Implement basic TCP sever funcionality

AzzyCode · AzzyCode · commit c834ff500058 · 2024-05-02T00:24:45.000+02:00
diff --git a/server.cpp b/server.cpp
@@ -0,0 +1,144 @@
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+#include <errno.h>
+#include <unistd.h>
+#include <arpa/inet.h>
+#include <sys/socket.h>
+#include <netinet/ip.h>
+#include <cassert>
+
+
+static void msg(const char *msg) {
+    fprintf(stderr, "%s\n", msg);
+}
+
+static void die(const char *msg) {
+    int err = errno;
+    fprintf(stderr, "[%d] %s\n", err, msg);
+    abort();
+}
+
+const size_t k_max_msg = 4096;
+
+// Function ensures that the full amount of data is read before proceeding.
+// Read a specified number of bytes('n') from a file descriptor ('fd') into buffer('buf'). 
+static int32_t read_full(int fd, char *buf, size_t n){
+    while (n > 0) {
+        ssize_t rv = read(fd, buf, n);
+        if (rv <= 0) {
+            return -1; // Error, or unexpected EOF
+        }
+        assert(static_cast<size_t>(rv) <= n); // Check the number of bytes read doesnt exceed the number of bytes requested
+        n -= static_cast<size_t>(rv);
+        buf += rv;
+    }
+
+    return 0;
+}
+
+static int32_t write_all(int fd, const char *buf, size_t n) {
+    while (n > 0) {
+        ssize_t rv = write(fd, buf, n);
+        if (rv <= 0) {
+            return -1; // Error
+        }
+        assert(static_cast<size_t>(rv) <= n);
+        n -= static_cast<size_t>(rv);
+        buf += rv;
+    }
+
+    return 0;
+}
+
+static int32_t one_request(int connfd) {
+    // 4 bytes header
+    char rbuf[4 + k_max_msg + 1];
+    errno = 0;
+    int32_t err = read_full(connfd, rbuf, 4);
+    if (err) {
+        if (errno == 0) {
+            msg("EOF");
+        } else {
+            msg("read() error");
+        }
+        return err;
+    }
+
+    uint32_t len = 0;
+    memcpy(&len, rbuf, 4); // assume little endian
+    if (len > k_max_msg) {
+        msg("too long");
+        return -1;
+    }
+
+    // request body
+    err = read_full(connfd, &rbuf[4], len);
+    if (err) {
+        msg("read() error");
+        return err;
+    }
+
+    // do something
+    rbuf[4 + len] = '\0';
+    printf("client says: %s\n", &rbuf[4]);
+
+    // replay using the same protocol
+    const char replay[] = "world";
+    char wbuf[4 + sizeof(replay)];
+    len = static_cast<uint32_t>(strlen(replay));
+    memcpy(wbuf, &len, 4);
+    memcpy(&wbuf[4], replay, len);
+
+    return write_all(connfd, wbuf, 4 + len);    
+
+}
+
+int main() {
+    int fd = socket(AF_INET, SOCK_STREAM, 0);
+    if (fd < 0) {
+        die("socket()");
+    }
+
+    // this is needed for most server applications
+    int val = 1;
+    setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &val, sizeof(val));
+
+    // bind
+    struct sockaddr_in addr = {};
+    addr.sin_family = AF_INET;
+    addr.sin_port = ntohs(1234);
+    addr.sin_addr.s_addr = ntohl(0);    // wildcard address 0.0.0.0
+    int rv = bind(fd, (const sockaddr *)&addr, sizeof(addr));
+    if (rv) {
+        die("bind()");
+    }
+
+    // listen
+    rv = listen(fd, SOMAXCONN);
+    if (rv) {
+        die("listen()");
+    }
+
+    while (true) {
+        // accept
+        struct sockaddr_in client_addr = {};
+        socklen_t socklen = sizeof(client_addr);
+        int connfd = accept(fd, (struct sockaddr *)&client_addr, &socklen);
+        if (connfd < 0) {
+            continue;   // error
+        }
+
+        // only serves one client connection at once
+        while (true) {
+            int32_t err = one_request(connfd);
+            if (err) {
+                break;
+            }
+        }
+        close(connfd);
+    }
+
+    return 0;
+}
