Add crt2-command.o to provide __main_void

cf. #485

libc-bottom-half/crt/crt2-command.c

@@ -0,0 +1,54 @@
+#include <wasi/api.h>
+#include <stdlib.h>
+#include <sysexits.h>
+
+// The user's `main` function, expecting arguments.
+int __main_argc_argv(int argc, char *argv[]);
+
+// If the user's `main` function expects arguments, the compiler will rename
+// it to `__main_argc_argv`, and this version will get linked in, which
+// initializes the argument data and calls `__main_argc_argv`.
+__attribute__((__weak__, nodebug))
+int __main_void(void) {
+    __wasi_errno_t err;
+
+    // Get the sizes of the arrays we'll have to create to copy in the args.
+    size_t argv_buf_size;
+    size_t argc;
+    err = __wasi_args_sizes_get(&argc, &argv_buf_size);
+    if (err != __WASI_ERRNO_SUCCESS) {
+        _Exit(EX_OSERR);
+    }
+
+    // Add 1 for the NULL pointer to mark the end, and check for overflow.
+    size_t num_ptrs = argc + 1;
+    if (num_ptrs == 0) {
+        _Exit(EX_SOFTWARE);
+    }
+
+    // Allocate memory for storing the argument chars.
+    char *argv_buf = malloc(argv_buf_size);
+    if (argv_buf == NULL) {
+        _Exit(EX_SOFTWARE);
+    }
+
+    // Allocate memory for the array of pointers. This uses `calloc` both to
+    // handle overflow and to initialize the NULL pointer at the end.
+    char **argv = calloc(num_ptrs, sizeof(char *));
+    if (argv == NULL) {
+        free(argv_buf);
+        _Exit(EX_SOFTWARE);
+    }
+
+    // Fill the argument chars, and the argv array with pointers into those chars.
+    // TODO: Remove the casts on `argv_ptrs` and `argv_buf` once the witx is updated with char8 support.
+    err = __wasi_args_get((uint8_t **)argv, (uint8_t *)argv_buf);
+    if (err != __WASI_ERRNO_SUCCESS) {
+        free(argv_buf);
+        free(argv);
+        _Exit(EX_OSERR);
+    }
+
+    // Call `__main_argc_argv` with the arguments!
+    return __main_argc_argv(argc, argv);
+}