8000 Fix mirb build under Cosmopolitan by katafrakt · Pull Request #6694 · mruby/mruby · GitHub
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Fix mirb build under Cosmopolitan#6694

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matz merged 2 commits intomruby:masterfrom
katafrakt:mirb-cosmopolitan
Jan 8, 2026
Merged

Fix mirb build under Cosmopolitan#6694
matz merged 2 commits intomruby:masterfrom
katafrakt:mirb-cosmopolitan

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@katafrakt katafrakt commented Jan 4, 2026

When I try to build mruby using Cosmopolitan (COSMO_ROOT=/path/to/cosmo ruby ./minirake MRUBY_CONFIG=cosmopolitan), I'm getting the following error:

CC    mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.c -> build/host/mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.o
/home/katafrakt/dev/github/mruby/mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.c: In function 'mirb_term_read_key':
/home/katafrakt/dev/github/mruby/mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.c:137:5: error: unknown type name 'fd_set'
  137 |     fd_set fds;
      |     ^~~~~~
/home/katafrakt/dev/github/mruby/mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.c:138:20: error: storage size of 'tv' isn't known
  138 |     struct timeval tv;
      |                    ^~
/home/katafrakt/dev/github/mruby/mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.c:141:5: error: implicit declaration of function 'FD_ZERO'; did you mean 'FP_ZERO'? [-Wimplicit-function-declaration]
  141 |     FD_ZERO(&fds);
      |     ^~~~~~~
      |     FP_ZERO
/home/katafrakt/dev/github/mruby/mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.c:142:5: error: implicit declaration of function 'FD_SET' [-Wimplicit-function-declaration]
  142 |     FD_SET(STDIN_FILENO, &fds);
      |     ^~~~~~
/home/katafrakt/dev/github/mruby/mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.c:146:9: error: implicit declaration of function 'select' [-Wimplicit-function-declaration]
  146 |     if (select(STDIN_FILENO + 1, &fds, NULL, NULL, &tv) <= 0) {
      |         ^~~~~~
rake aborted!

This seems to be related to the fact that Cosmopolitan is much mor strict when it comes to adding includes and you need to specifically include sys/select.h to access its functions.

I also added a Cosmopolitan build to the CI to avoid regressions in the future.

@katafrakt
Require sys/socket.h (for Cosmopolitan)
f856cf8 
Compilation of mirb with Cosmopolitan fails because of missing include
(Cosmopolitan seems to be more strict than "traditional" compilers.
@katafrakt katafrakt requested a review from matz as a code owner January 4, 2026 20:38
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Summary of Changes

Hello @katafrakt, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical build failure that occurred when compiling the mruby project, specifically the mirb tool, using the Cosmopolitan build environment. The error was traced to missing declarations for fd_set, timeval, FD_ZERO, FD_SET, and select, which are now resolved by explicitly including sys/select.h. To safeguard against similar issues in the future, a dedicated Cosmopolitan build step has been integrated into the continuous integration pipeline.

Highlights

  • Build Fix: Resolved a build error for mirb under Cosmopolitan by adding the necessary #include <sys/select.h> to mirb_term.c.
  • CI Integration: Introduced a new CI job specifically for Cosmopolitan builds to prevent future regressions and ensure continuous compatibility.

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Code Review

This pull request correctly identifies and fixes a build issue on the Cosmopolitan toolchain by explicitly including <sys/select.h>. This is a good practice for portability, as it avoids relying on transitive includes. The change is correct and directly addresses the compilation errors. I've added one minor suggestion to improve code style by sorting the include directives.

8000 mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.c
Comment on lines 13 to 17
#include <unistd.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <sys/select.h>
#include <errno.h>
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medium

While the added include is correct, for better long-term maintainability it's a good practice to keep header includes sorted alphabetically. This makes it easier to locate headers and helps prevent accidental duplicates. I'd suggest reordering this block of includes.

Suggested change
#include <unistd.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <sys/select.h>
#include <errno.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <sys/select.h>
#include <termios.h>
#include <unistd.h>

@matz matz merged commit ad53019 into mruby:master Jan 8, 2026
18 checks passed
@katafrakt katafrakt deleted the mirb-cosmopolitan branch January 8, 2026 06:07
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