WRITEV(3P)

💅 PROLOG

Okay bestie, real talk before we get into it: this page is part of the POSIX Programmer's Manual. The Linux version of this interface can be a lil different (so go check the corresponding Linux manual page for the deets on Linux behavior), or honestly it might not even be implemented on Linux at all. 🙈 No cap.

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✨ NAME ✨

writev — write a vector. That's it, that's the name, and it ate. 👑

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🎀 SYNOPSIS 🎀

#include <sys/uio.h>

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

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🌸 DESCRIPTION 🌸

So the way that writev() works is: it's the same as write(), except for everything listed below. That's the vibe. 💖

writev() gathers output data from the iovcnt buffers that the members of the iov array point at — so like, iov[0], iov[1], ..., all the way to iov[iovcnt-1]. She's collecting, she's assembling, she's the main character of scatter/gather I/O. 🌈

The iovcnt argument is valid if it's greater than 0 and less than or equal to {IOV_MAX}, as defined in <limits.h>. Respect the bounds, bestie, they're there for a reason.

Each iovec entry specifies the base address and the length of an area in memory that data should be written from. And here's the iconic part: writev() shall always write a complete area before proceeding to the next. One at a time, fully, no half-finished situations. Periodt. 💅

Now, if fildes refers to a regular file and all of the iov_len members in the array pointed to by iov are 0, then writev() shall return 0 and have no other effect. For other file types, the behavior is unspecified — so don't come for her, that's just the rules. 😭

And if the sum of the iov_len values is greater than {SSIZE_MAX}, the operation shall fail and no data shall be transferred. Nothing. Zero. Zip. The vibe check failed and nothing moved. 💀

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🔥 RETURN VALUE 🔥

On successful completion, writev() shall return the number of bytes actually written. Slay.

Otherwise? It shall return a value of -1, the file-pointer shall remain unchanged, and errno shall be set to indicate the error. So your offset didn't move, okay? Don't panic, just read errno. 🫶

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💜 ERRORS 💜

Go refer to write(). She's the source of truth for the general stuff. ✨

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

• EINVAL — the sum of the iov_len values in the iov array would overflow an ssize_t. Too much. Overflowing. Not the move. 😭

The writev() function may fail and set errno to:

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

Heads up: the following sections are informative — meaning they're the lil bonus content, not the hard rules. 🎀

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🦋 EXAMPLES 🦋

🌟 Writing Data from an Array 🌟

Okay so this example writes data from the buffers specified by members of the iov array to the file associated with the file descriptor fd. It's giving textbook, it's giving clean, we love her:

#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
...
ssize_t bytes_written;
int fd;
char *buf0 = "short string\n";
char *buf1 = "This is a longer string\n";
char *buf2 = "This is the longest string in this example\n";
int iovcnt;
struct iovec iov[3];

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

bytes_written = writev(fd, iov, iovcnt);
...

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🧚 APPLICATION USAGE 🧚

None.

(Yes really. Nothing here. She said "no thoughts, head empty" and honestly? Relatable. 💖)

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💀 RATIONALE 💀

Refer to write().

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⭐ FUTURE DIRECTIONS ⭐

None.

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👑 SEE ALSO 👑

• readv(), write()
• The Base Definitions volume of POSIX.1‐2017, <limits.h>, <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 — brought to you by IEEE/The Open Group, dated 2017, filed under WRITEV(3P). Now go gather those buffers, stay within {IOV_MAX}, and check your errno. Mwah. 💅💖✨