Question

这是一个简单的程序，其中函数abc返回一个数组。但输出是

Thanks
abcdefThanks

为什么这样？我希望 Thanks 只打印一次。此外，我需要将 a 的大小设为6.在这个程序中，它并不重要，但我正在进行原始套接字编程，我需要它。 size = 6在那里的预定义头文件中声明。我该如何实施呢？

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%s\n",a);
    printf("\n%s\n",(a+7));
    fflush(stdout);
    return 0;
}

谢谢:)

Answer 1

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

您应该执行以下操作（查看评论）

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] = '\0'; // Prevent garbage from uninitialized memory to pester your ch and strlen(a)
    strncpy(c,"abcdef",strlen("abcdef"));
    c[6] = '\0';
    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%s\n",a);
    printf("\n%s\n",(a+7));
    fflush(stdout);
    return 0;
}

以上内容是用C ++编译的，但几乎可以通过一些调整来实现。

这里有类似内存的转储，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] = '\0'; // Prevent garbage from uninitialized memory to pester your ch and strlen(a)
    // a = "Thanks\0############################"
    strncpy(c,"abcdef",strlen("abcdef"));
    // c = "abcdef############################"
    c[6] = '\0';
    // c = "abcdef\0############################"
    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

您忘记通过附加0字节来终止字符串。

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;

你的c字符串太短而且未明确地为0结尾。因此，strlen(c)为undefined behavior

struct returns character array两次打印一个值

2 个答案: