LSEEK(2)

🌸 NAME 🌸

lseek — reposition read/write file offset ✨

okay so welcome to the System Calls Manual, section 2, bestie 💖 and today's main character is lseek. short version: it moves the little "where am I reading/writing from" bookmark thingy in a file. that's it. that's the whole vibe. 👑

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📚 LIBRARY 📚

Standard C Library (libc, -lc) 💜

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✨ SYNOPSIS ✨

#include <unistd.h>

off_t
lseek(int fildes, off_t offset, int whence);

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🦋 DESCRIPTION 🦋

The lseek() system call repositions the offset of the file descriptor fildes to the argument offset, according to the directive whence. and like, fildes must be an open file descriptor — no closed doors around here, we don't do that 🙅‍♀️

so lseek() grabs the file position pointer that's attached to the file descriptor fildes and moves it like this:

• if whence is SEEK_SET, the offset is set to offset bytes. absolute. start-of-file energy 🌱
• if whence is SEEK_CUR, the offset is set to its current location plus offset bytes. relative to where you are rn 📍
• if whence is SEEK_END, the offset is set to the size of the file plus offset bytes. from the end, periodt 🔚
• if whence is SEEK_HOLE, the offset is set to the start of the next hole greater than or equal to the supplied offset. the definition of a hole is provided below (yes we're getting to it, be patient 🕳️)
• if whence is SEEK_DATA, the offset is set to the start of the next non-hole file region greater than or equal to the supplied offset. 📊

The lseek() system call allows the file offset to be set beyond the end of the existing end-of-file of the file. and if data is later written at this point, subsequent reads of the data in the gap return bytes of zeros (until data is actually written into the gap). BUT — and this is the part you cannot skip, bestie — the lseek() system call does not, by itself, extend the size of a file. it points. it does not commit. 🙈

🕳️ so what even IS a hole 🕳️

A "hole" is defined as a contiguous range of bytes in a file, all having the value of zero, but not all zeros in a file are guaranteed to be represented as holes returned with SEEK_HOLE. File systems are allowed to expose ranges of zeros with SEEK_HOLE, but not required to. Applications can use SEEK_HOLE to optimise their behavior for ranges of zeros, but must not depend on it to find all such ranges in a file. 🚨

Each file is presented as having a zero-size virtual hole at the very end of the file. The existence of a hole at the end of every data region allows for easy programming and also provides compatibility to the original implementation in Solaris. It also causes the current file size (i.e., end-of-file offset) to be returned to indicate that there are no more holes past the supplied offset. 🌈

Applications should use fpathconf(_PC_MIN_HOLE_SIZE) or pathconf(_PC_MIN_HOLE_SIZE) to determine if a file system supports SEEK_HOLE. See pathconf(2). 👀

For file systems that do not supply information about holes, the file will be represented as one entire data region. no holes, just vibes 💀

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💖 RETURN VALUES 💖

Upon successful completion, lseek() returns the resulting offset location as measured in bytes from the beginning of the file. ate and left no crumbs 🎉

Otherwise, a value of -1 is returned and errno is set to indicate the error. 😭

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🔥 ERRORS 🔥

The lseek() system call will fail and the file position pointer will remain unchanged if:

• [EBADF] — The fildes argument is not an open file descriptor. 🚫
• [EINVAL] — The whence argument is not a proper value or the resulting file offset would be negative for a non-character special file. 😬
• [ENXIO] — For SEEK_DATA, there are no more data regions past the supplied offset. Due to existence of the hole at the end of the file, for SEEK_HOLE this error is only returned when the offset already points to the end-of-file position. 🕳️❌
• [EOVERFLOW] — The resulting file offset would be a value which cannot be represented correctly in an object of type off_t. 📉
• [ESPIPE] — The fildes argument is associated with a pipe, socket, or FIFO. 🚰 pipes do not seek, bestie, they just flow

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👀 SEE ALSO 👀

dup(2), open(2), pathconf(2) 🫶

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📜 STANDARDS 📜

The lseek() system call is expected to conform to IEEE Std 1003.1-2008 (“POSIX.1”). ✅

The SEEK_HOLE and SEEK_DATA directives, along with the ENXIO error, are extensions to that specification. 🧚

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🎀 HISTORY 🎀

The lseek() function appeared in Version 7 AT&T UNIX. elder of the syscall family honestly, still iconic, still serving 👑

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💀 BUGS 💀

If the lseek() system call is operating on a device which is incapable of seeking, it will request the seek operation and return successfully, even though no seek was performed. Because the offset argument will be stored unconditionally in the file descriptor of that device, there is no way to confirm if the seek operation succeeded or not (e.g. using the ftell() function). Device types which are known to be incapable of seeking include tape drives. 📼😭

The lseek() system call will not detect whether media are present in changeable media devices such as DVD or Blu-ray devices. A requested seek operation will therefore return successfully when no medium is present. 💿🙈

This document's use of whence is incorrect English, but is maintained for historical reasons. grammatically it's giving chaos ✨ but we respect the heritage 💅

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and that's the whole page, bestie!! this lil masterpiece is brought to you by Debian, dated July 13, 2020, and filed under LSEEK(2) in the System Calls Manual 🫶 mwah, go seek responsibly 💖✨🌸