c` for example:
+
+```bash
+$ x86_64-linux-musl-cross/bin/x86_64-linux-musl-gcc printf.c -c -o printf.o
+$ x86_64-linux-musl-cross/bin/x86_64-linux-musl-ld -dynamic-linker /lib/ld-musl-x86_64.so.1 x86_64-linux-musl-cross/x86_64-linux-musl/lib/crt1.o x86_64-linux-musl-cross/x86_64-linux-musl/lib/crtn.o x86_64-linux-musl-cross/lib/gcc/x86_64-linux-musl/8.3.0/crtbeginS.o x86_64-linux-musl-cross/lib/gcc/x86_64-linux-musl/8.3.0/crtendS.o printf.o -o printf -lc -static
+```
+
+You can now run the produced program:
+
+```bash
+$ printf
+Built within rCore
+```
+
+Note: the long linker args can be replaced by invoking gcc instead later when we fix the problem. If you encountered `rcore-fs-fuse` panicking, consider upgrading it to latest version.