我们应该关注的术语是“流”。
“流”就像一座桥梁，负责以顺序方式传输数据。（程序内外流畅流的和谐由库/头文件管理，例如 stdio.h）
回到你的问题：
When you type input as 'a' and hit 'enter', you supply 2 values to input stream.
- a (ASCII Value : 97)
- enter (ASCII Value : 13)
/*This 'enter' as an input is the devil. To keep it simple, i will keep calling it
as Enter below, and the enter is usually not displayed on screen*/
继续之前的注意/重要/警告：直到您的流没有完全变空，您才能将新字符从控制台写入流中。（此场景仅暗示使用getchar和putchar，如下图）
这是您的代码：
for(i = 0; i < 10; i++)
{
printf("Enter a single character >> ");
ch = getchar();
putchar(ch);
}
循环通行证 1：
a) You ask user to enter a character. // printf statement
b) getchar reads only a single character from stream.
c) putchar renders/displays only a single character from stream.
d) At first pass you provide input as 'a' but you also hit 'Enter'
e) Now, your stream is like a ***QUEUE***, at first pass, and at 1st place of
the queue, you have 'a' and at 2nd place 'enter'.
f) Once you do putchar, the first character , i.e. 'a' from the stream/queue
gets displayed.
e) Loop ends.
g) Output of this pass:
Enter a single character >>a
循环通行证 2：
a) You ask user to enter a character. // printf() statement
b) Unfortunately your stream isn't empty. It has an "enter" value from the
previous pass.
c) So, getchar(), reads the next single character, i.e. 'enter' from stream.
(This is where you were expecting to manually enter the next character,
but the system did it for you. Read the NOTE/IMPORTANT/CAUTION section
mentioned above)
d) putchar() displays 'enter' on screen, but since 'enter' is no displayable
thing, nothing gets displayed.
e) Output of this pass:
Enter a single character >>
Loop Pass 3：
和loop 1类似，这次只输入'b'。
Loop Pass 4：
类似于loop 2
依此类推，直到 10 次通过。（因此，您可以输入的最后一个字符是“e”。）
推论/结论：
So, long story short, you were expecting to enter the next character,
so that getchar would pick your entered value, but since from your
previous pass,'enter' value was already waiting in the stream,
it got displayed first, giving you such an illusion.
谢谢你。如果您的想法不同，请告诉我。