feat: libc - add pread/pwrite

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_018Ae69wJS7sueQMUwNxV3nX

programs/lib/libc/libc.c

@@ -2047,6 +2047,24 @@ int write(int fd, const void *buf, size_t count) {
     return ret;
 }
 
+/*
+ * Positioned I/O. SYS_READ/SYS_FWRITE already take an explicit file position,
+ * so these pass the caller's offset straight through and never consult or
+ * update _fd_pos -- unlike an lseek/read/lseek-back emulation, the shared file
+ * offset is genuinely left alone.
+ */
+ssize_t pread(int fd, void *buf, size_t count, off_t offset) {
+    if (buf == NULL || offset < 0) return -1;
+    return (ssize_t)_zos_syscall4(SYS_READ, (long)fd, (long)buf,
+                                  (long)offset, (long)count);
+}
+
+ssize_t pwrite(int fd, const void *buf, size_t count, off_t offset) {
+    if (buf == NULL || offset < 0) return -1;
+    return (ssize_t)_zos_syscall4(SYS_FWRITE, (long)fd, (long)buf,
+                                  (long)offset, (long)count);
+}
+
 int close(int fd) {
     if (fd >= 0 && fd < _FD_POS_MAX)
         _fd_pos[fd] = 0;