__sync_synchronize not defined in older GCC versions

[Daniel Frankcom]

• My goal is:
  • I want to successfully compile Mongoose using an older g++ version for a legacy project, in order to facilitate the upgrade of that project gradually to a newer compiler version.
• My actions were:
  • Download Mongoose 7.12 source code
  • Attempt to compile using g++ -c mongoose.c
• My expectation was:
  • Successful compilation of .o file
• The result I saw:

Compilation failure:

g++ -c /tmp/mongoose-7.12/mongoose.c:
/tmp/mongoose-7.12/mongoose.c: In function 'size_t mg_queue_read_len(mg_queue*)':
/tmp/mongoose-7.12/mongoose.c:5849: error: '__sync_synchronize' was not declared in this scope
/tmp/mongoose-7.12/mongoose.c: In function 'void mg_queue_write_len(mg_queue*, size_t)':
/tmp/mongoose-7.12/mongoose.c:5858: error: '__sync_synchronize' was not declared in this scope

• My question is:

Can the definition of MG_MEMORY_BARRIER check the __GNUC__ version before attempting to use __sync_synchronize()?

The current check is the following:

#if defined(__GNUC__) || defined(__clang__)

This is the only barrier to the compilation succeeding, since the current code only checks for the presence of __GNUC__, and not the version. I am attempting to compile using g++ 3.2.3 (it's old, I know) and I am able to get everything to work by simply adding a version check like this:

#if (defined(__GNUC__) && (__GNUC__ > 4) || (defined(__GNUC__) && defined(__GNUC_MINOR__) && __GNUC__ == 4 && __GNUC_MINOR__ >= 1)) || defined(__clang__)

I have no expectation of ongoing support for such an old compiler version, but the current code happens to be compatible aside from this one small issue.

I have verified that the __sync_synchronize() call was added in GCC 4.1.0, and is not available before that. I've provided a Dockerfile at the bottom of this request which can be used to test changes before/after support for __sync_synchronize() was added.

I am happy to create a PR to modify this definition check if the proposed change is acceptable.

Environment

• Mongoose version: 7.12
• Compiler/IDE and SDK: GNU g++ version 4.0.4
• Target hardware/board: x86

Reproduction steps

Any GCC version before 4.1.0 will exhibit this issue. Below is a Dockerfile which showcases the issue and allows it to be reproduced as long as you have Docker installed.

Put these lines in a Dockerfile, and run docker build .

The Docker build will fail when using GCC 4.0.4, but will succeed and produce a .o file when using GCC 4.1.0. The environment variable near the top of the file can be modified to change the GCC version.

FROM centos:7

# Uncomment one of these to select a GCC version.
ENV GCC_VERSION 4.0.4
#ENV GCC_VERSION 4.1.0

RUN yum install -y \
    binutils \
    bzip2 \
    glibc-devel \
    glibc-devel.i686 \
    make \
    patch \
    unzip \
    wget

RUN rpm -i https://kojipkgs.fedoraproject.org/vol/fedora_koji_archive01/packages/compat-gcc-34/3.4.6/46.fc26/x86_64/compat-gcc-34-3.4.6-46.fc26.x86_64.rpm
RUN ln -s /usr/bin/gcc34 /usr/bin/cc

WORKDIR /tmp
RUN wget --no-check-certificate https://mirrorservice.org/sites/sourceware.org/pub/gcc/releases/gcc-${GCC_VERSION}/gcc-${GCC_VERSION}.tar.bz2
RUN tar -xvf gcc-${GCC_VERSION}.tar.bz2
WORKDIR /tmp/gcc-${GCC_VERSION}

# Patch the GCC source to fix a minor incompatibility with CentOS 7.
RUN wget https://gist.githubusercontent.com/Wheest/bdd5bc7d487061e5c973f0fba4b96c47/raw/31033a6c5e8d7811ae0a81c2e4c9adea2359a822/unwind.patch -O /tmp/unwind.patch
RUN patch -p0 < /tmp/unwind.patch

RUN mkdir /tmp/gcc-${GCC_VERSION}/build
WORKDIR /tmp/gcc-${GCC_VERSION}/build
RUN ../configure --enable-languages=c,c++

RUN make
RUN make install

RUN wget https://github.com/cesanta/mongoose/archive/refs/tags/7.12.tar.gz -O /tmp/7.12.tar.gz
RUN tar -xvzf /tmp/7.12.tar.gz -C /tmp

RUN g++ -c /tmp/mongoose-7.12/mongoose.c

[Sergey Lyubka]

@danielfrankcom Yes please make a PR.

Nitpick: in your check, one defined(__GNUC__) seems to be redundant.
Please sign https://cesanta.com/cla.html before we can integrate the PR.

[Sergey Lyubka]

Apparently I accidentally pushed that change in 01b612c#diff-e5e80dd7794e56fd96c9b5c781b500dda60555b7f8e68d7a5d7d3fa9384db6f2

Please verify that it does the job, and reopen this issue if it does not
Thank you for reporting it and suggesting the change!