This is used in various selftests and will be handy when integrating
those with nolibc.

Only the standard POSIX modes are supported.
No extensions nor the (noop) "b" from ISO C are accepted.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Acked-by: Willy Tarreau <w@1wt.eu>
---
 tools/include/nolibc/stdio.h                 | 20 ++++++++++++++++++++
 tools/testing/selftests/nolibc/nolibc-test.c | 24 ++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index df5717d591826c8468e3473581ac0d6e76629aa3..61b7252ab6d04f8758367f5265687146326456a3 100644
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -13,6 +13,7 @@
 #include "std.h"
 #include "arch.h"
 #include "errno.h"
+#include "fcntl.h"
 #include "types.h"
 #include "sys.h"
 #include "stdarg.h"
@@ -55,6 +56,25 @@ FILE *fdopen(int fd, const char *mode __attribute__((unused)))
 	return (FILE*)(intptr_t)~fd;
 }
 
+static __attribute__((unused))
+FILE *fopen(const char *pathname, const char *mode)
+{
+	int flags, fd;
+
+        switch (*mode) {
+                case 'r': flags = O_RDONLY; break;
+                case 'w': flags = O_WRONLY | O_CREAT | O_TRUNC; break;
+                case 'a': flags = O_WRONLY | O_CREAT | O_APPEND; break;
+                default : SET_ERRNO(EINVAL); return NULL;
+        }
+
+        if (mode[1] == '+')
+                flags = (flags & ~(O_RDONLY|O_WRONLY)) | O_RDWR;
+
+	fd = open(pathname, flags, 0666);
+	return fdopen(fd, mode);
+}
+
 /* provides the fd of stream. */
 static __attribute__((unused))
 int fileno(FILE *stream)
diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index d1157319b5d715dbc94ff9203b1d2a2e932aabcf..0391c7d01380ea2f20d1d07497ea1964bcb6a9f4 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -859,6 +859,29 @@ int test_getpagesize(void)
 	return !c;
 }
 
+int test_file_stream(void)
+{
+	FILE *f;
+	int r;
+
+	f = fopen("/dev/null", "r");
+	if (!f)
+		return -1;
+
+	errno = 0;
+	r = fwrite("foo", 1, 3, f);
+	if (r != 0 || errno != EBADF) {
+		fclose(f);
+		return -1;
+	}
+
+	r = fclose(f);
+	if (r == EOF)
+		return -1;
+
+	return 0;
+}
+
 int test_fork(void)
 {
 	int status;
@@ -1311,6 +1334,7 @@ int run_syscall(int min, int max)
 		CASE_TEST(dup3_0);            tmp = dup3(0, 100, 0);  EXPECT_SYSNE(1, tmp, -1); close(tmp); break;
 		CASE_TEST(dup3_m1);           tmp = dup3(-1, 100, 0); EXPECT_SYSER(1, tmp, -1, EBADF); if (tmp != -1) close(tmp); break;
 		CASE_TEST(execve_root);       EXPECT_SYSER(1, execve("/", (char*[]){ [0] = "/", [1] = NULL }, NULL), -1, EACCES); break;
+		CASE_TEST(file_stream);       EXPECT_SYSZR(1, test_file_stream()); break;
 		CASE_TEST(fork);              EXPECT_SYSZR(1, test_fork()); break;
 		CASE_TEST(getdents64_root);   EXPECT_SYSNE(1, test_getdents64("/"), -1); break;
 		CASE_TEST(getdents64_null);   EXPECT_SYSER(1, test_getdents64("/dev/null"), -1, ENOTDIR); break;

-- 
2.49.0