Docs Backend

Docs Backend is a multi-client collaborative Markdown editing server written in C. It was built to explore POSIX processes, real-time signals, named pipes, threads, mutexes, role-based permissions, document versioning, and synchronized command processing.

The server keeps one shared Markdown document in memory. Clients connect using the server PID and a username, receive the current document and version, and submit edit commands. A background server thread orders queued commands by timestamp, applies authorized changes, increments the document version, and broadcasts the results to every connected client.

Key features

• Concurrent client sessions managed with POSIX threads
• Bidirectional client/server communication through per-client FIFOs
• Real-time signal handshake for new connections
• Read and write roles loaded from roles.txt
• Mutex-protected client, command, document, and log state
• Versioned Markdown edits with cursor and stale-version validation
• Insert, delete, heading, bold, italic, blockquote, list, code, rule, and link operations
• Server and client document/log inspection commands
• AddressSanitizer-enabled development build

Architecture

client process ── SIGRTMIN + PID ──> server
client process <──── named FIFOs ───> client thread
                                      │
                                      ▼
                               command queue
                                      │
                                      ▼
                             synchronized editor
                                      │
                                      ▼
                           broadcast to all clients

source/server.c manages connections, authorization, command ordering, synchronization, and broadcasts. source/client.c maintains a local document view and sends commands. source/markdown.c implements the versioned Markdown editing model declared in libs/markdown.h.

Requirements

• Linux or another POSIX environment supporting SIGRTMIN
• GCC or a compatible C compiler
• POSIX threads and named pipes
• make

macOS does not provide POSIX real-time signals such as SIGRTMIN, so the current handshake is Linux-specific.

Build and run

make
./server 1000

The server prints its PID. In another terminal, connect a user listed in roles.txt:

./client <SERVER_PID> yuwon

The server argument is the command-processing interval in milliseconds. Add or update users in roles.txt using the format username read or username write before connecting.

Client commands

Read-only inspection commands are available to every authorized client:

DOC?
PERM?
LOG?
DISCONNECT

Users with the write role can submit editing commands such as:

INSERT <position> <content>
DELETE <position> <length>
NEWLINE <position>
HEADING <level> <position>
BOLD <start> <end>
ITALIC <start> <end>
BLOCKQUOTE <position>
ORDERED_LIST <position>
UNORDERED_LIST <position>
CODE <start> <end>
HORIZONTAL_RULE <position>
LINK <start> <end> <url>

The server console also accepts DOC?, LOG?, and QUIT. QUIT is rejected while clients remain connected.

Clean build artifacts

make clean

This removes compiled binaries, object files, and generated FIFO files.