simple http server in C

http-c/README.md

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+コンパイル・実行（Unix/Linux環境）:
+
+`gcc -o server server.c utils.c`
+`./server`
+
+動作確認:
+
+`curl http://127.0.0.1:8080/calc\?query\=1+11` など

http-c/handler.c

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+#include "handler.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#define INITIAL_READ_BUFFER_SIZE 1024
+
+static void respond_and_close(int connection_fd, int status_code, const char* body) {
+    char* status_message = NULL;
+    switch (status_code) {
+        case 200: status_message = "OK";                    break;
+        case 400: status_message = "Bad Request";           break;
+        case 404: status_message = "Not Found";             break;
+        case 500: status_message = "Internal Server Error"; break;
+        default:  status_message = "Unknown Status";        break;
+    }
+
+    char* response = NULL;
+    int response_len = asprintf(&response,
+        "HTTP/1.1 %d %s\r\n"
+        "Content-Length: %zu\r\n"
+        "\r\n"
+        "%s",
+        status_code, status_message, strlen(body), body
+    );
+
+    if (response_len == -1) {
+        perror("asprintf");
+        close(connection_fd);
+        return;
+    }
+
+    write(connection_fd, response, response_len);
+    free(response);
+    close(connection_fd);
+}
+
+static char* read_request(int connection_fd) {
+    size_t capacity = INITIAL_READ_BUFFER_SIZE;
+    char* read_buffer = malloc(capacity);
+    size_t total_read_size = 0;
+
+    while (1) {
+        ssize_t read_size = read(connection_fd, read_buffer + total_read_size, capacity - total_read_size - 1);
+        if (read_size < 0) {
+            perror("read");
+            free(read_buffer);
+            return NULL;
+        }
+        if (read_size == 0) {
+            break;
+        }
+
+        // "\r\n\r\n" (4文字) が読み込み境界を跨ぐ可能性を考慮し、3バイト戻ったところから探索する
+        size_t str_search_offset = total_read_size > 3 ? total_read_size - 3 : 0;
+
+        total_read_size += read_size;
+        read_buffer[total_read_size] = '\0';
+
+        if (strstr(read_buffer + str_search_offset, "\r\n\r\n")) {
+            break;
+        } 
+
+        if (total_read_size == capacity - 1) {
+            capacity *= 2;
+            char* new_read_buffer = realloc(read_buffer, capacity);
+            if (new_read_buffer == NULL) {
+                perror("realloc");
+                free(read_buffer);
+                return NULL;
+            }
+            read_buffer = new_read_buffer;
+        }
+    }
+
+    return read_buffer;
+}
+
+void handle_connection(int connection_fd) {
+    const char* path_prefix = "GET /calc";
+    const char* query_prefix = "?query=";
+
+    char* read_buffer = read_request(connection_fd);
+    if (read_buffer == NULL) {
+        respond_and_close(connection_fd, 500, "Something went wrong\n");
+        return;
+    }
+
+    if (strncmp(read_buffer, path_prefix, strlen(path_prefix)) != 0) {
+        respond_and_close(connection_fd, 404, "The requested URL was not found on this server\n");
+        return;
+    }
+
+    if (strncmp(read_buffer + strlen(path_prefix), query_prefix, strlen(query_prefix)) != 0) {
+        respond_and_close(connection_fd, 400, "Failed to find query");
+        return;
+    }
+
+    char* query_start = read_buffer + strlen(path_prefix) + strlen(query_prefix);
+    int a, b;
+    if (sscanf(query_start, "%d+%d", &a, &b) != 2) {
+        respond_and_close(connection_fd, 400, "Failed to parse as a valid expression\n");
+        free(read_buffer);
+        return;
+    }
+
+    int result = a + b;
+    char body[64];
+    snprintf(body, sizeof(body), "%d\n", result);
+    respond_and_close(connection_fd, 200, body);
+    free(read_buffer);
+}

http-c/handler.h

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+#ifndef UTILS_H
+#define UTILS_H
+
+void handle_connection(int connection_fd);
+
+#endif

http-c/server.c

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+#include "handler.h"
+
+#include <arpa/inet.h>
+#include <netinet/in.h>
+#include <stdio.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+#define PORT 8080
+#define BACKLOG_SIZE 8
+
+int main() {
+    int socket_fd = socket(PF_INET, SOCK_STREAM, 0);
+    if (socket_fd < 0) {
+        perror("socket");
+        return 1;
+    }
+
+    struct sockaddr_in addr = {
+        .sin_family = AF_INET,
+        .sin_port = htons(PORT),
+        .sin_addr.s_addr = INADDR_ANY,
+    };
+
+    if (bind(socket_fd, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
+        perror("bind");
+        return 1;
+    }
+
+    if (listen(socket_fd, BACKLOG_SIZE) < 0) {
+        perror("listen");
+        return 1;
+    }
+
+    while (1) {
+        struct sockaddr_in client_addr;
+        socklen_t client_addr_size = sizeof(client_addr);
+
+        int connection_fd = accept(socket_fd, (struct sockaddr*)&client_addr, &client_addr_size);
+        if (connection_fd < 0) {
+            perror("accept");
+            return 1;
+        }
+
+        handle_connection(connection_fd);
+    }
+
+    close(socket_fd);
+
+    return 0;
+}