libc: implement 'pread()' and 'pwrite()'

This patch implements the 'pread()' and 'pwrite()' functions in the libc.

A lock guard prevents the parallel execution of either or both functions.

Fixes #278.

libports/lib/mk/libc.mk

@@ -14,7 +14,7 @@ SRC_CC = atexit.cc dummies.cc rlimit.cc sysctl.cc readlink.cc munmap.cc \
          issetugid.cc errno.cc gai_strerror.cc clock_gettime.cc \
          gettimeofday.cc malloc.cc progname.cc fd_alloc.cc file_operations.cc \
          plugin.cc plugin_registry.cc select.cc exit.cc environ.cc nanosleep.cc \
-         libc_mem_alloc.cc writev.cc
+         libc_mem_alloc.cc writev.cc pread_pwrite.cc
 
 #
 # Files from string library that are not included in libc-raw_string because

libports/src/lib/libc/pread_pwrite.cc

@@ -0,0 +1,75 @@
+/*
+ * \brief  'pread()' and 'pwrite()' implementations
+ * \author Christian Prochaska
+ * \date   2012-07-11
+ */
+
+/*
+ * Copyright (C) 2012 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU General Public License version 2.
+ */
+
+/* Genode includes */
+#include <base/lock.h>
+
+/* libc includes */
+#include <sys/types.h>
+#include <sys/uio.h>
+#include <unistd.h>
+
+
+static Genode::Lock rw_lock;
+
+
+struct Read
+{
+     ssize_t operator()(int fd, void *buf, size_t count)
+     {
+    	 return read(fd, buf, count);
+     }
+};
+
+
+struct Write
+{
+     ssize_t operator()(int fd, const void *buf, size_t count)
+     {
+    	 return write(fd, buf, count);
+     }
+};
+
+
+template <typename Rw_func, typename Buf_type>
+static ssize_t pread_pwrite_impl(Rw_func rw_func, int fd, Buf_type buf, ::size_t count, ::off_t offset)
+{
+	Genode::Lock_guard<Genode::Lock> rw_lock_guard(rw_lock);
+
+	off_t old_offset = lseek(fd, 0, SEEK_CUR);
+
+	if (old_offset == -1)
+		return -1;
+
+	if (lseek(fd, offset, SEEK_SET) == -1)
+		return -1;
+
+	ssize_t result = rw_func(fd, buf, count);
+
+	if (lseek(fd, old_offset, SEEK_SET) == -1)
+		return -1;
+
+	return result;
+}
+
+
+extern "C" ssize_t pread(int fd, void *buf, ::size_t count, ::off_t offset)
+{
+	return pread_pwrite_impl(Read(), fd, buf, count, offset);
+}
+
+
+extern "C" ssize_t pwrite(int fd, const void *buf, ::size_t count, ::off_t offset)
+{
+	return pread_pwrite_impl(Write(), fd, buf, count, offset);
+}

libports/src/test/libc_ffat/main.cc

@@ -39,11 +39,15 @@
 int main(int argc, char *argv[])
 {
 	int ret, fd;
+	ssize_t count;
 
 	char const *dir_name      = "/testdir";
 	char const *file_name     = "test.tst";
+	char const *file_name2    = "test2.tst";
 	char const *pattern       = "a single line of text";
 
+	size_t      pattern_size  = strlen(pattern) + 1;
+
 	unsigned int iterations = 1;
 
 	try {
@@ -60,8 +64,7 @@ int main(int argc, char *argv[])
 
 		/* write pattern to a file */
 		CALL_AND_CHECK(fd, open(file_name, O_CREAT | O_WRONLY), fd >= 0, "file_name=%s", file_name);
-		size_t count = strlen(pattern);
-		CALL_AND_CHECK(ret, write(fd, pattern, count), ret > 0, "count=%zd", count);
+		CALL_AND_CHECK(count, write(fd, pattern, pattern_size), (size_t)count == pattern_size, "");
 		CALL_AND_CHECK(ret, close(fd), ret == 0, "");
 
 		/* query file status of new file */
@@ -76,7 +79,7 @@ int main(int argc, char *argv[])
 		/* read and verify file content */
 		CALL_AND_CHECK(fd, open(file_name, O_RDONLY), fd >= 0, "file_name=%s", file_name);
 		static char buf[512];
-		CALL_AND_CHECK(count, read(fd, buf, sizeof(buf)), count > 0, "");
+		CALL_AND_CHECK(count, read(fd, buf, sizeof(buf)), (size_t)count == pattern_size, "");
 		CALL_AND_CHECK(ret, close(fd), ret == 0, "");
 		printf("content of file: \"%s\"\n", buf);
 		if (strcmp(buf, pattern) != 0) {
@@ -86,6 +89,26 @@ int main(int argc, char *argv[])
 			printf("file content is correct\n");
 		}
 
+		/* test 'pread()' and 'pwrite()' */
+		CALL_AND_CHECK(fd, open(file_name2, O_CREAT | O_WRONLY), fd >= 0, "file_name=%s", file_name);
+		/* write "a single line of" */
+		CALL_AND_CHECK(count, pwrite(fd, pattern, (pattern_size - 6), 0), (size_t)count == (pattern_size - 6), "");
+		/* write "line of text" at offset 9 */
+		CALL_AND_CHECK(count, pwrite(fd, &pattern[9], (pattern_size - 9), 9), (size_t)count == (pattern_size - 9), "");
+		CALL_AND_CHECK(ret, close(fd), ret == 0, "");
+		CALL_AND_CHECK(fd, open(file_name2, O_RDONLY), fd >= 0, "file_name=%s", file_name);
+		memset(buf, 0, sizeof(buf));
+		/* read "single line of text" from offset 2 */
+		CALL_AND_CHECK(count, pread(fd, buf, sizeof(buf), 2), (size_t)count == (pattern_size - 2), "");
+		CALL_AND_CHECK(ret, close(fd), ret == 0, "");
+		printf("content of file: \"%s\"\n", buf);
+		if (strcmp(buf, &pattern[2]) != 0) {
+			printf("unexpected content of file\n");
+			return -1;
+		} else {
+			printf("file content is correct\n");
+		}
+
 		/* read directory entries */
 		DIR *dir;
 		CALL_AND_CHECK(dir, opendir(dir_name), dir, "dir_name=\"%s\"", dir_name);