Add comments to the code warning that _XOPEN_SOURCE might need to be

defined manually if using USE_PREAD or USE_PREAD64. (CVS 4509)

FossilOrigin-Name: d7ed7cd077fe5f9ffba2bca48b84b231ccfd85b0

1 files changed, 6 insertions, 0 deletions

diff --git a/src/os_unix.c b/src/os_unix.c
index e95435e74..f4dc24f04 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -772,6 +772,12 @@ static int transferOwnership(unixFile *pFile){
 /*
 ** Seek to the offset passed as the second argument, then read cnt 
 ** bytes into pBuf. Return the number of bytes actually read.
+**
+** NB:  If you define USE_PREAD or USE_PREAD64, then it might also
+** be necessary to define _XOPEN_SOURCE to be 500.  This varies from
+** one system to another.  Since SQLite does not define USE_PREAD
+** any any form by default, we will not attempt to define _XOPEN_SOURCE.
+** See tickets #2741 and #2681.
 */
 static int seekAndRead(unixFile *id, sqlite3_int64 offset, void *pBuf, int cnt){
   int got;