Nim "execution of an external program failed: gcc" in CI

Nim compiles to C and then invokes a C compiler to produce the binary. This error means that gcc step failed, either because gcc is absent on the runner or because compiling or linking the generated C errored.

What this error means

After Nim finishes its own pass, the build fails with "Error: execution of an external program failed: 'gcc ...'". The real gcc output appears just above the line.

nim

Error: execution of an external program failed: 'gcc -c -w ... @m..@smain.nim.c.o ...'

Common causes

No C compiler on the runner

A slim image without build-essential has no gcc, so Nim cannot run the backend compile step.

The generated C failed to compile or link

A missing system header, a missing C library, or a link error in the generated C makes gcc exit non-zero; Nim reports it as an external program failure.

How to fix it

Install a C toolchain on the runner

Provide gcc and the standard headers before the Nim build.

Terminal

sudo apt-get update && sudo apt-get install -y build-essential

Read the captured gcc output

Scroll up to the gcc error printed above the Nim line.
Install the missing dev header or library it names (for example "cannot find -lpcre" needs libpcre3-dev).
Re-run; the Nim line clears once gcc succeeds.

How to prevent it

Install build-essential before Nim builds on slim images.
Add required -dev system packages for C-linking dependencies.
Read the gcc output above the Nim summary to find the real cause.