What is the difference between `read` and `sysread`?

Question

read and sysread have very similar documentation. What are the differences between the two?

Answer 1

About read:

• read supports PerlIO layers.
• read works with any Perl file handle^[1].
• read buffers.
• read obtains data from the system in fixed sized blocks of 8 KiB^[2].
• read may block if less data than requested is available^[3].

About sysread:

• sysread doesn't support PerlIO layers (meaning it requires a raw a.k.a. binary handle).
• sysread only works with Perl file handles that map to a system file handle/descriptor^[4].
• sysread doesn't buffer.
• sysread performs a single system call.
• sysread returns immediately if data is available to be returned, even if the amount of data is less than the amount requested.

Summary and conclusions:

• read works with any Perl file handle, while sysread is limited to Perl file handles mapped to a system file handle/descriptor.
• read isn't compatible with select^[5], while sysread is compatible with select.
• read can perform decoding for you, while sysread requires that you do your own decoding.
• read should be faster for very small reads, while sysread should be faster for very large reads.

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Notes:

1. These include, for example, tied file handles and those created using open(my $fh, '<', \$var).

2. Before 5.14, Perl read in 4 KiB blocks. Since 5.14, the size of the blocks is configurable when you build perl, with a default of 8 KiB.

3. In my experience, read will return exactly the amount requested (if possible) when reading from a plain file, but may return less when reading from a pipe. These results are by no means guaranteed.

4. fileno returns a non-negative number for these. These include, for example, handles that read from plain files, from pipes and from sockets, but not those mentioned in [1].

5. I'm referring to the 4-argument one called by IO::Select.