posix: device_io: implement pread() and pwrite()

Add pread() and pwrite() implementations, which are nearly
identical to read() and write() but differ in that they do not
update the file-descriptor offset and instead read from a
specific file offset.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

lib/os/fdtable.c

@@ -308,34 +308,62 @@ int zvfs_alloc_fd(void *obj, const struct fd_op_vtable *vtable)
 	return fd;
 }
 
-static ssize_t zvfs_rw(int fd, void *buf, size_t sz, bool is_write)
+static bool supports_pread_pwrite(uint32_t mode)
 {
+	switch (mode & ZVFS_MODE_IFMT) {
+	case ZVFS_MODE_IFSHM:
+		return true;
+	default:
+		return false;
+	}
+}
+
+static ssize_t zvfs_rw(int fd, void *buf, size_t sz, bool is_write, const size_t *from_offset)
+{
+	bool prw;
 	ssize_t res;
+	const size_t *off;
 
 	if (_check_fd(fd) < 0) {
 		return -1;
 	}
 
 	(void)k_mutex_lock(&fdtable[fd].lock, K_FOREVER);
 
+	prw = supports_pread_pwrite(fdtable[fd].mode);
+	if (from_offset != NULL && !prw) {
+		/*
+		 * Seekable file types should support pread() / pwrite() and per-fd offset passing.
+		 * Otherwise, it's a bug.
+		 */
+		errno = ENOTSUP;
+		res = -1;
+		goto unlock;
+	}
+
+	/* If there is no specified from_offset, then use the current offset of the fd */
+	off = (from_offset == NULL) ? &fdtable[fd].offset : from_offset;
+
 	if (is_write) {
-		if (fdtable[fd].vtable->write_offset == NULL) {
+		if (fdtable[fd].vtable->write_offs == NULL) {
 			res = -1;
 			errno = EIO;
 		} else {
-			res = fdtable[fd].vtable->write_offset(fdtable[fd].obj, buf, sz,
-							       fdtable[fd].offset);
+			res = fdtable[fd].vtable->write_offs(fdtable[fd].obj, buf, sz, *off);
 		}
 	} else {
-		if (fdtable[fd].vtable->read == NULL) {
+		if (fdtable[fd].vtable->read_offs == NULL) {
 			res = -1;
 			errno = EIO;
 		} else {
-			res = fdtable[fd].vtable->read_offset(fdtable[fd].obj, buf, sz,
-							      fdtable[fd].offset);
+			res = fdtable[fd].vtable->read_offs(fdtable[fd].obj, buf, sz, *off);
 		}
 	}
-	if (res > 0) {
+	if (res > 0 && prw && from_offset == NULL) {
+		/*
+		 * only update the fd offset when from_offset is not specified
+		 * See pread() / pwrite()
+		 */
 		fdtable[fd].offset += res;
 	}
 
@@ -345,14 +373,14 @@ static ssize_t zvfs_rw(int fd, void *buf, size_t sz, bool is_write)
 	return res;
 }
 
-ssize_t zvfs_read(int fd, void *buf, size_t sz)
+ssize_t zvfs_read(int fd, void *buf, size_t sz, const size_t *from_offset)
 {
-	return zvfs_rw(fd, buf, sz, false);
+	return zvfs_rw(fd, buf, sz, false, from_offset);
 }
 
-ssize_t zvfs_write(int fd, const void *buf, size_t sz)
+ssize_t zvfs_write(int fd, const void *buf, size_t sz, const size_t *from_offset)
 {
-	return zvfs_rw(fd, (void *)buf, sz, true);
+	return zvfs_rw(fd, (void *)buf, sz, true, from_offset);
 }
 
 int zvfs_close(int fd)
@@ -494,7 +522,7 @@ static ssize_t stdinout_read_vmeth(void *obj, void *buffer, size_t count)
 static ssize_t stdinout_write_vmeth(void *obj, const void *buffer, size_t count)
 {
 #if defined(CONFIG_BOARD_NATIVE_POSIX)
-	return zvfs_write(1, buffer, count);
+	return zvfs_write(1, buffer, count, NULL);
 #elif defined(CONFIG_NEWLIB_LIBC) || defined(CONFIG_ARCMWDT_LIBC)
 	return z_impl_zephyr_write_stdout(buffer, count);
 #else

lib/posix/options/device_io.c

@@ -15,8 +15,8 @@
 /* prototypes for external, not-yet-public, functions in fdtable.c or fs.c */
 int zvfs_close(int fd);
 int zvfs_open(const char *name, int flags);
-ssize_t zvfs_read(int fd, void *buf, size_t sz);
-ssize_t zvfs_write(int fd, const void *buf, size_t sz);
+ssize_t zvfs_read(int fd, void *buf, size_t sz, size_t *from_offset);
+ssize_t zvfs_write(int fd, const void *buf, size_t sz, size_t *from_offset);
 
 int close(int fd)
 {
@@ -41,9 +41,33 @@ int poll(struct pollfd *fds, int nfds, int timeout)
 	return zsock_poll(fds, nfds, timeout);
 }
 
+ssize_t pread(int fd, void *buf, size_t count, off_t offset)
+{
+	size_t off = (size_t)offset;
+
+	if (offset < 0) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	return zvfs_read(fd, buf, count, (size_t *)&off);
+}
+
+ssize_t pwrite(int fd, void *buf, size_t count, off_t offset)
+{
+	size_t off = (size_t)offset;
+
+	if (offset < 0) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	return zvfs_write(fd, buf, count, (size_t *)&off);
+}
+
 ssize_t read(int fd, void *buf, size_t sz)
 {
-	return zvfs_read(fd, buf, sz);
+	return zvfs_read(fd, buf, sz, NULL);
 }
 #ifdef CONFIG_POSIX_DEVICE_IO_ALIAS_READ
 FUNC_ALIAS(read, _read, ssize_t);
@@ -57,7 +81,7 @@ int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struc
 
 ssize_t write(int fd, const void *buf, size_t sz)
 {
-	return zvfs_write(fd, buf, sz);
+	return zvfs_write(fd, buf, sz, NULL);
 }
 #ifdef CONFIG_POSIX_DEVICE_IO_ALIAS_WRITE
 FUNC_ALIAS(write, _write, ssize_t);