打印已声明但未分配的变量在C中会发生什么？

Question

Why does the following snippet cause random numbers to print to the screen with printf , but putchar always outputs 1?

#include <stdio.h>

int main() {
    char c;

    printf("%d\n", c );
    putchar(c);
}

Answer 1

According to C99 standard, this is undefined behavior . Let's see why:

Section 6.7.8.9 says that

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.

This applies to your variable c , because it has automatic storage duration, and it is not initialized explicitly.

Section J.2 says that

The behavior is undefined in the following circumstances:

...

The value of an object with automatic storage duration is used while it is indeterminate

This applies to your code as well, because you read c when you pass it as a parameter to both printf and putchar , and the value of c is still indeterminate, because it has not been assigned.

Answer 2

1) The c variable first has a random value(default/garbage value) to itself as you declared-but-did-not-initialize your char c to any defined letter or value of ur interest(character).
2) Next you tried to printf the %d(digit/decimal/numerical value) of the char c, so now it is giving you a converted value of the garbage which was earlier assigned to c when you declared the char c in the first place.
3) Finally you tried to use putchar(c), which again behaves similarly because your char c is uninitialized and is still being read thereby re-trying to manage with an undetermined value to be printed onto the screen. (since the same un-initialized character variable c is being passed to both kind of printing as a parameter).

Yes these 3 statements are a bit clumsy to understand but they are as layman as it can get to help speed-up some understanding regarding this query of yours.

Pay attention to the 1st comment response to your question by @bluemoon. Those 3 words alone litterally have a huge amount of sensibility and meaningfull-ness to them, to a point that it also tells you what you have done erroneous in your own code(your actions)."UNDEFINED"(try relating the same with UNINITIALIZED).