Reading and Writing, but not Sequentially

Thus far, we’ve discussed how to read and write files, but all access has been sequential; that is, we have either read a file from the beginning to the end, or written a file out from beginning to end.

Sometimes, however, it is useful to be able to read or write to a specific offset within a file; for example, if you build an index over a text document, and use it to look up a specific word, you may end up reading from some random offsets within the document. To do so, we will use the lseek() system call. Here is the function prototype:

The first argument is familiar (a file descriptor). The second argument is the offset, which positions the file offset to a particular location within the file. The third argument, called whence for historical reasons, determines exactly how the seek is performed. From the man ...