首先snprintf()的第二個參數代表的是緩衝區的大小，在這裡應該是LENGTH，sizeof(LENGTH)的值是4
（我猜你想寫的應該是size (cache)吧）。那麼，改成snprintf(cache, LENGTH, "%s/ruaruarua", cache);之後就行了嗎？我們來看看這個例子：

#include <stdio.h>

int main()
{
    char buf[20] = "hello";
    snprintf(buf, 20, "%s world", buf);
    puts(buf);
    return 0;
}

這個範例企圖給buf末尾加上一個字串，看看輸出

 world

並沒有達到期望的結果。這是為什麼呢？ snprintf()的手冊裡有這麼一段：

Some programs imprudently rely on code such as the following

sprintf(buf, "%s some further text", buf);

to append text to buf. However, the standards explicitly note that the results are undefined if source and destination buffers overlap when calling sprintf(), (), vsnprintf( ). Depending on the version of gcc(1) used, and the compiler options employed, calls such as the above will not produce the expected results. 即如果原始和目標緩衝區重疊的話，這些printf()

家族的函數執行結果將是未定義的。

那該如何「向一個陣列中印出自己原有的內容和一些新的內容」呢？一種方式是把傳給

snprintf()

snprintf(buf + strlen(buf), 20, " world");

#include <stdio.h>
#include <string.h>

int main()
{
    char buf[40];
    char *p = buf;

    p += sprintf(p, "hello %s\n", "world");
    p += sprintf(p, "hello %s", "again");
    /* and more similar calls... */

    puts(buf);
    return 0;
}

hello world
hello again