pinMode() segfaults on every call with the new libgpiod 2.x API path (RPi/Linux) — sprintf() called with a string literal as the destination buffer

[mauricio-camayo]

Environment

• Raspberry Pi 3 Model B Plus, Raspberry Pi OS (Debian 13 "trixie")
• libgpiod 2.2.1 (Debian trixie ships the 2.x API, so the #else // new 2.x API branch is the one compiled)
• Built via usetrmnl/trmnl-display's build.sh, which clones and builds bb_epaper as a dependency
• Commit tested against: bitbank2/bb_epaper@4896bae (2026-08-22)

Summary

Any call into pinMode() on Linux/RPi with the new libgpiod 2.x API immediately segfaults, because sprintf() is called with a string literal as its destination buffer instead of the local szChip buffer that was declared right above it for that purpose.

Root cause

src/rpi_io.inl, inside pinMode(), #else // new 2.x API branch:

   char szChip[16];
   sprintf("/dev/%s", szChipName);

sprintf's first argument must be the destination buffer. Here it's passed the format string "/dev/%s" — a string literal living in read-only memory (.rodata). sprintf then tries to write into that literal, which segfaults (SIGSEGV). The freshly-declared szChip[16] buffer is left completely unused.

Backtrace (gdb):

Program received signal SIGSEGV, Segmentation fault.
0x0000007ff7892964 in ?? () from /lib/aarch64-linux-gnu/libc.so.6
#0  0x0000007ff7892964 in ?? () from /lib/aarch64-linux-gnu/libc.so.6
#1  0x0000007ff7872060 in sprintf () from /lib/aarch64-linux-gnu/libc.so.6
#2  0x000000555556f8f8 in pinMode (iPin=<optimized out>, iMode=iMode@entry=0) at ../src/rpi_io.inl:181
#3  0x0000005555557f74 in TRMNL_EPAPER () at main.cpp:1018
#4  0x00000055555559c8 in main (argc=1, argv=0x7ffffffab8) at main.cpp:1382

Fix

    char szChip[16];
-   sprintf("/dev/%s", szChipName);
+   sprintf(szChip, "/dev/%s", szChipName);
    chip = gpiod_chip_open(szChip);

This is a one-character-argument-order fix; verified locally that it resolves the crash and gpiod_chip_open() succeeds against /dev/gpiochip0 afterward.

Impact

This affects every Linux/RPi user on a distro shipping libgpiod 2.x (e.g. any current Raspberry Pi OS / Debian trixie install) — the old-API (GPIOD_API) branch is unaffected, but that's not the one that gets compiled on modern systems. In practice this means bb_epaper-based clients (e.g. usetrmnl/trmnl-display) crash on their very first GPIO pin write on any up-to-date Raspberry Pi OS install.

[bitbank2]

Thanks for finding this; I'll fix the omission. It's interesting that a different version of GPIOD is used for different RPI boards with the same OS version.

[bitbank2]

I just pushed a fix; please let me know if you're working again.