FAQ 5.14 How come when I open a file read-write it wipes it out?

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5.14: How come when I open a file read-write it wipes it out?

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

            open(FH, "+> /path/name");              # WRONG (almost always)

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

            open(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

            use Fcntl;

    To open file for reading:

            open(FH, "< $path")                                 || die $!;
            sysopen(FH, $path, O_RDONLY)                        || die $!;

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

            open(FH, "> $path") || die $!;
            sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT)        || die $!;
            sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666)  || die $!;

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

            sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT)         || die $!;
            sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666)   || die $!;

    To open file for appending, create if necessary:

            open(FH, ">> $path") || die $!;
            sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT)       || die $!;
            sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;

    To open file for appending, file must exist:

            sysopen(FH, $path, O_WRONLY|O_APPEND)               || die $!;

    To open file for update, file must exist:

            open(FH, "+< $path")                                || die $!;
            sysopen(FH, $path, O_RDWR)                          || die $!;

    To open file for update, create file if necessary:

            sysopen(FH, $path, O_RDWR|O_CREAT)                  || die $!;
            sysopen(FH, $path, O_RDWR|O_CREAT, 0666)            || die $!;

    To open file for update, file must not exist:

            sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT)           || die $!;
            sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666)     || die $!;

    To open a file without blocking, creating if necessary:

            sysopen(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 if you have it (new for 5.6).


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