Question

I am working on serial port application on Linux , when select () api is checked for data on waiting handles it returns positive value once data arrives on port but read() api call returns -1 , how can this possible?

Answer 1

Here is what man pages say about select:

select() and pselect() allow a program to monitor multiple file
descriptors, waiting until one or more of the file descriptors become
“ready” for some class of I/O operation (e.g., input possible). A file
descriptor is considered ready if it is possible to perform the
corresponding I/O operation (e.g., read(2)) without blocking.

If there is an error condition on a file descriptor (RST segment received for a network sockets, for example), the only way to notify the process that is currently blocked in select is to make this descriptor ready for IO. The subsequent read/write function will return an error so that the application can handle it.

This behavior looks compliant with what select promises – flag a descriptor as ready if you can perform a non-blocking IO on it (however, there are some cases with network sockets, when a descriptor is flagged as ready for read, but the subsequent read operation blocks). You don’t block on read – an error is returned immediately.

As for the return status of select, it should only return the number of “ready events” for the file descriptors in all sets. It can be negative if select itself fails (interrupted by a system call, for example – errno == EINTR), but not when some errors on the descriptors being watched occur.