[{"_1":2,"_51":-5,"_52":-5},"loaderData",{"_3":4,"_7":8,"_50":-5},"root",{"_5":6},"path","/blogs/tags/C","pages/Tag",{"_9":10,"_46":47,"_19":21,"_48":49},"posts",[11],{"_12":13,"_14":15,"_16":17,"_19":20,"_24":25,"_26":27,"_28":29,"_32":33,"_36":37,"_38":39},"id","cm92tiice04tw07mpcmwmhzre","publishDate","2025-04-04","category",[18],"Guide","tag",[21,22,23],"C","Socket Programming","Networking","slug","building-a-simple-chat-app-in-c-a-linux-socket-programming-guide-for-beginners","title","Building a Simple Chat App in C: A Linux Socket Programming Guide for Beginners","excerpt",{"_30":31},"text","Learn to build a simple chat app in C using Linux sockets! This beginner-friendly guide covers client-server communication, multiplexing with poll(), and hands-on coding.","coverImg",{"_34":35},"url","https://ap-south-1.graphassets.com/AHDh3MjtFRAmkhgRu025Qz/cm92t7fxh04gq07mpub5ndkqz","content","## Architecture Diagram\n```ascii\n+------------+ +-----------+\n| Client | <------> | Server |\n+------------+ +-----------+\n ^ ^\n | stdin | stdin\n v v\n+----------+ +----------+\n| User | | User |\n| Input | | Input |\n+----------+ +----------+\n```\n\n## Server Implementation Breakdown\n\n### Server Code Structure\n```c\n#include \n#include \n#include \n#include \n#include \n\nint main() {\n // Socket creation and setup\n // Main loop with poll()\n // Communication handling\n return 0;\n}\n```\n\n### Step-by-Step Explanation\n\n**Step 1: Create Socket**\n```c\nint sockfd = socket(AF_INET, SOCK_STREAM, 0);\n```\n- `AF_INET`: IPv4 address family\n- `SOCK_STREAM`: TCP socket type\n- Returns file descriptor for the socket\n\n**Step 2: Bind Socket**\n```c\nstruct sockaddr_in address = {\n AF_INET,\n htons(9999),\n 0\n};\nbind(sockfd, &address, sizeof(address));\n```\n- `htons(9999)`: Convert port number to network byte order\n- `0` in address field means bind to all interfaces\n\n**Step 3: Listen for Connections**\n```c\nlisten(sockfd, 10);\n```\n- Backlog of 10 pending connections\n- Server is now waiting for client connections\n\n**Step 4: Accept Client Connection**\n```c\nint clientfd = accept(sockfd, 0, 0);\n```\n- Blocks until client connects\n- Returns new file descriptor for client communication\n\n**Step 5: Setup poll() Structure**\n```c\nstruct pollfd fds[2] = {\n {0, POLLIN, 0}, // stdin\n {clientfd, POLLIN, 0} // client socket\n};\n```\n- Monitor 2 file descriptors:\n 1. Standard input (keyboard)\n 2. Client socket\n\n**Step 6: Main poll() Loop**\n```c\nfor (;;) {\n char buffer[256] = {0};\n poll(fds, 2, 50000);\n \n // Handle events...\n}\n```\n- `poll()` waits for 50 seconds (50000 milliseconds)\n- Checks for input from either source\n\n**Step 7: Handle Input Events**\n```c\nif (fds[0].revents & POLLIN) { // stdin input\n read(0, buffer, 255);\n send(clientfd, buffer, 255, 0);\n} \nelse if (fds[1].revents & POLLIN) { // client message\n if (recv(clientfd, buffer, 255, 0) == 0) return 0;\n printf(\"%s\\n\", buffer);\n}\n```\n- Forward stdin input to client\n- Print received messages from client\n\n## Client Implementation Breakdown\n\n### Client Code Structure\n```c\n#include \n#include \n#include \n#include \n#include \n\nint main() {\n // Socket creation and connection\n // Main loop with poll()\n // Communication handling\n return 0;\n}\n```\n\n### Step-by-Step Explanation\n\n**Step 1: Create Socket**\n```c\nint sockfd = socket(AF_INET, SOCK_STREAM, 0);\n```\n- Same as server socket creation\n\n**Step 2: Connect to Server**\n```c\nstruct sockaddr_in address = {\n AF_INET,\n htons(9999),\n 0\n};\nconnect(sockfd, &address, sizeof(address));\n```\n- Connect to server at port 9999\n- `0` in address field means localhost (127.0.0.1)\n\n**Step 3: Setup poll() Structure**\n```c\nstruct pollfd fds[2] = {\n {0, POLLIN, 0}, // stdin\n {sockfd, POLLIN, 0} // server socket\n};\n```\n- Similar to server's poll setup\n\n**Step 4: Main poll() Loop**\n```c\nfor (;;) {\n char buffer[256] = {0};\n poll(fds, 2, 50000);\n \n // Handle events...\n}\n```\n- Same logic as server's main loop\n\n**Step 5: Handle Input Events**\n```c\nif (fds[0].revents & POLLIN) { // stdin input\n read(0, buffer, 255);\n send(sockfd, buffer, 255, 0);\n} \nelse if (fds[1].revents & POLLIN) { // server message\n if (recv(sockfd, buffer, 255, 0) == 0) return 0;\n printf(\"%s\\n\", buffer);\n}\n```\n- Mirror of server's message handling\n\n## Communication Flow Diagram\n\n```ascii\nClient Process Server Process\n | |\n |-------- TCP Connection ---------->|\n | |\n |------- stdin Input -------------->|\n |<----- Received Message -----------|\n | |\n |<----- stdin Input ----------------|\n |------- Received Message --------->| \n```\n\n## Key Concepts Explained\n\n### poll() System Call\n```ascii\n+----------------+\n| poll() |\n+----------------+\n| | +------------+\n| File Descriptors <----| stdin (0) |\n| | | socket fd |\n+----------------+ +------------+\n |\n v\n+---------------+\n| Event Checking|\n| POLLIN |\n| POLLOUT |\n| etc. |\n+---------------+\n```\n- Monitors multiple file descriptors simultaneously\n- Returns when any fd is ready for I/O\n- More efficient than busy-waiting\n\n### Network Byte Order\n```ascii\nHost Byte Order (e.g., 1234) -> htons() -> Network Byte Order (e.g., 3412)\nNetwork Byte Order (e.g., 3412) -> ntohs() -> Host Byte Order (e.g., 1234)\n```\n- Ensures consistent data representation across different architectures\n- Critical for port numbers and IP addresses\n\n## Execution Flow\n\n1. Start server first:\n```bash\n./server\n```\n\n2. Start client in another terminal:\n```bash\n./client\n```\n\n3. Communication test:\n- Type message in server terminal -> appears in client\n- Type message in client terminal -> appears in server\n\n## Important Notes\n\n1. **Port Number**: Both use port 9999 (ensure it's available)\n2. **Localhost Only**: `0` in address field binds to all interfaces\n3. **Buffer Size**: Fixed 256-byte buffer (255 chars + null terminator)\n4. **Blocking Behavior**: `poll()` with 50s timeout prevents busy-waiting\n\n## Limitations and Improvements\n\n1. **Single Client**: Server handles only one connection at a time\n2. **No Error Handling**: Add checks for socket/bind/listen errors\n3. **Message Truncation**: Messages over 255 bytes will be split\n4. **Exit Condition**: Closing connection terminates both ends\n\n## Full Code with Comments\n\n### server.c (Enhanced Comments)\n```c\n#include \n#include \n#include \n#include \n#include \n\nint main() {\n // Create TCP socket\n int sockfd = socket(AF_INET, SOCK_STREAM, 0);\n \n // Configure server address\n struct sockaddr_in address = {\n AF_INET, // IPv4\n htons(9999), // Port\n 0 // Listen on all interfaces\n };\n \n // Bind socket to address\n bind(sockfd, (struct sockaddr*)&address, sizeof(address));\n \n // Start listening\n listen(sockfd, 10);\n \n // Accept incoming connection\n int clientfd = accept(sockfd, 0, 0);\n \n // Configure poll structure\n struct pollfd fds[2] = {\n {0, POLLIN, 0}, // stdin\n {clientfd, POLLIN, 0} // client socket\n };\n \n // Main communication loop\n for (;;) {\n char buffer[256] = {0};\n \n // Wait for events (50s timeout)\n poll(fds, 2, 50000);\n \n // Handle stdin input\n if (fds[0].revents & POLLIN) {\n read(0, buffer, 255);\n send(clientfd, buffer, 255, 0);\n }\n // Handle client message\n else if (fds[1].revents & POLLIN) {\n if (recv(clientfd, buffer, 255, 0) == 0) {\n // Connection closed\n return 0;\n }\n printf(\"Received: %s\\n\", buffer);\n }\n }\n \n return 0;\n}\n```\n\n### client.c (Enhanced Comments)\n```c\n#include \n#include \n#include \n#include \n#include \n\nint main() {\n // Create TCP socket\n int sockfd = socket(AF_INET, SOCK_STREAM, 0);\n \n // Configure server address\n struct sockaddr_in address = {\n AF_INET, // IPv4\n htons(9999), // Port\n 0 // Connect to localhost\n };\n \n // Connect to server\n connect(sockfd, (struct sockaddr*)&address, sizeof(address));\n \n // Configure poll structure\n struct pollfd fds[2] = {\n {0, POLLIN, 0}, // stdin\n {sockfd, POLLIN, 0} // server socket\n };\n \n // Main communication loop\n for (;;) {\n char buffer[256] = {0};\n \n // Wait for events (50s timeout)\n poll(fds, 2, 50000);\n \n // Handle stdin input\n if (fds[0].revents & POLLIN) {\n read(0, buffer, 255);\n send(sockfd, buffer, 255, 0);\n }\n // Handle server message\n else if (fds[1].revents & POLLIN) {\n if (recv(sockfd, buffer, 255, 0) == 0) {\n // Connection closed\n return 0;\n }\n printf(\"Received: %s\\n\", buffer);\n }\n }\n \n return 0;\n}\n```\n\n## References & Further Reading\n\n### Essential Socket Programming Resources:\n1. **\"UNIX Network Programming\" by W. Richard Stevens** \n *The definitive guide to network programming in UNIX environments*\n\n2. **\"Beej's Guide to Network Programming\"** (Free Online Resource) \n [https://beej.us/guide/bgnet/](https://beej.us/guide/bgnet/) \n *Excellent beginner-friendly tutorial on socket programming*\n\n3. **Linux Manual Pages** \n Use `man 2 socket`, `man 2 bind`, `man poll` in terminal \n *Official Linux system call documentation*\n\n4. **\"The Linux Programming Interface\" by Michael Kerrisk** \n *Comprehensive guide to Linux system programming*\n\n5. **GNU C Library Documentation** \n [https://www.gnu.org/software/libc/manual/](https://www.gnu.org/software/libc/manual/) \n *Official C standard library documentation*\n\n## Support & Contact\n\n**☕ Support the Developer** \nIf you found this guide helpful, consider supporting my work: \n[buymeacoffee.com/trish07](https://buymeacoffee.com/trish07)\n\n**🐦 Connect on Twitter/X** \nFor updates and programming tips follow : \n[Trish](https://x.com/_trish_07) & [Paul](https://x.com/nilotpaul_n)\n\n## Learning Path Recommendation \n\n![Screenshot 2025-04-04 182657.png](https://ap-south-1.graphassets.com/AHDh3MjtFRAmkhgRu025Qz/cm92uiuos05mj08piosogr707)\n*Visual roadmap from basic sockets to advanced network protocols*\n\n**Path Explanation** \n1. **Socket Basics**: Understand fundamental socket APIs \n2. **TCP Client/Server**: Master connection-oriented communication \n3. **Multiplexing**: Learn efficient I/O handling with poll()/select() \n4. **Multi-client**: Scale to handle concurrent connections \n5. **Advanced Protocols**: Implement TLS, HTTP, or custom protocols \n\nThis visual roadmap helps you: \n- Track your progress through networking concepts \n- Understand prerequisite relationships between topics \n- Identify key milestones in network programming \n\nFor any questions or queries, feel free to [reach out](https://x.com/_trish_07)!\n\n---","author",{"_40":41,"_42":43,"_44":45},"name","Dexter","img","https://avatars.githubusercontent.com/u/71167911?v=4","social","https://github.com/dexter-xD","hasMorePosts",false,"page",1,"layouts/HomeLayout","actionData","errors"]