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Home Page: http://blog.tangzhixiong.com/notes/

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notes's Issues

位运算

#include <stdio.h>

#define PRINT(x) printf("hex: 0x%08x, dec: %12d, str: \"%s\"\n", x, x, #x)

int main()
{
    puts( "全 0 & 全 1" );
    PRINT( 0 );
    PRINT( -1 );

    puts( "\n全 1 的左右 shift" );
    PRINT( -1>>1 );
    PRINT( -1<<1 );

    puts( "\n减法优先级更高" );
    PRINT( 1<<31-1 );
    PRINT( 1<<30 );

    puts( "\n怎么得到 int 的最大正数" );
    PRINT( 1<<31 );
    PRINT( (1<<31)-1 );
    PRINT( ((unsigned int)1<<31)-1 );
    PRINT( ((unsigned int)-1)>>1 );
    PRINT( -1>>1 );
    PRINT( 0x7FFFFFFF );
}

编译：gcc test.c -o test，

运行：./test

结果：

全 0 & 全 1
hex: 0x00000000, dec:            0, str: "0"
hex: 0xffffffff, dec:           -1, str: "-1"

全 1 的左右 shift
hex: 0xffffffff, dec:           -1, str: "-1>>1"
hex: 0xfffffffe, dec:           -2, str: "-1<<1"

减法优先级更高
hex: 0x40000000, dec:   1073741824, str: "1<<31-1"
hex: 0x40000000, dec:   1073741824, str: "1<<30"

怎么得到 int 的最大正数
hex: 0x80000000, dec:  -2147483648, str: "1<<31"
hex: 0x7fffffff, dec:   2147483647, str: "(1<<31)-1"
hex: 0x7fffffff, dec:   2147483647, str: "((unsigned int)1<<31)-1"
hex: 0x7fffffff, dec:   2147483647, str: "((unsigned int)-1)>>1"
hex: 0xffffffff, dec:           -1, str: "-1>>1"
hex: 0x7fffffff, dec:   2147483647, str: "0x7FFFFFFF"

不要被别人的 memset(buf, -1, sizeof(buf)) 迷惑了

有文件 test.c

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

int main()
{
    int m[2];
    printf( "sizeof(m): %lu\n", sizeof(m) );

    memset( m, -1, sizeof(m) );
    printf( "after memset(m,   -1, sizeof(m)): %20d (0x%08x), %20d (0x%08x)\n", m[0], m[0], m[1], m[1] );

    memset( m,  1, sizeof(m) );
    printf( "after memset(m,    1, sizeof(m)): %20d (0x%08x), %20d (0x%08x)\n", m[0], m[0], m[1], m[1] );

    memset( m, 0xFF, sizeof(m) );
    printf( "after memset(m, 0xFF, sizeof(m)): %20d (0x%08x), %20d (0x%08x)\n", m[0], m[0], m[1], m[1] );

    memset( m, 0xA5, sizeof(m) );
    printf( "after memset(m, 0xA5, sizeof(m)): %20d (0x%08x), %20d (0x%08x)\n", m[0], m[0], m[1], m[1] );
}

编译运行

$ gcc test.c -o test
$ ./test
sizeof(m): 8
after memset(m,   -1, sizeof(m)):                   -1 (0xffffffff),                   -1 (0xffffffff)
after memset(m,    1, sizeof(m)):             16843009 (0x01010101),             16843009 (0x01010101)
after memset(m, 0xFF, sizeof(m)):                   -1 (0xffffffff),                   -1 (0xffffffff)
after memset(m, 0xA5, sizeof(m)):          -1515870811 (0xa5a5a5a5),          -1515870811 (0xa5a5a5a5)

所以，要小心了。别人用 memset(buf, -1, sizeof(buf)) 来初始化 buf，你用 -2 可不是 -2 啊！

这涉及到 two's compliment 编码。

另一个不符合直觉的是 int buf[50] = { 0 }; 确实会把所有元素初始化为 0，但是 int buf[50] = { 3 }; 只回初始化第一个元素为 3，其它都是 0……至少在我的 gcc 和 g++ 上都是如此。

MathJax

$$x^2=\frac{y}{z}$$

district10 / notes Goto Github PK

notes's Issues

位运算

不要被别人的 memset(buf, -1, sizeof(buf)) 迷惑了

MathJax

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