kern: filedesc: Implement lseek() function

The lseek() function allows the call to resposition the file offset
using a specific whence value to govern offsetting semantics

Signed-off-by: Ian Moffett <ian@osmora.org>

3 files changed, 85 insertions, 0 deletions

diff --git a/src/sys/include/os/filedesc.h b/src/sys/include/os/filedesc.h
index a6a2ac5..b773de9 100644
--- a/src/sys/include/os/filedesc.h
+++ b/src/sys/include/os/filedesc.h
@@ -30,6 +30,7 @@
 #ifndef _OS_FILEDESC_H_
 #define _OS_FILEDESC_H_ 1
 
+#include <sys/seek.h>
 #include <sys/types.h>
 #include <os/vnode.h>
 
@@ -117,4 +118,13 @@ ssize_t write(int fd, const void *buf, size_t count);
  */
 ssize_t read(int fd, void *buf, size_t count);
 
+/*
+ * Reposition the file offset of a file
+ *
+ * @fd: File descriptor to reposition
+ * @offset: Offset to move `fd' to
+ * @whence: How it should be repositioned
+ */
+off_t lseek(int fd, off_t offset, int whence);
+
 #endif  /* !_OS_FILEDESC_H_ */
diff --git a/src/sys/include/sys/seek.h b/src/sys/include/sys/seek.h
new file mode 100644
index 0000000..1edc50b
--- /dev/null
+++ b/src/sys/include/sys/seek.h
@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2025 Ian Marco Moffett and L5 engineers
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the project nor the names of its
+ *    contributors may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef _SYS_SEEK_H_
+#define _SYS_SEEK_H_ 1
+
+#define SEEK_SET 0
+#define SEEK_CUR 1
+#define SEEK_END 2
+
+#endif  /* !_SYS_SEEK_H_ */
diff --git a/src/sys/os/os_filedes.c b/src/sys/os/os_filedes.c
index ca56fc3..3cbbc6f 100644
--- a/src/sys/os/os_filedes.c
+++ b/src/sys/os/os_filedes.c
@@ -359,6 +359,44 @@ read(int fd, void *buf, size_t count)
     return retval;
 }
 
+off_t
+lseek(int fd, off_t offset, int whence)
+{
+    struct filedesc *fdp;
+    struct proc *self = proc_self();
+    struct vattr attr;
+    int error;
+
+    if (fd < 0) {
+        return -EBADF;
+    }
+
+    fdp = fd_get(self, fd);
+    if (fdp == NULL) {
+        return -EBADF;
+    }
+
+    /* Could not get vnode attributes */
+    error = vop_getattr(fdp->vp, &attr);
+    if (error < 0) {
+        return 0;
+    }
+
+    switch (whence) {
+    case SEEK_SET:
+        fdp->off = offset;
+        break;
+    case SEEK_CUR:
+        fdp->off += offset;
+        break;
+    case SEEK_END:
+        fdp->off = attr.size;
+        break;
+    }
+
+    return fdp->off;
+}
+
 /*
  * ARG0: Path
  * ARG1: Mode