c-struct返回两次打印值的字符数组

Question

这是一个简单的程序，函数abc在其中返回一个数组。但是输出是

Thanks
abcdefThanks

为什么会这样？我想要谢谢只打印一次。此外，我需要将a的大小取为6。在这个程序中，这并不重要，但我在需要的地方进行原始套接字编程。size＝6在那里的预定义头文件中声明。我该如何实现它？

char *abc()
{
    unsigned char *ch;
    unsigned char a[7],c[6];
    strncpy(a,"Thanks",strlen("Thanks"));
    strncpy(c,"abcdef",strlen("abcdef"));
    ch=malloc(50);
    memset(ch,0,50);
    memcpy(ch,&a,strlen(a));
    memcpy(ch+strlen(a)+1,&c,strlen(c));
    return ch;
}
int main()
{
    char *a;
    a=abc();
    printf("n%sn",a);
    printf("n%sn",(a+7));
    fflush(stdout);
    return 0;
}

感谢：(

Answer 1

调用strlen(a)不会停止在您认为应该停止的地方，因为没有零终止符，垃圾内存会破坏您的结果strlen(字符串(不包括零终止符计数

你应该做以下(看看评论(

char *abc()
{
    char *ch;
    char a[7],c[7];
    strncpy(a,"Thanks",strlen("Thanks")); // Watch out, strlen(string) doesn't include null terminator
    a[6] = ''; // Prevent garbage from uninitialized memory to pester your ch and strlen(a)
    strncpy(c,"abcdef",strlen("abcdef"));
    c[6] = '';
    ch=malloc(50);
    memset(ch,0,50);
    memcpy(ch,&a,strlen(a));
    memcpy(ch+strlen(a),&c,strlen(c)); // No -1 because you want to cut the terminator off
    return ch;
}
int main()
{
    char *a;
    a=abc();
    printf("n%sn",a);
    printf("n%sn",(a+7));
    fflush(stdout);
    return 0;
}

上面的内容是用C++编译的，但经过一些调整后应该基本相同。

这里有一个类似转储的内存，其中#表示垃圾

char *abc()
{
    char *ch;
    char a[7],c[7];
    strncpy(a,"Thanks",strlen("Thanks")); // Watch out, strlen(string) doesn't include null terminator
    // a = "Thanks##################################.."
    a[6] = ''; // Prevent garbage from uninitialized memory to pester your ch and strlen(a)
    // a = "Thanks############################"
    strncpy(c,"abcdef",strlen("abcdef"));
    // c = "abcdef############################"
    c[6] = '';
    // c = "abcdef############################"
    ch=malloc(50);
    // ch = "###############################"
    memset(ch,0,50);
    // ch = "000000000000000000000000000000"
    memcpy(ch,&a,strlen(a));
    // ch = "Thanks000000000000000000000000"
    memcpy(ch+strlen(a),&c,strlen(c)); // No -1 because you want to cut the terminator off
    // ch = "Thanksabcdef00000000000000000"
    return ch;
}

Answer 2

您忘记了通过追加0字节来零终止字符串。

unsigned char a[7], c[7]; // 7 = 6+1 since "Thanks" and "abcdef" have 6 bytes
strncpy (a, "Thanks", 6);
a[6] = (char)0;
strncpy (c, "abcdef", 6);
c[6] = (char)0;

您的c字符串太短，并且没有明确地以0结尾。所以你的strlen(c)是未定义的行为