msvcirt: Fix implementation of strstreambuf::underflow

[Iván Matellanes]

Spotted while testing for ostream printing.

Signed-off-by: Iván Matellanes <matellanes.ivan at gmail.com>
---
 dlls/msvcirt/msvcirt.c       |  5 ++++-
 dlls/msvcirt/tests/msvcirt.c | 28 ++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/dlls/msvcirt/msvcirt.c b/dlls/msvcirt/msvcirt.c
index fe5901a..e4fb3fb 100644
--- a/dlls/msvcirt/msvcirt.c
+++ b/dlls/msvcirt/msvcirt.c
@@ -1469,8 +1469,11 @@ int __thiscall strstreambuf_underflow(strstreambuf *this)
     if (this->base.gptr < this->base.egptr)
         return *this->base.gptr;
     /* extend the get area to include the characters written */
-    if (this->base.egptr < this->base.pptr)
+    if (this->base.egptr < this->base.pptr) {
+        this->base.gptr = this->base.base + (this->base.gptr - this->base.eback);
+        this->base.eback = this->base.base;
         this->base.egptr = this->base.pptr;
+    }
     return (this->base.gptr < this->base.egptr) ? *this->base.gptr : EOF;
 }
 
diff --git a/dlls/msvcirt/tests/msvcirt.c b/dlls/msvcirt/tests/msvcirt.c
index a6916c1..428ddf3 100644
--- a/dlls/msvcirt/tests/msvcirt.c
+++ b/dlls/msvcirt/tests/msvcirt.c
@@ -1745,8 +1745,36 @@ static void test_strstreambuf(void)
     ssb2.base.pbase = ssb2.base.pptr = ssb2.base.epptr = NULL;
     ret = (int) call_func1(p_strstreambuf_underflow, &ssb2);
     ok(ret == EOF, "expected EOF got %d\n", ret);
+    ssb2.base.pbase = ssb2.base.pptr = ssb2.base.base;
+    ret = (int) call_func1(p_strstreambuf_underflow, &ssb2);
+    ok(ret == EOF, "expected EOF got %d\n", ret);
+    ok(ssb2.base.eback == ssb2.base.base, "wrong get base, expected %p got %p\n", ssb2.base.base, ssb2.base.eback);
+    ok(ssb2.base.gptr == ssb2.base.base, "wrong get pointer, expected %p got %p\n", ssb2.base.base, ssb2.base.gptr);
+    ok(ssb2.base.egptr == ssb2.base.base, "wrong get end, expected %p got %p\n", ssb2.base.base, ssb2.base.egptr);
+    ssb2.base.pptr = ssb2.base.base + 3;
+    ret = (int) call_func1(p_strstreambuf_underflow, &ssb2);
+    ok(ret == 'C', "expected 'C' got %d\n", ret);
+    ok(ssb2.base.eback == ssb2.base.base, "wrong get base, expected %p got %p\n", ssb2.base.base, ssb2.base.eback);
+    ok(ssb2.base.gptr == ssb2.base.base, "wrong get pointer, expected %p got %p\n", ssb2.base.base, ssb2.base.gptr);
+    ok(ssb2.base.egptr == ssb2.base.base + 3, "wrong get end, expected %p got %p\n", ssb2.base.base + 3, ssb2.base.egptr);
+    ssb2.base.gptr = ssb2.base.base + 2;
+    ssb2.base.egptr = NULL;
+    ret = (int) call_func1(p_strstreambuf_underflow, &ssb2);
+    ok(ret == 'e', "expected 'e' got %d\n", ret);
+    ok(ssb2.base.eback == ssb2.base.base, "wrong get base, expected %p got %p\n", ssb2.base.base, ssb2.base.eback);
+    ok(ssb2.base.gptr == ssb2.base.base + 2, "wrong get pointer, expected %p got %p\n", ssb2.base.base, ssb2.base.gptr);
+    ok(ssb2.base.egptr == ssb2.base.base + 3, "wrong get end, expected %p got %p\n", ssb2.base.base + 3, ssb2.base.egptr);
+    ssb2.base.eback = ssb2.base.egptr = ssb2.base.base + 1;
+    ssb2.base.gptr = ssb2.base.base + 3;
+    ssb2.base.pptr = ssb2.base.base + 5;
+    ret = (int) call_func1(p_strstreambuf_underflow, &ssb2);
+    ok(ret == 'e', "expected 'e' got %d\n", ret);
+    ok(ssb2.base.eback == ssb2.base.base, "wrong get base, expected %p got %p\n", ssb2.base.base, ssb2.base.eback);
+    ok(ssb2.base.gptr == ssb2.base.base + 2, "wrong get pointer, expected %p got %p\n", ssb2.base.base, ssb2.base.gptr);
+    ok(ssb2.base.egptr == ssb2.base.base + 5, "wrong get end, expected %p got %p\n", ssb2.base.base + 5, ssb2.base.egptr);
     ssb2.base.eback = ssb2.base.base;
     ssb2.base.gptr = ssb2.base.egptr = ssb2.base.ebuf;
+    ssb2.base.pbase = ssb2.base.pptr = NULL;
     ret = (int) call_func1(p_strstreambuf_underflow, &ssb2);
     ok(ret == EOF, "expected EOF got %d\n", ret);
 
-- 
2.7.4