x-yuri

c/c++: jumping over declarations

In C one can't have a labeled variable declaration. This quirk probably appeared in C99. Before that variable declarations had to be at the beginnings of blocks, so there could never be a variable declaration after a label. C99 allowed intermixing variable declarations and statements, but the grammar [remained the same][a], allowing only statements after labels.

cases

$ gcc --version
gcc (Alpine 13.2.1_git20240309) 13.2.1 20240309

x-yuri

A poor man's http server

The server process handles a request and exits. But the server is running in a docker container with --restart always, so after each request docker restarts the server.

Dockerfile:

FROM alpine:3.20
COPY a.c a.c
RUN apk add gcc musl-dev \

x-yuri

Reading an HTTP request without blocking

It's assumed that only valid requests are received. And that read() doesn't block until \r\n\r\n is received.

a.c:

#include <unistd.h>
#include <err.h>
#include <string.h>

x-yuri

mecab

Dockerfile (git):

FROM alpine:3.20
RUN apk add git build-base curl \
    && git clone https://github.com/taku910/mecab \
    && cd mecab/mecab \
    && ./configure \

x-yuri

BRE/ERE: escape special chars

a.bats:

setup() {
    load bats-support/load
    load bats-assert/load
}

x-yuri

iptables-save != iptables-save + iptables-xml + iptables.xslt

$ sudo iptables-save | iptables-xml > /tmp/iptables.xml
$ diff -u <(sudo iptables-save) <(xsltproc /usr/share/xtables/iptables.xslt /tmp/iptables.xml)

Output:

x-yuri

XSLT: swap 2 elements

a.xml:

<a>
    <b/>
    <c/>
    <d/>

x-yuri

xargs

tl;dr By default quotes and backslash quote blanks (spaces), and only backslash can quote newlines. With -d and -0, all of those are not special (unless one of those is passed with -d). With -I the separator is a newline and it uses one argument per invocation. Quotes and backslash are stripped (1). Quotes must be balanced (2). Use along with -d\\n to avoid (1) and (2).

By default single, double quotes and backslash quote blanks (e.g. spaces):

$ echo "'a b' c" | xargs -t
echo 'a b' c
a b c

x-yuri

docker run: determining if --init is needed

Normally, if a program doesn't set a SIGTERM handler, it needs --init:

Dockerfile:

FROM alpine:3.20
COPY a.c .
RUN apk add build-base \

x-yuri

go runtime sets its own signal handlers

By default go sets its own [signal handlers][ae]:

• runtime.mstart0() -> runtime.mstart1()
• runtime.mstart1() -> [runtime.mstartm0()][ab]
• runtime.mstartm0() -> [runtime.initsig()][ac]
• runtime.initsig() -> [runtime.setsig(i, abi.FuncPCABIInternal(sighandler))][ad]