splice(2)

💖 splice(2) — the System Calls Manual glow-up 💖

okay bestie, welcome to splice(2) from the System Calls Manual ✨ this one is a real one and she does NOT get the attention she deserves, so buckle up 🎀

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✨ NAME ✨

splice — splice data to/from a pipe 🫶

📚 LIBRARY 📚

Standard C library (libc, -lc) 💅

🩷 SYNOPSIS 🩷

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#define _FILE_OFFSET_BITS 64
#include <fcntl.h>

ssize_t splice(int fd_in, off_t *_Nullable off_in,
               int fd_out, off_t *_Nullable off_out,
               size_t size, unsigned int flags);

🌸 DESCRIPTION 🌸

so here's the tea, bestie: splice() moves data between two file descriptors without copying between kernel address space and user address space. No round-tripping through userspace, no extra copies, it's giving efficiency 👑

It transfers up to size bytes of data from the file descriptor fd_in to the file descriptor fd_out, and one of those file descriptors must refer to a pipe. That's the rule, no cap 💀

Here's how it goes for fd_in and off_in:

• If fd_in refers to a pipe, then off_in must be NULL.
• If fd_in does NOT refer to a pipe and off_in is NULL, then bytes are read from fd_in starting from the file offset, and the file offset gets adjusted appropriately ✨
• If fd_in does NOT refer to a pipe and off_in is NOT NULL, then off_in must point to a buffer which specifies the starting offset from which bytes will be read from fd_in. In this case the file offset of fd_in is not changed, and instead the offset pointed to by off_in is adjusted appropriately. Iconic behavior honestly 🎀

Analogous statements apply for fd_out and off_out — same energy, other end of the pipe 🦋

The flags argument is a bit mask that you compose by ORing together zero or more of the following values:

• SPLICE_F_MOVE — Attempt to move pages instead of copying. This is only a hint to the kernel, so don't get too attached: pages may still be copied if the kernel cannot move the pages from the pipe, or if the pipe buffers don't refer to full pages. The initial implementation of this flag was buggy, so starting in Linux 2.6.21 it's a no-op (but it's still permitted in a splice() call). In the future, a correct implementation may be restored 👀
• SPLICE_F_NONBLOCK — Do not block on I/O. This makes the splice pipe operations nonblocking, but heads up: splice() may nevertheless block, because the file descriptors that are spliced to/from may block (unless they have the O_NONBLOCK flag set) 🙈
• SPLICE_F_MORE — More data will be coming in a subsequent splice. This is a helpful hint when the fd_out refers to a socket (see also the description of MSG_MORE in send(2), and the description of TCP_CORK in tcp(7)) 💌
• SPLICE_F_GIFT — Unused for splice(); see vmsplice(2) 🎁

🫶 RETURN VALUE 🫶

Upon successful completion, splice() returns the number of bytes spliced to or from the pipe. Slay.

A return value of 0 means end of input. If fd_in refers to a pipe, then this means there was no data to transfer, and it would not make sense to block, because there are no writers connected to the write end of the pipe 😭

On error, splice() returns -1 and errno is set to indicate the error. Rude, but okay.

💀 ERRORS 💀

• EAGAIN — SPLICE_F_NONBLOCK was specified in flags, or one of the file descriptors had been marked as nonblocking (O_NONBLOCK), and the operation would block.
• EBADF — One or both file descriptors are not valid, or do not have proper read-write mode.
• EINVAL — The target filesystem doesn't support splicing.
• EINVAL — The target file is opened in append mode.
• EINVAL — Neither of the file descriptors refers to a pipe.
• EINVAL — An offset was given for nonseekable device (e.g., a pipe).
• EINVAL — fd_in and fd_out refer to the same pipe.
• ENOMEM — Out of memory.
• ESPIPE — Either off_in or off_out was not NULL, but the corresponding file descriptor refers to a pipe.

bestie not the EINVAL stack 💀 that's FIVE whole EINVAL cases, she's an overachiever fr fr

🌈 STANDARDS 🌈

Linux. That's it. That's the standard. 💅

🦋 HISTORY 🦋

Linux 2.6.17, glibc 2.5 🎀

In Linux 2.6.30 and earlier, exactly one of fd_in and fd_out was required to be a pipe. Since Linux 2.6.31, both arguments may refer to pipes. Glow-up era ✨

👑 NOTES 👑

Okay this part is the lore, and I need you to be seated for it 🫖

The three system calls splice(), vmsplice(2), and tee(2) provide user-space programs with full control over an arbitrary kernel buffer, implemented within the kernel using the same type of buffer that is used for a pipe. In overview, these system calls perform the following tasks:

• splice() — moves data from the buffer to an arbitrary file descriptor, or vice versa, or from one buffer to another.
• tee(2) — "copies" the data from one buffer to another.
• vmsplice(2) — "copies" data from user space into the buffer.

Though we talk of copying, actual copies are generally avoided. The kernel does this by implementing a pipe buffer as a set of reference-counted pointers to pages of kernel memory. The kernel creates "copies" of pages in a buffer by creating new pointers (for the output buffer) referring to the pages, and increasing the reference counts for the pages: only pointers are copied, not the pages of the buffer. Literally the main character energy of memory management 🧚

_FILE_OFFSET_BITS should be defined to be 64 in code that uses non-null off_in or off_out, or that takes the address of splice, if the code is intended to be portable to traditional 32-bit x86 and ARM platforms where off_t's width defaults to 32 bits. Don't skip this, bestie, your 32-bit friends will suffer 😤

⭐ EXAMPLES ⭐

See tee(2) for another example 👀

#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64
#include <err.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int
main(void)
{
        int         fd;
        int         pfd[2];
        off_t       off;
        [[gnu::nonstring]]
     const char  s[12] = "Hello, world";

        fd = open("out", O_WRONLY | O_CREAT | O_EXCL, 0666);
        if (fd == -1)
                err(EXIT_FAILURE, "open");

        if (pipe(pfd) == -1)
                err(EXIT_FAILURE, "pipe");

        if (write(pfd[1], s, sizeof(s)) != sizeof(s))
                err(EXIT_FAILURE, "write");
        if (close(pfd[1]) == -1)
                err(EXIT_FAILURE, "close");

        off = 10;
        if (splice(pfd[0], NULL, fd, &off, sizeof(s), 0) != sizeof(s))
                err(EXIT_FAILURE, "splice");
        if (close(pfd[0]) == -1)
                err(EXIT_FAILURE, "close");

        printf("New offset is %jd\n", (intmax_t) off);

        if (close(fd) == -1)
                err(EXIT_FAILURE, "close");
        exit(EXIT_SUCCESS);
}

this one is actually so clean — write into a pipe, drop the read end, splice it straight into a file with an explicit off, and print the new offset. Obsessed 💖

🌟 SEE ALSO 🌟

copy_file_range(2), sendfile(2), tee(2), vmsplice(2), pipe(7)

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and that's the page, babe 💜 this one is from Linux man-pages 6.20, dated 2026-02-08 — drink some water, go splice something without copying it, and I'll see you in the next man page 🍓🫶✨