(head_lines): If we have read too much data, try

to seek back to the position we would have gotten to had we
been reading one byte at a time.  POSIX currently doesn't
require this, but it's easy to do and some software relies on it.

diff --git a/src/head.c b/src/head.c
index 6f7496f..dd39cbd 100644 (file)
--- a/src/head.c
+++ b/src/head.c
@@ -182,7 +182,20 @@ head_lines (const char *filename, int fd, uintmax_t lines_to_write)
        break;
       while (bytes_to_write < bytes_read)
        if (buffer[bytes_to_write++] == '\n' && --lines_to_write == 0)
-         break;
+         {
+           /* If we have read more data than that on the specified number
+              of lines, try to seek back to the position we would have
+              gotten to had we been reading one byte at a time.  */
+           if (lseek (fd, bytes_to_write - bytes_read, SEEK_CUR) < 0)
+             {
+               int e = errno;
+               struct stat st;
+               if (fstat (fd, &st) != 0 || S_ISREG (st.st_mode))
+                 error (0, e, _("cannot reposition file pointer for %s"),
+                        filename);
+             }
+           break;
+         }
       if (fwrite (buffer, 1, bytes_to_write, stdout) == 0)
        error (EXIT_FAILURE, errno, _("write error"));
     }