What does p stand for in function names pwrite and pread?

Question

I'm trying to learn linux api but some function names seem (at least to me) cumbersome.

Could anybody explain to me what does the initial letter p stands for in pwrite/pread?

Answer 1

I am only guessing, but since pread()and pwrite() read or write at a specified position in the file, I would assume that the "p" stands for "positional", as in "positional read".

Both read() and pread() are part of the POSIX standard (see e.g. http://www.unix.com/man-page/POSIX/3posix/pread/), therefore I don't think that "p" stands for "POSIX".

But perhaps this is completely wrong!

Answer 2

p in pread and pwrite refers to POSIX.

The two differences of "p" variant as opposed to read and write are:

• The "p" variants take offset to read from, so they are independent of the current file pointer. That makes it easier to read/write from multiple threads concurrently.
• The "p" variants only work on seekable files (i.e. real files, not pipes, sockets or devices).