Which libc/stdio functions does Perl use to open and read a file (e.g. for perl -e 'open(F,"<","test.txt");print(<F>);')?
I tried running this command under strace and it gave me a sequence open -> fcntl -> ioctl -> lseek -> fstat -> mmap -> read. strace intercepts syscalls, so I’m wondering what are the higher-level libc/stdio functions are actually being called in https://github.com/Perl/perl5/blob/blead/perlio.c and https://github.com/Perl/perl5/blob/blead/doio.c? The Perl I/O codebase has many layers of indirection, so I’m struggling to understand it.
I’m not a Perl pro, so not understanding https://perldoc.perl.org/PerlIO and the codebase sufficiently enough 🙁
Thanks!
3
Depends on the layer.
There are two options for the base layer of ordinary handles: :unix and :stdio.
-
The
:unixlayer is is defined inperlio.casPerlIO_unix.-
For opening,
:unixusesPerlIOUnix_open, which callsPerlLIO_open3_cloexec, which callsPerlLIO_open3, which is defined asopen(except maybe on MVS). -
For writing,
:unixusesPerlIOUnix_write, which callsPerlLIO_write, which is defined aswrite.
-
-
The
:stdiolayer is is defined inperlio.casPerlIO_stdio.-
For opening,
:stdiousesPerlIOStdio_open. Follow from there. -
For writing,
:stdiousesPerlIOStdio_write, which callsPerlSIO_fwrite, which is defined asfwrite.
-
The selection of the base layer appears to be:
- If a layer is provided to
open, use that one. - If a process-wide default is provided (e.g. using the
PERLIOenv var), use that one. - Otherwise, use the default selected when Perl was built. This can apparently be
:stdioin some circumstances, but it’s usually:perlio, and:perlioadds an underlying:unixlayer.
Note that :unix is used on Windows as well. It refers to the libc functions rather than anything specific to Unix.
[All links for v5.40.0.]
4