diff --git a/include/posix/unistd.h b/include/posix/unistd.h
index ca93eb4af1a..3195de3048e 100644
--- a/include/posix/unistd.h
+++ b/include/posix/unistd.h
@@ -13,7 +13,7 @@ extern "C" {
 #include "sys/types.h"
 #include "sys/stat.h"
 
-#ifdef CONFIG_POSIX_FS
+#ifdef CONFIG_POSIX_API
 #include <fs.h>
 
 typedef unsigned int mode_t;
@@ -21,9 +21,9 @@ typedef unsigned int mode_t;
 /* File related operations */
 extern int open(const char *name, int flags);
 extern int close(int file);
-extern ssize_t write(int file, char *buffer, unsigned int count);
-extern ssize_t read(int file, char *buffer, unsigned int count);
-extern int lseek(int file, int offset, int whence);
+extern ssize_t write(int file, const void *buffer, size_t count);
+extern ssize_t read(int file, void *buffer, size_t count);
+extern off_t lseek(int file, off_t offset, int whence);
 
 /* File System related operations */
 extern int rename(const char *old, const char *newp);
diff --git a/lib/posix/fs.c b/lib/posix/fs.c
index 803a34d523f..3d5743e728d 100644
--- a/lib/posix/fs.c
+++ b/lib/posix/fs.c
@@ -151,7 +151,7 @@ int close(int fd)
  *
  * See IEEE 1003.1
  */
-ssize_t write(int fd, char *buffer, unsigned int count)
+ssize_t write(int fd, const void *buffer, size_t count)
 {
 	ssize_t rc;
 	struct fs_file_t *ptr = NULL;
@@ -175,7 +175,7 @@ ssize_t write(int fd, char *buffer, unsigned int count)
  *
  * See IEEE 1003.1
  */
-ssize_t read(int fd, char *buffer, unsigned int count)
+ssize_t read(int fd, void *buffer, size_t count)
 {
 	ssize_t rc;
 	struct fs_file_t *ptr = NULL;
@@ -199,7 +199,7 @@ ssize_t read(int fd, char *buffer, unsigned int count)
  *
  * See IEEE 1003.1
  */
-int lseek(int fd, int offset, int whence)
+off_t lseek(int fd, off_t offset, int whence)
 {
 	int rc;
 	struct fs_file_t *ptr = NULL;