malloc的堆内存，内存越界导致挂死。
还有内存被覆盖挂死。(本内存被别人写了)
访问没有申请的内存，挂死。
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0xffffffff -1
0xfffffffE -2
0xfffffffD -3
0xfffffffC -4
0xfffffffB -5
0xfffffffA -6
0xfffffff9 -7
0xfffffff8 -8
0xfffffff7 -9
0xfffffff6 -10
0xfffffff5 -11
0xfffffff4 -12
The C standard doesn't mandate any particular way of representing negative signed numbers.
In most implementations that you are likely to encounter, negative signed integers are stored in what is called two's complement. The other major way of storing negative signed numbers is called one's complement.
The two's complement of an N-bit number x is defined as 2^N - x. For example, the two's complement of 8-bit 1 is 2^8 - 1, or 1111 1111. The two's complement of 8-bit 8 is 2^8 - 8, which in binary is 1111 1000. This can also be calculated by flipping the bits of x and adding one. For example:
1 = 0000 0001
~1 = 1111 1110
~1 + 1 = 1111 1111
-1 = 1111 1111
21 = 0001 0101
~21 = 1110 1010
~21 + 1 = 1110 1011
-21 = 1110 1011
The one's complement of an N-bit number x is defined as x with all its bits flipped, basically.
1 = 0000 0001
-1 = 1111 1110
21 = 0001 0101
-21 = 1110 1010
Two's complement has several advantages over one's complement. For example, it doesn't have the concept of 'negative zero', which for good reason is confusing to many people. Addition, multiplication and subtraction work the same with signed integers implemented with two's complemented as they do with unsigned integers as well.
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answered May 14 '14 at 9:44
Miles Rout
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There are three well known methods for representing negative values in binary:
Signed magnitude. This is the easiest to understand, because it works the same as we are used to when dealing with negative decimal values: The first position (bit) represents the sign (0 for positive, 1 for negative), and the other bits represent the number. Although it is easy for us to understand, it is hard for computers to work with, especially when doing arithmetic with negative numbers.
In 8-bit signed magnitude, the value 8 is represented as 0 0001000 and -8 as 1 0001000.
One's complement. In this representation, negative numbers are created from the corresponding positive number by flipping all the bits and not just the sign bit. This makes it easier to work with negative numbers for a computer, but has the complication that there are two distinct representations for +0 and -0. The flipping of all the bits makes this harder to understand for humans.
In 8-bit one's complement, the value 8 is represented as 00001000 and -8 as 11110111.
Two's complement. This is the most common representation used nowadays for negative integers because it is the easiest to work with for computers, but it is also the hardest to understand for humans. When comparing the bit patterns used for negative values between one's complement and two's complement, it can be observed that the same bit pattern in two's complement encodes for the next lower number. For example 11111111 stands for -0 in one's complement and for -1 in two's complement, and similarly for 10000000 (-127 vs -128).
In 8-bit two's complement, the value 8 is represented as 00001000 and -8 as 11111000.