比较C中的字符串-strcmp [英] Comparing strings in C - strcmp

问题描述

我在比较C语言中的字符串时遇到了麻烦(我还不太熟悉).我在此服务器应用程序上有套接字，正在等待接受来自客户端的数据.在程序的这一特定部分中，我希望能够基于从客户端接收的数据执行MySQL查询.我希望能够知道何时接收到的数据具有"newuser"值，以启动简单的注册过程. Strcmp返回一个正1值，我相信我应该得到一个0，因为这些值应该相等.

I'm having trouble comparing strings in C (with which I am fairly new to). I have socket on this server application waiting to accept data from a client. In this particular portion of my program I want to be able to execute a MySQL query based on the data received from the client. I want to be able to know when the received data has the value of "newuser" to initiate a simple registration procedure. Strcmp is returning a positive 1 value where I believe I should be getting a 0 because the values should be equal.

源代码:

//setup socket
//loop and select structure to handle multiple connections

if ((nbytes = recv(i, buf, sizeof buf, 0)) <= 0) {
// got error or connection closed by client
    if (nbytes == 0) {
        // connection closed
        printf("selectserver: socket %d hung up\n", i);
    } else {
        perror("recv");
    }
    close(i); // bye!
    FD_CLR(i, &master); // remove from master set
} else {

    char check[] = "newuser";
    char fromUser[sizeof check];

    strncpy(fromUser,buf, sizeof check);
    printf("length of fromUser: %d\n", sizeof fromUser);
    printf("length of check: %d\n", sizeof check);
    printf("message from user: %s\n", fromUser);
    printf("check = %s \n", check);
    int diff = strcmp(fromUser, check);
    printf("compare fromUser to check: %d\n", diff);
    if ( strcmp(fromUser, check) == 0) {
        printf("aha! new user");
    }

输出:

length of fromUser: 8
length of check: 8
newuser from user: newuser
check = newuser 
compare fromUser to check:

我感觉我没有正确处理传入的缓冲区或错误地复制了缓冲区.

I have a feeling I'm not handling the incoming buffer correctly or erroneously copying the buffer.

推荐答案

strncpy最多复制-在这种情况下-sizeof个校验字节.如果nul字节不在该范围内，则不会复制该字节.您可能会将"newuser"一词作为一个较长句子的一部分，例如"newuser blah blah"，因此您需要自己放置该nul

strncpy copies at most - in this case - sizeof check bytes. If the nul byte is not in that range it is not copied. You probably are getting the word "newuser" as part of a longer sentence, like "newuser blah blah" so you need to place that nul yourself

strncpy(fromUser, buf, sizeof check);
fromUser[sizeof check - 1] = '\0';

或使用strlcpy(如果可用).

这篇关于比较C中的字符串-strcmp的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！