READV(3P)

💖 READV(3P) — POSIX Programmer's Manual 💖

okay bestie, so this one's from the POSIX Programmer's Manual — the readv(3P) page specifically. Come sit with me, we're gonna read a vector together and it's gonna be iconic. 🎀

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🚨 PROLOG 🚨

Heads up before we even start: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ — so if you're on Linux, go peek at the corresponding Linux manual page for the tea on Linux behavior. Or… the interface might not even be implemented on Linux at all. 🙈 Like, no cap, check first.

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✨ NAME ✨

readv — read a vector 💅

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🌸 SYNOPSIS 🌸

#include <sys/uio.h>

ssize_t readv(int fildes, const struct iovec *iov, int iovcnt);

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💜 DESCRIPTION 💜

So readv() is basically read()'s cooler sibling — it shall be equivalent to read(), except for the stuff we're about to spill below. 🫶

What it does: it takes the input data and places it into the iovcnt buffers specified by the members of the iov array. That's iov[0], iov[1], …, iov[iovcnt-1]. A whole lil lineup. 👑

The iovcnt argument is valid if it's greater than 0 and less than or equal to {IOV_MAX}. That's the vibe check. ✅

Every single iovec entry specifies the base address and length of an area in memory where data should be placed. And here's the important bit, bestie: readv() shall always fill an area completely before proceeding to the next. No half-filled buffers, no skipping ahead. It's giving main character energy with commitment. 💖

And upon successful completion? readv() shall mark for update the last data access timestamp of the file. Timestamps updated, slay. ✨

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🔥 RETURN VALUE 🔥

Refer to read(). (Yeah that's it, go read that one, it's the source of truth fr fr.)

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💀 ERRORS 💀

Refer to read().

In addition, the readv() function shall fail if:

• EINVAL — the sum of the iov_len values in the iov array overflowed an ssize_t. 😭 Yeah, that's an overflow, we do not love that.

The readv() function may fail if:

• EINVAL — the iovcnt argument was less than or equal to 0, or greater than {IOV_MAX}. 💀

The following sections are informative. (Read: the required bits are done, everything from here is bonus lore. 🦋)

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🌈 EXAMPLES 🌈

🍓 Reading Data into an Array

okay so the following example reads data from the file associated with the file descriptor fd into the buffers specified by members of the iov array. Watch and learn:

#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
...
ssize_t bytes_read;
int fd;
char buf0[20];
char buf1[30];
char buf2[40];
int iovcnt;
struct iovec iov[3];

iov[0].iov_base = buf0;
iov[0].iov_len = sizeof(buf0);
iov[1].iov_base = buf1;
iov[1].iov_len = sizeof(buf1);
iov[2].iov_base = buf2;
iov[2].iov_len = sizeof(buf2);
...
iovcnt = sizeof(iov) / sizeof(struct iovec);

bytes_read = readv(fd, iov, iovcnt);
...

Three buffers, one syscall. It's giving efficiency, it's giving organized. 🎀

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🧚 APPLICATION USAGE 🧚

None. (Nothing to add, bestie, the example said it all. 💅)

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👑 RATIONALE 👑

Refer to read().

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⭐ FUTURE DIRECTIONS ⭐

None.

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💜 SEE ALSO 💜

• read()
• writev()

The Base Definitions volume of POSIX.1‐2017, <sys_uio.h>

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📜 COPYRIGHT 📜

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page, bestie!! 🌈 this one was brought to you by IEEE/The Open Group, dated 2017, and the manual name was READV(3P). go read some vectors, ily, bye!! 💖✨🎀🦋