Popen and pclose bufer

In C, popen and pclose are system functions in the standard C library that allow users to create a pipe and invoke the shell. We ran into a subtle bug related to correct usage of popen and pclose.

Issue

libCEED can use either NVRTC or Clang to do JiT compilation of programmatically generated code to produce specific kernels that match the user parameters in order to offer the best performance. This feature was specifically designed to support kernel fusion of user code written in Rust with programmatically generated wrapping code written in CUDA. In order to support this feature, both the Rust and CUDA code is compiled to LLVM IR and then combined via link time optimization to produce similar performance to code written in a single generated kernel containing only C/CUDA.

The CUDA backends use popen to run commands to determine the LLVM version used in the Rust toolchain for compilation of the user Rust code to LLVM IR so that the same version of clang++ is invoked while compiling the generated CUDA code to LLVM IR. This process was not working as intended, resulting in incompatible versions of Rust and clang++ being used together.

Diagnosis

In order to determine the version of LLVM used in the Rust toolchain, we first read the environment variable RUST_TOOLCHAIN (or assume nightly if not defined, as we rely upon unstable features for this process) and then use popen to execute the command $(find $(rustup run [RUST_TOOLCHAIN] rustc –print sysroot) -name llvm-link) –version. This command produces a large amount of output, which we read from the buffer of the pipe and search for the string “LLVM version”, followed by the version number, given as [major].[minor].

Once we have LLVM major version number, we attempt to invoke the associated C++ compiler via clang++-[major] with popen. We assume that if we pclose with a clean error code, then the invocation was successful and can use clang++-[major], otherwise we default to clang++.

When we used pclose, we got a non-zero error code and defaulted to clang++. Unfortunately, in our CI environment clang++ defaulted to clang++-21 while Rust was using LLVM version 22. clang++-21 errored while attempting to compile our generated code while clang++-22 did not.

Root Cause

We were using the command clang++-[major] –version 2>&1 to determine if the version of clang++ specified was valid. As we did not care about the output of the command, only the exit code, we did not check the contents of the buffer for the pipe.

The documentation for pclose states that the termination status of the command language interpreter is returned if successful, otherwise pclose returns -1 on error and sets errno to set the error.

This means that return values other than zero can indicate a successful execution. Specifically, when we were calling pclose without reading the contents of the buffer, then a positive number will be returned from pclose instead of zero or -1.

Fix

We could either read the contents of the buffer or modify our check for a non-zero return value to be a check that the return value is not -1. We opted to read the buffer and check for a non-zero return value. Then we optionally write the contents out as debugging information if debugging output is enabled for libCEED.

The full details of the fix are in this pull request.

Metadata

Started: 20 May 2026

Last edited: 20 May 2026