musl - Sign conversion warning in FD

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Message-ID: <2e176da1-75d4-4823-9ae9-8ec6a168f695@pgbackrest.org>
Date: Fri, 23 May 2025 15:28:23 +0000 (UTC)
From: David Steele <david@...ackrest.org>
To: musl@...ts.openwall.com
Subject: Sign conversion warning in FD_ISSET()

Greetings,

I'm getting a warning when compiling on Alpine 3.21 with musl libc 1.2.5:

# /lib/libc.musl-aarch64.so.1
musl libc (aarch64)
Version 1.2.5

The code looks like this (simplified for this example, see original at 
(https://github.com/pgbackrest/pgbackrest/blob/release/2.55.1/src/protocol/parallel.c#L163):

int fd = getFdFunc();

if (FD_ISSET(fd, &selectSet))

And I get this warning when -Wsign-conversion is enabled on gcc:

../../pgbackrest/src/protocol/parallel.c:164:25: error: conversion to 
'long unsigned int' from 'int' may change the sign of the result 
[-Werror=sign-conversion]
   164 |                     if (FD_ISSET(fd, &selectSet))

Here's the cc output from ninja:

[3/4] cc -Isrc/pgbackrest.p -Isrc -I../../pgbackrest/src 
-Isrc/command/help -Isrc/postgres -I/usr/include/postgresql 
-I/usr/include/libxml2 -fdiagnostics-color=always -DNDEBUG 
-D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -Werror -std=c99 -O3 
-Wcast-qual -Wconversion -Wduplicated-cond -Wduplicated-branches 
-Wformat=2 -Wformat-overflow=2 -Wformat-signedness -Winit-self 
-Wmissing-prototypes -Wmissing-variable-declarations -Wpointer-arith 
-Wredundant-decls -Wstrict-prototypes -Wvla -Wwrite-strings 
-Wno-clobbered -Wno-missing-field-initializers -funroll-loops 
-ftree-vectorize -fmacro-prefix-map=../../pgbackrest/src/= 
-fmacro-prefix-map=../../pgbackrest/test/src/=test/ 
-D_POSIX_C_SOURCE=200809L -MD -MQ src/pgbackrest.p/protocol_parallel.c.o 
-MF src/pgbackrest.p/protocol_parallel.c.o.d -o 
src/pgbackrest.p/protocol_parallel.c.o -c 
../../pgbackrest/src/protocol/parallel.c

I can fix this by casting to long unsigned int, but it seems like 
FD_ISSET() should accept int without complaint like the associate macros 
and functions.

Please CC me on any response since I am not subscribed to the mailing list.

Thanks,
-David

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