pciconfig_read(2)

💖 pciconfig_read(2) — the manual, but make it slay 💖

okay bestie, gather round, because today we're talking about three syscalls from the System Calls Manual: pciconfig_read, pciconfig_write, and pciconfig_iobase 🫶 They're all about pci device information handling, and ngl, most of you will never need them. But knowledge is free, so let's go 💅

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🌸 LIBRARY 🌸

Standard C library (libc, -lc) — yes queen, it's just libc, we love a basic-in-a-good-way moment ✨

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🎀 SYNOPSIS 🎀

#include <pci.h>

int pciconfig_read(unsigned long bus, unsigned long dfn,
                   unsigned long off, unsigned long size,
                   unsigned char *buf);
int pciconfig_write(unsigned long bus, unsigned long dfn,
                   unsigned long off, unsigned long size,
                   unsigned char *buf);
int pciconfig_iobase(int which, unsigned long bus,
                   unsigned long devfn);

look at those signatures, they're giving very ✨ low-level ✨ energy, and honestly? iconic 👑

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🦋 DESCRIPTION 🦋

So here's the tea 🍓: most of the talking-to-PCI-devices business is already handled by the kernel PCI layer. Which means these calls should not normally need to be accessed from user space. Like, they exist, but they're not the main characters — they're the background extras, iykyk.

pciconfig_read()

Reads to buf from device dev at offset off value. That's the whole vibe: you point at a device, an offset, and a buffer, and the info flows into your buf 🧚

pciconfig_write()

Writes from buf to device dev at offset off value. Same energy, opposite direction 💫

pciconfig_iobase()

You hand it a bus/devfn pair and you get back a physical address for whichever of these you asked about:

• the memory offset (for things like prep, this is 0xc0000000 👀)
• the IO base for PIO cycles
• or the ISA holes, if any

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💀 RETURN VALUE 💀

pciconfig_read()

On success, zero is returned. On error, -1 is returned and errno is set to indicate the error.

pciconfig_write()

On success, zero is returned. On error, -1 is returned and errno is set to indicate the error.

pciconfig_iobase()

Returns information on locations of various I/O regions in physical memory according to the which value. The values for which are: IOBASE_BRIDGE_NUMBER, IOBASE_MEMORY, IOBASE_IO, IOBASE_ISA_IO, IOBASE_ISA_MEM. Pick your fighter, bestie 🌈

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😭 ERRORS 😭

• EINVAL — the size value is invalid. This does not apply to pciconfig_iobase().
• EIO — I/O error. classic.
• ENODEV — for pciconfig_iobase(), the "hose" value is NULL. For the other calls, it means the slot could not be found 🔍
• ENOSYS — the system has not implemented these calls (CONFIG_PCI not defined). the kernel said "not today" 🙈
• EOPNOTSUPP — this return value is valid only for pciconfig_iobase(). It's returned if the value for which is invalid.
• EPERM — the user does not have the CAP_SYS_ADMIN capability. This does not apply to pciconfig_iobase().

fr fr kings and queens, CAP_SYS_ADMIN is not a vibe you can just manifest. either you have it or you don't 💅

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🌟 STANDARDS 🌟

Linux. that's it, that's the standard 🇫🇮✨

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📜 HISTORY 📜

Linux 2.0.26 / 2.1.11. ancient. vintage. lowkey a museum piece 🏛️

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👑 SEE ALSO 👑

• capabilities(7)

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and that's the whole page, bestie — go forth and don't casually poke at PCI devices from user space, the kernel's got it 🫶

mwah 💖 stay slay, stay on the physical memory,

✨ Linux man-pages 6.20 — 2026-02-08 ✨