C语言循环的实现 在C语言中采用3中语法来实现循环，它们分别是while、for、do while，本文将分别说明这三种循环的实现，并对它们的运行效率进行比较。do while首先来看do while的实现：下面是简单的代码：int nCount = 0; int nMax = 10; do { nCount++; } while (nCount < nMax); return 0; 下面对应的是它的汇编代码：9: int nCount = 0; 00401268 mov dword ptr [ebp-4],0 10: int nMax = 10; 0040126F mov dword ptr [ebp-8],0Ah 11: do 12: { 13: nCount++; 00401276 mov eax,dword ptr [ebp-4] 00401279 add eax,1 0040127C mov dword ptr [ebp-4],eax 14: } while (nCount < nMax); 0040127F mov ecx,dword ptr [ebp-4];exc = nCount 00401282 cmp ecx,dword ptr [ebp-8];比较nCount 和 nMax的值 00401285 jl main+26h (00401276);跳转到循环体中 15: return 0; 00401287 xor eax,eax在汇编代码中首先执行了一次循环体中的操作，然后判断，当条件满足时会跳转回循环体，然后再次执行，当条件不满足时会接着执行后面的语句。这个过程可以用goto来模拟： int nCount = 0; int nMax = 10; __WHILE: nCount++; if(nCount < nMax) goto __WHILE;while循环不同于do while的先执行再比较，while采取的是先比较再循环的方式，下面是一个while的例子： int nCount = 0; int nMax = 10; while (nCount < nMax) { nCount++; }00401268 mov dword ptr [ebp-4],0 10: int nMax = 10; 0040126F mov dword ptr [ebp-8],0Ah 11: while (nCount < nMax) 00401276 mov eax,dword ptr [ebp-4] 00401279 cmp eax,dword ptr [ebp-8] 0040127C jge main+39h (00401289) 12: { 13: nCount++; 0040127E mov ecx,dword ptr [ebp-4] 00401281 add ecx,1 00401284 mov dword ptr [ebp-4],ecx 14: } 00401287 jmp main+26h (00401276) 15: return 0; 00401289 xor eax,eax 从汇编代码上可以看出，执行while循环时会有两次跳转，当条件不满足时会执行一次跳转，跳转到循环体外，而条件满足，执行完一次循环后，会再次跳转到循环体中，再次进行比较。相比于do while来说，while执行了两次跳转，效率相对较低。for 循环for循环是首先进行初始化操作然后进行比较，条件满足时执行循环，再将循环变量递增，最后再次比较，执行循环或者跳出。下面是for的简单例子： int nMax = 10; for (int i = 0; i < nMax; i++) { printf("%d\n", i); }下面是它对应的汇编代码：9: int nMax = 10; 00401268 mov dword ptr [ebp-4],0Ah 10: for (int i = 0; i < nMax; i++) 0040126F mov dword ptr [ebp-8],0 ;初始化循环变量 00401276 jmp main+31h (00401281);跳转到比较操作处 00401278 mov eax,dword ptr [ebp-8] 0040127B add eax,1 0040127E mov dword ptr [ebp-8],eax;这三句话实现的是循环变量自增操作 00401281 mov ecx,dword ptr [ebp-8];ecx = i 00401284 cmp ecx,dword ptr [ebp-4];比较ecx与i 00401287 jge main+4Ch (0040129c);跳转到循环体外 11: { 12: printf("%d\n", i); 00401289 mov edx,dword ptr [ebp-8] 0040128C push edx 0040128D push offset string "%d\n" (0042e01c) 00401292 call printf (00401540) 00401297 add esp,8 13: } 0040129A jmp main+28h (00401278);跳转到i++位置 14: return 0; 0040129C xor eax,eax从上面的汇编代码可以看出for循环的效率最低，它经过了3次跳转，生成对应的汇编代码上，初始化操作后面紧接着是循环变量自增操作，所以首先在完成初始化后会进行一次跳转，跳转到判断，然后根据判断条件再次跳转或者接着执行循环体，最后当循环完成后会再次跳转到循环变量自增的位置，同样采用goto语句来模拟这个操作： int nMax = 10; int i = 0; goto __CMP; __ADD: i++; __CMP: if (i >= nMax) { goto __RETURN; } __LOOP: printf("%d\n", i); goto __ADD; __RETURN: return 0;continue语句continue用于结束这次循环进入下一次循环，下面采用最复杂的for循环来说明continue语句：int nMax = 10; int i = 0; for(;i < nMax; i++) { if (i == 6) { continue; } }下面是它对应的汇编代码：00401268 mov dword ptr [ebp-4],0Ah 10: int i = 0; 0040126F mov dword ptr [ebp-8],0 11: for(;i < nMax; i++) 00401276 jmp main+31h (00401281) 00401278 mov eax,dword ptr [ebp-8] 0040127B add eax,1 0040127E mov dword ptr [ebp-8],eax 00401281 mov ecx,dword ptr [ebp-8] 00401284 cmp ecx,dword ptr [ebp-4] 00401287 jge main+43h (00401293) 12: { 13: if (i == 6) 00401289 cmp dword ptr [ebp-8],6; 0040128D jne main+41h (00401291);条件不满足组跳转到循环结束处 14: { 15: continue; 0040128F jmp main+28h (00401278) 16: } 17: } 00401291 jmp main+28h (00401278) 18: return 0; 00401293 xor eax,eax 从上面的汇编代码可以看到，continue语句也是一个跳转语句，它会直接跳转到循环体的开始位置。对于for来说相对特殊一些（我觉得循环变量自增并不属于循环体），由于第一次进入循环时并没有执行循环变量自增，所以它会跳转到循环变量自增的位置，其他则直接到循环开始处。慎用gotogoto 语句就像汇编中的 jmp 一样，是直接跳转到对应的标识位置，从上面我们使用goto来模拟各种循环来看，goto语句的可读性不强，而且有可能跳过变量的初始化等过程造成一些难以察觉的问题，但有些时候goto确实好用，例如在写socket或者其他需要清理资源的代码时，goto可以显著的增加程序的可读性并且也会减少相关代码的编写，例如一个典型的服务端socket例子#include <winsock2.h> #include <stdio.h> #include <stdlib.h> #pragma comment(lib, "ws2_32.lib") // Winsock Library #define PORT 8080 #define BUFFER_SIZE 1024 int main() { WSADATA wsaData; SOCKET serverSocket, clientSocket; struct sockaddr_in serverAddr, clientAddr; int addrLen = sizeof(clientAddr); char buffer[BUFFER_SIZE]; // 初始化 Winsock if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) { printf("Failed to initialize Winsock. Error Code: %d\n", WSAGetLastError()); return EXIT_FAILURE; } // 创建 socket serverSocket = socket(AF_INET, SOCK_STREAM, 0); if (serverSocket == INVALID_SOCKET) { printf("Could not create socket. Error Code: %d\n", WSAGetLastError()); WSACleanup(); return EXIT_FAILURE; } // 设置服务器地址结构 serverAddr.sin_family = AF_INET; serverAddr.sin_addr.s_addr = INADDR_ANY; // 监听所有可用的接口 serverAddr.sin_port = htons(PORT); // 转换为网络字节序 // 绑定 socket if (bind(serverSocket, (struct sockaddr *)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR) { printf("Bind failed. Error Code: %d\n", WSAGetLastError()); closesocket(serverSocket); WSACleanup(); return EXIT_FAILURE; } // 开始监听 if (listen(serverSocket, 3) == SOCKET_ERROR) { printf("Listen failed. Error Code: %d\n", WSAGetLastError()); closesocket(serverSocket); WSACleanup(); return EXIT_FAILURE; } printf("Server is listening on port %d...\n", PORT); // 接受客户端连接 clientSocket = accept(serverSocket, (struct sockaddr *)&clientAddr, &addrLen); if (clientSocket == INVALID_SOCKET) { printf("Accept failed. Error Code: %d\n", WSAGetLastError()); closesocket(serverSocket); WSACleanup(); return EXIT_FAILURE; } printf("Client connected.\n"); // 发送消息给客户端 const char *message = "Hello from server!"; send(clientSocket, message, strlen(message), 0); // 关闭 sockets closesocket(clientSocket); closesocket(serverSocket); WSACleanup(); return EXIT_SUCCESS; }中间有好几次执行了closesocket、以及最后的WSACleanup操作、前面每一步出错都要写一次这些清理资源的操作。如果使用goto将会简单的多#include <winsock2.h> #include <stdio.h> #include <stdlib.h> #pragma comment(lib, "ws2_32.lib") // Winsock Library #define PORT 8080 #define BUFFER_SIZE 1024 int main() { WSADATA wsaData; SOCKET serverSocket, clientSocket; struct sockaddr_in serverAddr, clientAddr; int addrLen = sizeof(clientAddr); char buffer[BUFFER_SIZE]; int err = EXIT_SUCCESS; // 初始化 Winsock if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) { printf("Failed to initialize Winsock. Error Code: %d\n", WSAGetLastError()); err = EXIT_FAILURE; goto __CLEANUP; } // 创建 socket serverSocket = socket(AF_INET, SOCK_STREAM, 0); if (serverSocket == INVALID_SOCKET) { printf("Could not create socket. Error Code: %d\n", WSAGetLastError()); err = EXIT_FAILURE; goto __CLEANUP; } // 设置服务器地址结构 serverAddr.sin_family = AF_INET; serverAddr.sin_addr.s_addr = INADDR_ANY; // 监听所有可用的接口 serverAddr.sin_port = htons(PORT); // 转换为网络字节序 // 绑定 socket if (bind(serverSocket, (struct sockaddr *)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR) { printf("Bind failed. Error Code: %d\n", WSAGetLastError()); err = EXIT_FAILURE; goto __CLEANUP; } // 开始监听 if (listen(serverSocket, 3) == SOCKET_ERROR) { printf("Listen failed. Error Code: %d\n", WSAGetLastError()); err = EXIT_FAILURE; goto __CLEANUP; } printf("Server is listening on port %d...\n", PORT); // 接受客户端连接 clientSocket = accept(serverSocket, (struct sockaddr *)&clientAddr, &addrLen); if (clientSocket == INVALID_SOCKET) { printf("Accept failed. Error Code: %d\n", WSAGetLastError()); err = EXIT_FAILURE; goto __CLEANUP; } printf("Client connected.\n"); // 发送消息给客户端 const char *message = "Hello from server!"; send(clientSocket, message, strlen(message), 0); // 关闭 sockets __CLEANUP: if(clientSocket != INVALID_SOCKET) { closesocket(clientSocket) } if(serverSocket != INVALID_SOCKET) { closesocket(serverSocket); } WSACleanup(); return err; }如果在不允许使用goto的情况下，可以考虑使用 do while 来模拟这种情况，上面的代码可以修改为#include <winsock2.h> #include <stdio.h> #include <stdlib.h> #pragma comment(lib, "ws2_32.lib") // Winsock Library #define PORT 8080 #define BUFFER_SIZE 1024 int main() { WSADATA wsaData; SOCKET serverSocket, clientSocket; struct sockaddr_in serverAddr, clientAddr; int addrLen = sizeof(clientAddr); char buffer[BUFFER_SIZE]; int err = EXIT_SUCCESS; do{ // 初始化 Winsock if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) { printf("Failed to initialize Winsock. Error Code: %d\n", WSAGetLastError()); err = EXIT_FAILURE; break; } // 创建 socket serverSocket = socket(AF_INET, SOCK_STREAM, 0); if (serverSocket == INVALID_SOCKET) { printf("Could not create socket. Error Code: %d\n", WSAGetLastError()); err = EXIT_FAILURE; break; } // 设置服务器地址结构 serverAddr.sin_family = AF_INET; serverAddr.sin_addr.s_addr = INADDR_ANY; // 监听所有可用的接口 serverAddr.sin_port = htons(PORT); // 转换为网络字节序 // 绑定 socket if (bind(serverSocket, (struct sockaddr *)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR) { printf("Bind failed. Error Code: %d\n", WSAGetLastError()); err = EXIT_FAILURE; break; } // 开始监听 if (listen(serverSocket, 3) == SOCKET_ERROR) { printf("Listen failed. Error Code: %d\n", WSAGetLastError()); err = EXIT_FAILURE; break; } printf("Server is listening on port %d...\n", PORT); // 接受客户端连接 clientSocket = accept(serverSocket, (struct sockaddr *)&clientAddr, &addrLen); if (clientSocket == INVALID_SOCKET) { printf("Accept failed. Error Code: %d\n", WSAGetLastError()); err = EXIT_FAILURE; break; } printf("Client connected.\n"); // 发送消息给客户端 const char *message = "Hello from server!"; send(clientSocket, message, strlen(message), 0); }while (FALSE); // 关闭 sockets if(clientSocket != INVALID_SOCKET) { closesocket(clientSocket) } if(serverSocket != INVALID_SOCKET) { closesocket(serverSocket); } WSACleanup(); return err; }这里的while不是为了循环，而是利用了do while 无论如何都会先执行循环体中代码的特性，只执行一次上述主体代码，利用break来跳转到最后的清理模块，实现与goto 类似的效果。使用goto 的方案比do while的方案要显得简洁易懂，goto使用的好，也能使得程序简单易懂。具体使用哪种方案是个见仁见智的事情，看个人喜好。如果遇上公司要求不能使用 goto，那么就可以采用do while的实现方案