Recipe 18.15 Reading from or Writing to a Specific Location in a File
18.15.1 Problem
You want to read from (or
write to) a specific place in a
file. For example, you want to replace the third record in a file of
80-byte records, so you have to write starting at the 161st byte.
18.15.2 Solution
Use fseek( ) to move to a specific number of bytes after
the beginning of the file, before the end of the file, or from the
current position in the file:
fseek($fh,26); // 26 bytes after the beginning of the file
fseek($fh,26,SEEK_SET); // 26 bytes after the beginning of the file
fseek($fh,-39,SEEK_END); // 39 bytes before the end of the file
fseek($fh,10,SEEK_CUR); // 10 bytes ahead of the current position
fseek($fh,0); // beginning of the file
The rewind( ) function moves to the beginning of a file:
rewind($fh); // the same as fseek($fh,0)
18.15.3 Discussion
The function fseek( ) returns 0 if it can move to
the specified position, otherwise it returns -1. Seeking beyond the
end of the file isn't an error for fseek(
). Contrastingly, rewind( ) returns 0 if
it encounters an error.
You can use fseek( ) only with local files, not
HTTP or FTP files opened with fopen( ). If you
pass a file handle of a remote file to fseek( ),
it throws an E_NOTICE error.
To get the current file position, use ftell(
)
:
if (0 === ftell($fh)) {
print "At the beginning of the file.";
}
Because ftell( ) returns false
on error, you need to use the === operator to make
sure that its return value is really the integer 0.
18.15.4 See Also
Documentation on fseek( ) at
http://www.php.net/fseek, ftell(
) at http://www.php.net/ftell, and
rewind( ) at
http://www.php.net/rewind.
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