lib: add unread() syscall; use it to implement ungetc()

Add an unread() syscall, which allows some read bytes to be pushed
back into the read buffer.  This allows us to read a header from a
file and then push it back.

This also makes ungetc() implementable.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

9 files changed, 114 insertions, 7 deletions

diff --git a/com32/include/stdio.h b/com32/include/stdio.h
index f37bdd9e..cbba55e8 100644
--- a/com32/include/stdio.h
+++ b/com32/include/stdio.h
@@ -88,6 +88,8 @@ __extern size_t fwrite(const void *, size_t, size_t, FILE *);
 /* No seek, but we can tell */
 __extern long ftell(FILE *);
 
+__extern int ungetc(int, FILE *);
+
 __extern int printf(const char *, ...);
 __extern int vprintf(const char *, va_list);
 __extern int fprintf(FILE *, const char *, ...);
diff --git a/com32/include/unistd.h b/com32/include/unistd.h
index c0b52d60..8b1b93d5 100644
--- a/com32/include/unistd.h
+++ b/com32/include/unistd.h
@@ -18,6 +18,8 @@ __extern int close(int);
 __extern ssize_t read(int, void *, size_t);
 __extern ssize_t write(int, const void *, size_t);
 
+__extern ssize_t unread(int, const void *, size_t);
+
 __extern int isatty(int);
 
 __extern int getscreensize(int, int *, int *);
diff --git a/com32/lib/Makefile b/com32/lib/Makefile
index 1806ec20..f00b27a9 100644
--- a/com32/lib/Makefile
+++ b/com32/lib/Makefile
@@ -10,7 +10,7 @@ include MCONFIG
 LIBOBJS = \
 	abort.o atexit.o atoi.o atol.o atoll.o calloc.o creat.o		\
 	ctypes.o errno.o fgetc.o fgets.o fopen.o fprintf.o fputc.o	\
-	fclose.o putchar.o setjmp.o					\
+	fclose.o putchar.o ungetc.o setjmp.o				\
 	fputs.o fread2.o fread.o free.o fwrite2.o fwrite.o getopt.o	\
 	lrand48.o malloc.o stack.o memccpy.o memchr.o memcmp.o		\
 	memcpy.o mempcpy.o memmem.o memmove.o memset.o memswap.o	\
@@ -38,7 +38,7 @@ LIBOBJS = \
 	sys/entry.o sys/exit.o sys/argv.o sys/times.o			\
 	sys/fileinfo.o sys/opendev.o sys/read.o sys/write.o sys/ftell.o \
 	sys/close.o sys/open.o sys/fileread.o sys/fileclose.o		\
-	sys/openmem.o							\
+	sys/openmem.o sys/unread.o					\
 	sys/isatty.o sys/fstat.o					\
 	\
 	sys/zfile.o sys/zfopen.o					\
diff --git a/com32/lib/sys/file.h b/com32/lib/sys/file.h
index 12492264..4bcca2e5 100644
--- a/com32/lib/sys/file.h
+++ b/com32/lib/sys/file.h
@@ -69,7 +69,7 @@ struct output_dev {
     ssize_t (*write)(struct file_info *, const void *, size_t);
     int (*close)(struct file_info *);
     int (*open)(struct file_info *);
-    const struct output_dev *fallback;	/* Fallback option for certain consoles */
+    const struct output_dev *fallback;	/* Fallback option for some consoles */
 };
 
 /* File structure */
@@ -97,9 +97,21 @@ struct file_info {
 	char *datap;		/* Current data pointer */
 	void *pvt;		/* Private pointer for driver */
 	char *buf;		/* Data buffer */
+	size_t unread_bytes;	/* Valid bytes in unread_buf */
+	char unread_buf[64];	/* unread() data */
     } i;
 };
 
+static inline size_t unread_free(struct file_info *fp)
+{
+    return sizeof fp->i.unread_buf - fp->i.unread_bytes;
+}
+
+static inline char *unread_data(struct file_info *fp)
+{
+    return fp->i.unread_buf + unread_free(fp);
+}
+
 extern struct file_info __file_info[NFILES];
 
 /* Line input discipline */
diff --git a/com32/lib/sys/fileread.c b/com32/lib/sys/fileread.c
index 54ff7115..f1fd3eb1 100644
--- a/com32/lib/sys/fileread.c
+++ b/com32/lib/sys/fileread.c
@@ -58,12 +58,12 @@ int __file_get_block(struct file_info *fp)
     fp->i.filedes = oreg.esi.w[0];
     fp->i.nbytes = oreg.ecx.l;
     fp->i.datap = fp->i.buf;
-    memcpy(fp->i.buf, __com32.cs_bounce, fp->i.nbytes);
+    memcpy(fp->i.datap, __com32.cs_bounce, fp->i.nbytes);
 
     return 0;
 }
 
-ssize_t __file_read(struct file_info * fp, void *buf, size_t count)
+ssize_t __file_read(struct file_info *fp, void *buf, size_t count)
 {
     char *bufp = buf;
     ssize_t n = 0;
diff --git a/com32/lib/sys/ftell.c b/com32/lib/sys/ftell.c
index 5c1a9442..d8090087 100644
--- a/com32/lib/sys/ftell.c
+++ b/com32/lib/sys/ftell.c
@@ -12,5 +12,5 @@ long ftell(FILE * stream)
     int fd = fileno(stream);
     struct file_info *fp = &__file_info[fd];
 
-    return fp->i.offset;
+    return fp->i.offset - fp->i.unread_bytes;
 }
diff --git a/com32/lib/sys/read.c b/com32/lib/sys/read.c
index e097ade1..388f6987 100644
--- a/com32/lib/sys/read.c
+++ b/com32/lib/sys/read.c
@@ -41,11 +41,29 @@
 ssize_t read(int fd, void *buf, size_t count)
 {
     struct file_info *fp = &__file_info[fd];
+    size_t n0, n1;
 
     if (fd >= NFILES || !fp->iop) {
 	errno = EBADF;
 	return -1;
     }
 
-    return fp->iop->read(fp, buf, count);
+    if (!count)
+	return 0;
+
+    n0 = min(fp->i.unread_bytes, count);
+    if (n0) {
+	memcpy(buf, unread_data(fp), n0);
+	fp->i.unread_bytes -= n0;
+	count -= n0;
+	if (!count)
+	    return n0;
+    }
+
+    n1 = fp->iop->read(fp, buf, count);
+
+    if (n1 == -1 && n0 > 0)
+	return n0;
+    else
+	return n0 + n1;
 }
diff --git a/com32/lib/sys/unread.c b/com32/lib/sys/unread.c
new file mode 100644
index 00000000..08f8b3bf
--- /dev/null
+++ b/com32/lib/sys/unread.c
@@ -0,0 +1,55 @@
+/* ----------------------------------------------------------------------- *
+ *
+ *   Copyright 2004-2008 H. Peter Anvin - All Rights Reserved
+ *
+ *   Permission is hereby granted, free of charge, to any person
+ *   obtaining a copy of this software and associated documentation
+ *   files (the "Software"), to deal in the Software without
+ *   restriction, including without limitation the rights to use,
+ *   copy, modify, merge, publish, distribute, sublicense, and/or
+ *   sell copies of the Software, and to permit persons to whom
+ *   the Software is furnished to do so, subject to the following
+ *   conditions:
+ *
+ *   The above copyright notice and this permission notice shall
+ *   be included in all copies or substantial portions of the Software.
+ *
+ *   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ *   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+ *   OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ *   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ *   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ *   WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ *   FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ *   OTHER DEALINGS IN THE SOFTWARE.
+ *
+ * ----------------------------------------------------------------------- */
+
+/*
+ * unread.c
+ *
+ * "Unreading" from a file descriptor
+ */
+
+#include <errno.h>
+#include <string.h>
+#include <com32.h>
+#include <minmax.h>
+#include <klibc/compiler.h>
+#include "file.h"
+
+ssize_t unread(int fd, const void *buf, size_t count)
+{
+    struct file_info *fp = &__file_info[fd];
+
+    if (fd >= NFILES || !fp->iop) {
+	errno = EBADF;
+	return -1;
+    }
+
+    count = min(count, unread_free(fp));
+    fp->i.unread_bytes += count;
+    memcpy(unread_data(fp), buf, count);
+
+    return count;
+}
diff --git a/com32/lib/ungetc.c b/com32/lib/ungetc.c
new file mode 100644
index 00000000..b8ffca98
--- /dev/null
+++ b/com32/lib/ungetc.c
@@ -0,0 +1,18 @@
+/*
+ * ungetc.c
+ */
+
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <errno.h>
+
+int ungetc(int c, FILE *f)
+{
+    unsigned char ch = c;
+
+    if (unread(fileno(f), &ch, 1) == 1)
+	return ch;
+    else
+	return EOF;
+}