Random File Access

Random File Access

If you're adventurous, you can do random reads and writes within a file, as previously mentioned. The following sections briefly cover some tools you'll need to do random reads and writes; they are not covered more extensively because you shouldn't need them often.

Opening Files for Read and Write

Until now, you've looked at three methods for opening files. Files could be opened for reading, they could be opened for writing, or they could be opened for appending. Files can also be opened for both reading and writing at the same time. Table 15.1 lists the various modes available for opening files.

Table 15.1. Summary of Open Modes

open Command	Reading?	Writing?	Append?	Creates If Does Not Exist?	Truncates Any Existing Data?
open(F, "<file") or open(F, "file")	Yes	No	No	No	No
open(F, ">file")	No	Yes	No	Yes	Yes
open(F, ">>file")	No	Yes	Yes	Yes	No
open(F, "+<file")	Yes	Yes	No	No	No
open(F, "+>file")	Yes	Yes	No	Yes	Yes
open(F, "+>>file")	Yes	Yes	Yes	Yes	No

Notice the following:

• Modes that specify "Append" can be tricky. On some systems such as Unix, data written to the file is always written at the end of the file, regardless of where the read pointer is. (You'll read more about that in a moment.)

• You should almost never use +>. The contents of the file are erased as soon as it's opened.

Moving Around in a Read/Write File

When a file is opened, the operating system keeps track of where in the file you happen to be. This pointer is called the read pointer. For example, when a file is first opened for reading, the read pointer is at the beginning of the file, as shown here:

After you've read through the whole file, the read pointer is at the end of the file, as shown here:

To reposition the pointer to any spot within the file, you must use the seek function. The seek function takes two arguments: The first is an open filehandle, and the second is the offset in the file that you want to seek. The last argument is what the offset is relative to: 0, the beginning of the file; 1, the current position in the file; or 2, the end of the file. The following are some examples of seeking within a file:

# open existing file for reading and writing

open(F, "+<file.txt") || die "file.txt error: $!"; 

seek(F, 0, 2);                  # Seek to the end of the file

print F "On the end";           # Appended to the end of the file

seek(F, 0, 0);                  # Seek back to the beginning of the file

print F "This is at the beginning";

The tell function returns the position of the current read pointer in a file. For example, after running the preceding snippet, tell(F) would return 24—the length of "This is at the beginning"—because the read pointer is sitting immediately after that text.