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GitHub Actions · Ubuntu 26.04 · Checked November 4, 2026

GCC 15 on ubuntu-latest (Ubuntu 26.04): C and C++ builds that now fail

ubuntu-latest is moving from Ubuntu 24.04 to 26.04 between October 19 and November 19, 2026 (what else changes). The default gcc and g++ go from GCC 13 to GCC 15 (13.3.0 on the 24.04 image, 15.2.0 on the 26.04 image). The default skips GCC 14, so the changes of two releases arrive at once. Code that built yesterday can fail in make, in pip install of a package with a C extension, or in a Rust or Go build that compiles C underneath.

Which change it is

Three groups of errors account for most of it. The flag that fixes one group does nothing for the others, so look at the first error first.

1. C is C23 by default (GCC 15)

GCC 15 compiles C as -std=gnu23; GCC 13 and 14 used -std=gnu17 (Porting to GCC 15). In C23 an empty parameter list () means (void), and bool, true, false, nullptr and thread_local are keywords. From HDF-EOS5 on ubuntu-latest (hdf5 #6841) and the guide:

error: too many arguments to function 'pakr2dm'; expected 0, have 1
error: passing argument 8 of 'inv_init' from incompatible pointer type [-Wincompatible-pointer-types]
error: 'bool' cannot be defined via 'typedef'

Less obvious ones come from macros that switch on the C version. libmdbx expands to [[maybe_unused]] in C23 and fails with "expected identifier or '(' before 'static'" (lighthouse #10518).

2. Warnings that became errors (GCC 14)

GCC 14 made six old warnings into errors: implicit function declarations, implicit int, int-conversion, incompatible-pointer-types, return-mismatch and declaration-missing-parameter-type (Porting to GCC 14):

error: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]

pylzma 0.5.0 (2018) fails in pip install with "makes integer from pointer without a cast" (hera #1140). The quiet case is configure: its test programs stop compiling, so it decides a library you installed is missing. In hdf5 #6841 it said "szlib is not functional".

3. C++ headers that are no longer pulled in (GCC 15)

libstdc++ headers include less of each other, and <cstdint> is the usual one (Porting to GCC 15):

error: 'uint64_t' does not name a type
note: 'uint64_t' is defined in header '<cstdint>'; this is probably fixable by adding '#include <cstdint>'

It is often in a dependency, not your code: an ITK pinned before the fix (ITKR #79), or RocksDB 8.10.0 under the Rust rocksdb crate 0.22.0, built by cc-rs.

Fix for group 1: build C as C17

The real fix is to give the functions their parameters and rename the keywords. To get the build green first, ask for the old default:

env:
  CFLAGS: -g -O2 -std=gnu17

Keep -g -O2: a CFLAGS you set replaces the default that configure would use. Autotools, setuptools (so pip install from source) and Rust's cc crate read CFLAGS. Elsewhere:

lighthouse #10518 does this for its slasher tests until the libmdbx fix lands.

Fix for group 2: add the declarations

Include the header the note names, or declare the function. If the code is not yours, first look for a newer release: pylzma has had 0.6.0 and 0.6.1 since January 2026. As a stopgap, -fpermissive turns these errors back into warnings in C, or one at a time:

env:
  CFLAGS: -g -O2 -Wno-error=implicit-function-declaration -Wno-error=int-conversion

hdf5 passed -Wno-error=implicit-function-declaration to configure to get its szlib check working again. Some build systems do not pass CFLAGS to every step; the GCC 14 guide suggests CC="gcc -fpermissive" for those.

Fix for group 3: update the dependency, or add the include

No -std option brings the header back. Update the library that has the missing include: drasi moved the rocksdb crate from 0.22.0 to 0.24.0, which brings RocksDB 10.4.2 (drasi-core #1000), and ITK fixed its header in June 2025. If you cannot update yet, CXXFLAGS: -include cstdint adds the header to every C++ file.

Or use the GCC you had on 24.04

The 26.04 image still has gcc-13 and gcc-14 (with g++-13 13.4.0 and g++-14 14.3.0). Pointing the build at GCC 13 avoids all three groups:

env:
  CC: gcc-13
  CXX: g++-13

Autotools, CMake (on a fresh build directory), setuptools, cc-rs and cgo read CC and CXX. The other way out is runs-on: ubuntu-24.04 for that job, until GitHub retires the 24.04 image. Either one buys time; the code still has to change for the next compiler.

Telling it apart

Other 26.04 failures

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