Re: Read a fixed length of input each time

[Andrew J. Schorr]

On Tue, Jun 23, 2020 at 01:27:18AM -0600, arnold@skeeve.com wrote:
> It is fairly easy to write a C extension that would do this.

It should be quite trivial to do this. The get_file API makes it easy to open a
file or pipe or coprocess. For example, you can take a look at how the
gawkextlib select extension uses the get_file call in various functions such as
set_non_blocking() or input_fd(). Here's a simple example:

bash-4.2$ gawk -lselect 'BEGIN {print input_fd("/etc/passwd", "<")}'
3
bash-4.2$ strace -eopen gawk -lselect 'BEGIN {print input_fd("/etc/passwd", 
"<")}'
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libsigsegv.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libreadline.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libmpfr.so.4", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libgmp.so.10", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libm.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libtinfo.so.5", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
open("/proc/self/maps", O_RDONLY)       = 3
open("/usr/lib64/gconv/gconv-modules.cache", O_RDONLY) = 3
open("/usr/lib64/gawk/select.so", O_RDONLY|O_CLOEXEC) = 3
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libgawkextlib.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/etc/passwd", O_RDONLY)           = 3
3
+++ exited with 0 +++

The only thing missing is a thin wrapper around the read() system call. Maybe I
should just add read() and write() to the select extension. It just didn't seem
terribly useful for awk scripts which typically use getline for input and
print/printf for output.  What's the usage case? I thought maybe for socket I/O
it could be useful to have read and write, but we don't have a socket library
at this point...

Regards,
Andy