[PATCH v2 04/12] tools/nolibc: crt.h: add _start_c

[Zhangjin Wu]

As the environ and _auxv support added for nolibc, the assembly _start
function becomes more and more complex and therefore makes the porting
of nolibc to new architectures harder and harder.

To simplify portability, this c version of _start_c() is added to do
most of the assembly start operations in C, which reduces the complexity
a lot and will eventually simplify the porting of nolibc to the new
architectures.

The new _start_c() only requires a stack pointer argument, it will find
argv, envp and _auxv for us, and then call main(), finally, it exit()
with main's return status. With this new _start_c(), the future new
architectures only require to add very few assembly instructions.

Signed-off-by: Zhangjin Wu <falcon@xxxxxxxxxxx>
---
 tools/include/nolibc/crt.h | 44 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/tools/include/nolibc/crt.h b/tools/include/nolibc/crt.h
index 221b7c5346ca..b269294e9664 100644
--- a/tools/include/nolibc/crt.h
+++ b/tools/include/nolibc/crt.h
@@ -13,4 +13,48 @@
 char **environ __attribute__((weak));
 const unsigned long *_auxv __attribute__((weak));
 
+int main(int argc, char *argv[], char **envp);
+static void exit(int);
+
+void _start_c(long *sp)
+{
+	int argc, i;
+	char **argv;
+	char **envp;
+
+	/*
+	 * sp  :  argc          <-- argument count, required by main()
+	 * argv:  argv[0]       <-- argument vector, required by main()
+	 *        argv[1]
+	 *        ...
+	 *        argv[argc-1]
+	 *        null
+	 * envp:  envp[0]       <-- environment variables, required by main() and getenv()
+	 *        envp[1]
+	 *        ...
+	 *        null
+	 * _auxv: auxv[0]       <-- auxiliary vector, required by getauxval()
+	 *        auxv[1]
+	 *        ...
+	 *        null
+	 */
+
+	/* assign argc and argv */
+	argc = sp[0];
+	argv = (void *)(sp + 1);
+
+	/* find envp */
+	envp = argv + argc + 1;
+	environ = envp;
+
+	/* find auxv */
+	i = 0;
+	while (envp[i])
+		i++;
+	_auxv = (void *)(envp + i + 1);
+
+	/* go to application */
+	exit(main(argc, argv, envp));
+}
+
 #endif /* _NOLIBC_CRT_H */
-- 
2.25.1