CODEWITHSUNDEEP
perlfile-handling
Dasha Sham
Dasha Sham

Reputation: 73

perl print to a file and STDOUT which is the file

My program is trying to print to a file which for it is the STDOUT. To say, print "text here"; prints to a file x.log , while I am also trying to print to the file x.log using file-handler method as in print FH1 "text here"; . I notice that when the file-handler method statement is provided first and then followed by the STDOUT procedure. My second print can override the first.I would like to know more on why this occurs.

This makes me think of a race condition or that the file-handler is relatively slow (if it goes through buffer?) than the STDOUT print statements. This I am not sure on how if that is the way Perl works. Perl version - 5.22.0

Upvotes: 0

Views: 1026

Answers (1)

Steffen Ullrich
Steffen Ullrich

Reputation: 123551

As far as I understand your program basically looks like this:

open(my $fh,'>','foobar.txt');
print $fh "foo\n";
print "bar\n"; # prints to STDOUT

And then you use it in a way that STDOUT is redirected in the shell to the same file which is already opened in your program:

$ perl test.pl > foobar.txt

This will open two independent file handles to the same file: one within your program and the other within the shell where you start the program. Both file handles manage their own file position for writing, start at position 0 and advance the position after each write.

Since these file handles are independent from each other they will not care if there are other file handles dealing currently with this file, no matter if these other file handles are inside or outside the program. This means that these writes will overwrite each other.

In addition to this there is also internal buffering done, i.e. each print will first result into a write into some internal buffer and might result immediately into a write to the file handle. When data are written to the file handle depends on the mode of the file handle, i.e. unbuffered, line-buffered or a buffer of a specific size. This makes the result kind of unpredictable.

If you don't want this behavior but still want to write to the same file using multiple file handle you better use the append-mode, i.e. open with >> instead of > in both Perl code and shell. This will make sure that all data will be appended to the end of the file instead of written to the file position maintained by the file handle. This way data will not get overwritten. Additionally you might want to make the file handles unbuffered so that data in the file end up in the same order as the print statements where done:

open(my $fh,'>>','foobar.txt');
$fh->autoflush(1); # make $fh unbuffered
$|=1;   # make STDOUT unbuffered
print $fh "foo\n";
print "bar\n"; # prints to STDOUT

$ perl test.pl >> foobar.txt

Upvotes: 5

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