为什么我们需要memset始终为零？

Question

Whenever we use memset we set it with zero.

Why? Why not with 1 or 2 or something else.

Also, setting a struct to 0 seems to work but setting to 1 doesn't:

typedef struct abc{
    int a;
} abc;

int main()
{
    abc* ab;
    memset(ab, 0, sizeof(abc));// it sets abc->a = 0; correct
}

But instead of 0 if I use 1 like:

memset(ab, 1, sizeof(abc));

then the value of abc->a = garbage or not equals to 1

Why?

Answer 1

You don't always need to memset to 0, this is just the most common (and useful) thing to do.

memset sets each byte to some given value. An int consists of 4 bytes, so, when memset ing to 1 , you'd set each of those 4 to 1 , then you'd have 00000001 | 00000001 | 00000001 | 00000001 00000001 | 00000001 | 00000001 | 00000001 00000001 | 00000001 | 00000001 | 00000001 ₂ = 16843009 ₁₀ (the first numbers are in binary, the last in decimal).

Also - note that you're never allocating memory for ab . Even though your code may work now, it's not safe. This would be:

abc ab;
memset(&ab, 0, sizeof(abc));

Answer 2

As others have mentioned, 0 is largely arbitrary. However, if we had to pick reasons, it's for convenience (at the cost of safety - you should prefer to explicitly initialise variables if you ever depend on them):

• 0 is mostly understood as an initial value for integers, signed or unsigned, as it represents 0 no matter the width or endianness
• 0 is an end-of-string for char* s
• 0 is often 0.0 for floating point numbers
• 0 is often NULL in C, guaranteed to be NULL in C++

Answer 3

The value you use with memset() to set memory to depends on your needs. That's all. No one keeps you from using any other value to initialise memory.

Answer 4

As shown in your program When variable is defined, by default it will point to any location and any junk data can be previously available there.

we generally memset to (0)zero to make sure that variable is initialized to zero, so that later on in the program we can trust that the value assigned to the variable is genuinely assigned value and not any garbage.

hope it helps.....

Answer 5

Firstly your program got an undefined behavior the reaosn is memory for abc* ab; is never allocated!

Secondly it is required that string should be terminated with "null", which mean it is the end, so inspired by this we use null in memset. But you can use any value you want...