FAQ 5.16 How come when I open a file read-write it wipes it out?


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This is an excerpt from the latest version perlfaq5.pod, which
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5.16: How come when I open a file read-write it wipes it out?

Because you're using something like this, which truncates the file
*then* gives you read-write access:

open my $fh, '+>', '/path/name'; # WRONG (almost always)

Whoops. You should instead use this, which will fail if the file doesn't
exist:

open my $fh, '+<', '/path/name'; # open for update

Using ">" always clobbers or creates. Using "<" never does either. The
"+" doesn't change this.

Here are examples of many kinds of file opens. Those using "sysopen" all
assume that you've pulled in the constants from "Fcntl":

use Fcntl;

To open file for reading:

open my $fh, '<', $path or die $!;
sysopen my $fh, $path, O_RDONLY or die $!;

To open file for writing, create new file if needed or else truncate old
file:

open my $fh, '>', $path or die $!;
sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT or die $!;
sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666 or die $!;

To open file for writing, create new file, file must not exist:

sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT or die $!;
sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT, 0666 or die $!;

To open file for appending, create if necessary:

open my $fh, '>>' $path or die $!;
sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT or die $!;
sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT, 0666 or die $!;

To open file for appending, file must exist:

sysopen my $fh, $path, O_WRONLY|O_APPEND or die $!;

To open file for update, file must exist:

open my $fh, '+<', $path or die $!;
sysopen my $fh, $path, O_RDWR or die $!;

To open file for update, create file if necessary:

sysopen my $fh, $path, O_RDWR|O_CREAT or die $!;
sysopen my $fh, $path, O_RDWR|O_CREAT, 0666 or die $!;

To open file for update, file must not exist:

sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT or die $!;
sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT, 0666 or die $!;

To open a file without blocking, creating if necessary:

sysopen my $fh, '/foo/somefile', O_WRONLY|O_NDELAY|O_CREAT
or die "can't open /foo/somefile: $!":

Be warned that neither creation nor deletion of files is guaranteed to
be an atomic operation over NFS. That is, two processes might both
successfully create or unlink the same file! Therefore O_EXCL isn't as
exclusive as you might wish.

See also the new perlopentut.



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