請教這篇Secure, Efficient and Easy C programming的原理

看板Programming作者yajun.時間13年前 (2012/04/06 02:01)推噓4(4推 0噓 7→)

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http://irccrew.org/~cras/security/c-guide.html "Secure, Efficient and Easy C programming" 這篇文章看大意是在介紹利用記憶體佈局中的 stack 來達成不需手動經由malloc / free 動態配置/回收記憶體的技巧，但想必是我的 C 語言學得太差，看不懂他範例程式是如何使用到 stack。如果有 C 達人讀過這篇文章，能否為在下解惑，說明一下作者用了什麼方式來操控 stack 在自己的程式中模擬「動態記憶分配」的功能，謝謝 // 以下是網頁上的第一個程式，我讀得一頭霧水，看不出「動態分配」在哪？ /* asprintf()-like functon, but allocates the memory from data stack */ const char *t_printf(const char *fmt, ...) __attribute__((format (printf, 1, 2))); void client_input(struct client *client) { const char *cmd; int i; cmd = read_command(client); if (cmd == NULL) return; if (strcasecmp(cmd, "status") == 0) { time_t now = time(NULL); client_send(client, t_printf("Time: %s", ctime(&now))); client_send(client, t_printf("Clients: %u", clients_count)); for (i = 0; i < 10000; i++) { /* without an extra stack frame here, we'd allocate the t_printf() 10000 times before freeing them all. That's probably not be very good. */ t_push(); client_send(client, t_printf("%d: %u", i, stuff[i])); t_pop(); } } else { client_send(client, t_printf("Unknown command: %s", cmd)); } } void main_loop(void) { unsigned int i, id; fd_set rfds; if (select(max_fd, &rfds, NULL, NUL, NULL) <= 0) return; for (i = 0; i < clients_count; i++) { if (FD_ISSET(clients[i].fd, &rds)) { id = t_push(); client_input(&clients[i]); if (t_pop() != id) { /* we could simply call the missing t_pop()s, but this usually indicates a problem which we want to know. for example we might have leaked it inside a for loop which caused unnecessarily large memory usage. */ panic("missing t_pop()"); } } } } -- ☆ [Origin:椰林風情] [From: host-219-70-179-157.dynamic] [Login: 3] [Post: 0]