Programming Perl

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3.2.132 seek


This function positions the file pointer for FILEHANDLE, just like the fseek(3) call of standard I/O. The first position in a file is at offset 0, not offset 1, and offsets refer to byte positions, not line numbers. (In general, since line lengths vary, it's not possible to access a particular line number without examining the whole file up to that line number, unless all your lines are known to be of a particular length, or you've built an index that translates line numbers into byte offsets.) FILEHANDLE may be an expression whose value gives the name of the filehandle or a reference to a filehandle object. The function returns 1 upon success, 0 otherwise. For handiness, the function can calculate offsets from various file positions for you. The value of WHENCE specifies which file position your OFFSET is relative to: 0, the beginning of the file; 1, the current position in the file; or 2, the end of the file. OFFSET may be negative for a WHENCE of 1 or 2.

One interesting use for this function is to allow you to follow growing files, like this:

for (;;) {
    while (<LOG>) {
        ...           # Process file.
    sleep 15;
    seek LOG,0,1;      # Reset end-of-file error.

The final seek clears the end-of-file error without moving the pointer. If that doesn't work (depending on your C library's standard I/O implementation), then you may need something more like this:

for (;;) {
    for ($curpos = tell FILE; $_ = <FILE>; $curpos = tell FILE) {
        # search for some stuff and put it into files
    sleep $for_a_while;
    seek FILE, $curpos, 0;

Similar strategies could be used to remember the seek addresses of each line in an array.

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