{"id":3492,"date":"2017-08-06T08:34:54","date_gmt":"2017-08-06T00:34:54","guid":{"rendered":"https:\/\/ixyzero.com\/blog\/?p=3492"},"modified":"2017-08-06T08:34:54","modified_gmt":"2017-08-06T00:34:54","slug":"c%e8%af%ad%e8%a8%80%e5%ad%97%e8%8a%82%e5%af%b9%e9%bd%90","status":"publish","type":"post","link":"https:\/\/ixyzero.com\/blog\/archives\/3492.html","title":{"rendered":"C\u8bed\u8a00\u5b57\u8282\u5bf9\u9f50"},"content":{"rendered":"

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\u4e8c\u3001\u5b57\u8282\u5bf9\u9f50\u7684\u6548\u679c\u6f14\u793a\u4ee3\u7801<\/h6>\n
#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <string.h>\r\n\r\ntypedef struct {\r\n    char  c1; \/\/ 2\r\n    short s;  \/\/ 2\r\n    char  c2; \/\/ 4\r\n    int   i;  \/\/ 4\r\n} T_FOO;\r\n\r\ntypedef struct\r\n{\r\n  int a;    \/\/ 4\r\n  char b;   \/\/ 2\r\n  short c;  \/\/ 2\r\n} A;\r\n\r\ntypedef struct\r\n{\r\n  char b;   \/\/ 4\r\n  int a;    \/\/ 4\r\n  short c;  \/\/ 4\r\n} B;\r\n\r\n#pragma pack (2) \/*\u6307\u5b9a\u63092\u5b57\u8282\u5bf9\u9f50*\/\r\ntypedef struct {\r\n    char b;   \/\/ 2\r\n    int a;    \/\/ 4\r\n    short c;  \/\/ 2\r\n} C;\r\n#pragma pack () \/*\u53d6\u6d88\u6307\u5b9a\u5bf9\u9f50\uff0c\u6062\u590d\u7f3a\u7701\u5bf9\u9f50*\/\r\n\r\n#pragma pack (1) \/*\u6307\u5b9a\u63091\u5b57\u8282\u5bf9\u9f50*\/\r\ntypedef struct {\r\n    char b;   \/\/ 1\r\n    int a;    \/\/ 4\r\n    short c;  \/\/ 2\r\n} D;\r\n#pragma pack ()\/*\u53d6\u6d88\u6307\u5b9a\u5bf9\u9f50\uff0c\u6062\u590d\u7f3a\u7701\u5bf9\u9f50*\/\r\n\r\nint main(int argc, char const *argv[]) {\r\n    A xx;\r\n    \/\/      a -> 0, b -> 4, c -> 6\r\n    printf(\"a -> %d, b -> %d, c -> %d\\n\",\r\n          (unsigned int)(void*)&xx.a - (unsigned int)(void*)&xx,\r\n          (unsigned int)(void*)&xx.b - (unsigned int)(void*)&xx,\r\n          (unsigned int)(void*)&xx.c - (unsigned int)(void*)&xx);\r\n    printf(\"sizeof(A) = %d\\n\", sizeof(A));  \/\/ 8\r\n\r\n    B yy;\r\n    \/\/      b -> 0, a -> 4, c -> 8\r\n    printf(\"b -> %d, a -> %d, c -> %d\\n\",\r\n          (unsigned int)(void*)&yy.b - (unsigned int)(void*)&yy,\r\n          (unsigned int)(void*)&yy.a - (unsigned int)(void*)&yy,\r\n          (unsigned int)(void*)&yy.c - (unsigned int)(void*)&yy);\r\n    printf(\"sizeof(B) = %d\\n\", sizeof(B));  \/\/ 12\r\n\r\n    C cc;\r\n    \/\/      b -> 0, a -> 2, c -> 6\r\n    printf(\"b -> %d, a -> %d, c -> %d\\n\",\r\n          (unsigned int)(void*)&cc.b - (unsigned int)(void*)&cc,\r\n          (unsigned int)(void*)&cc.a - (unsigned int)(void*)&cc,\r\n          (unsigned int)(void*)&cc.c - (unsigned int)(void*)&cc);\r\n    printf(\"sizeof(C) = %d\\n\", sizeof(C));  \/\/ 8\r\n\r\n    D dd;\r\n    \/\/      b -> 0, a -> 1, c -> 5\r\n    printf(\"b -> %d, a -> %d, c -> %d\\n\",\r\n          (unsigned int)(void*)&dd.b - (unsigned int)(void*)&dd,\r\n          (unsigned int)(void*)&dd.a - (unsigned int)(void*)&dd,\r\n          (unsigned int)(void*)&dd.c - (unsigned int)(void*)&dd);\r\n    printf(\"sizeof(D) = %d\\n\", sizeof(D));  \/\/ 7\r\n\r\n    return 0;\r\n}<\/pre>\n
\u4e09\u3001\u7f16\u8bd1\u5668\u662f\u6309\u7167\u4ec0\u4e48\u6837\u7684\u539f\u5219\u8fdb\u884c\u5bf9\u9f50\u7684?<\/h6>\n
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    \n
  1. \u6570\u636e\u7c7b\u578b\u81ea\u8eab\u7684\u5bf9\u9f50\u503c<\/span><\/strong>\uff1a\u5bf9\u4e8echar\u578b\u6570\u636e\uff0c\u5176\u81ea\u8eab\u5bf9\u9f50\u503c\u4e3a1\uff0c\u5bf9\u4e8eshort\u578b\u4e3a2\uff0c\u5bf9\u4e8eint,float,double\u7c7b\u578b\uff0c\u5176\u81ea\u8eab\u5bf9\u9f50\u503c\u4e3a4\uff0c\u5355\u4f4d\u662f\u5b57\u8282\u3002<\/li>\n
  2. \u7ed3\u6784\u4f53\u6216\u8005\u7c7b\u7684\u81ea\u8eab\u5bf9\u9f50\u503c\uff1a\u5176\u6210\u5458\u4e2d\u81ea\u8eab\u5bf9\u9f50\u503c\u6700\u5927\u7684\u90a3\u4e2a\u503c\u3002<\/li>\n
  3. \u6307\u5b9a\u5bf9\u9f50\u503c\uff1a#pragma pack (value)<\/code>\u65f6\u7684\u6307\u5b9a\u5bf9\u9f50\u503cvalue\u3002<\/li>\n
  4. \u6570\u636e\u6210\u5458\u3001\u7ed3\u6784\u4f53\u548c\u7c7b\u7684\u6709\u6548\u5bf9\u9f50\u503c\uff1a\u81ea\u8eab\u5bf9\u9f50\u503c\u548c\u6307\u5b9a\u5bf9\u9f50\u503c\u4e2d\u5c0f\u7684\u90a3\u4e2a\u503c\u3002<\/li>\n<\/ol>\n
    \u6709\u4e86\u8fd9\u4e9b\u503c\uff0c\u6211\u4eec\u5c31\u53ef\u4ee5\u5f88\u65b9\u4fbf\u7684\u6765\u8ba8\u8bba\u5177\u4f53\u6570\u636e\u7ed3\u6784\u7684\u6210\u5458\u548c\u5176\u81ea\u8eab\u7684\u5bf9\u9f50\u65b9\u5f0f\u3002\u6709\u6548\u5bf9\u9f50\u503cN\u662f\u6700\u7ec8\u7528\u6765\u51b3\u5b9a\u6570\u636e\u5b58\u653e\u5730\u5740\u65b9\u5f0f\u7684\u503c\uff0c\u6700\u91cd\u8981\u3002\u6709\u6548\u5bf9\u9f50\u503cN\uff0c\u5c31\u662f\u8868\u793a\u201c\u5bf9\u9f50\u5728N\u4e0a\u201d\uff0c\u4e5f\u5c31\u662f\u8bf4\u8be5\u6570\u636e\u7684\u201c\u5b58\u653e\u8d77\u59cb\u5730\u5740%N=0\u201d\u3002\u800c\u6570\u636e\u7ed3\u6784\u4e2d\u7684\u6570\u636e\u53d8\u91cf\u90fd\u662f\u6309\u5b9a\u4e49\u7684\u5148\u540e\u987a\u5e8f\u6765\u6392\u653e\u7684\u3002\u7b2c\u4e00\u4e2a\u6570\u636e\u53d8\u91cf\u7684\u8d77\u59cb\u5730\u5740\u5c31\u662f\u6570\u636e\u7ed3\u6784\u7684\u8d77\u59cb\u5730\u5740\u3002\u7ed3\u6784\u4f53\u7684\u6210\u5458\u53d8\u91cf\u8981\u5bf9\u9f50\u6392\u653e\uff0c\u7ed3\u6784\u4f53\u672c\u8eab\u4e5f\u8981\u6839\u636e\u81ea\u8eab\u7684\u6709\u6548\u5bf9\u9f50\u503c\u5706\u6574(\u5c31\u662f\u7ed3\u6784\u4f53\u6210\u5458\u53d8\u91cf\u5360\u7528\u603b\u957f\u5ea6\u9700\u8981\u662f\u7ed3\u6784\u4f53\u6709\u6548\u5bf9\u9f50\u503c\u7684\u6574\u6570\u500d)\u3002\u8fd9\u6837\u5c31\u4e0d\u96be\u7406\u89e3\u4e0a\u9762\u7684\u4f8b\u5b50\u4e86\u3002<\/p>\n
    \u56db\u3001\u5982\u4f55\u4fee\u6539\u7f16\u8bd1\u5668\u7684\u9ed8\u8ba4\u5bf9\u9f50\u503c\uff1f<\/h6>\n
    \u5728\u7f16\u7801\u65f6\uff0c\u53ef\u4ee5\u901a\u8fc7 #pragma pack (value)<\/code> \u7684\u65b9\u5f0f\u52a8\u6001\u4fee\u6539\u3002\u6ce8\u610f\uff1a\u662f pragma \u800c\u4e0d\u662f progma \u3002<\/strong><\/span><\/p>\n
    \u4e94\u3001\u9488\u5bf9\u5b57\u8282\u5bf9\u9f50\uff0c\u6211\u4eec\u5728\u7f16\u7a0b\u4e2d\u6709\u4ec0\u4e48\u9700\u8981\u6ce8\u610f\u7684\uff1f<\/h6>\n
    \u57fa\u672c\u7684\u539f\u5219\u5c31\u662f\u628a\u7ed3\u6784\u4e2d\u7684\u53d8\u91cf\u6309\u7167\u7c7b\u578b\u5927\u5c0f\u4ece\u5c0f\u5230\u5927\u58f0\u660e\uff0c\u5c3d\u91cf\u51cf\u5c11\u4e2d\u95f4\u7684\u586b\u8865\u7a7a\u95f4\u3002<\/strong><\/span><\/p>\n
    \u8fd8\u6709\u4e00\u79cd\u5c31\u662f\u4e3a\u4e86\u4ee5\u7a7a\u95f4\u6362\u65f6\u95f4\uff0c\u6211\u4eec\u663e\u793a\u7684\u586b\u8865\u7a7a\u95f4\u4ee5\u8fdb\u884c\u5bf9\u9f50\uff0c\u6bd4\u5982\uff1a\u663e\u5f0f\u7684\u63d2\u5165reserved\u6210\u5458\uff1a<\/p>\n
    typedef struct {\r\n  char a;\r\n  char reserved[3]; \/\/\u4f7f\u7528\u7a7a\u95f4\u6362\u65f6\u95f4\r\n  int b;\r\n} A;<\/pre>\n
    reserved\u6210\u5458\u5bf9\u6211\u4eec\u7684\u7a0b\u5e8f\u6ca1\u6709\u4ec0\u4e48\u610f\u4e49\uff0c\u5b83\u53ea\u662f\u8d77\u5230\u586b\u8865\u7a7a\u95f4\u4ee5\u8fbe\u5230\u5b57\u8282\u5bf9\u9f50\u7684\u76ee\u7684\uff0c\u5f53\u7136\u5373\u4f7f\u4e0d\u52a0\u8fd9\u4e2a\u6210\u5458\u901a\u5e38\u7f16\u8bd1\u5668\u4e5f\u4f1a\u7ed9\u6211\u4eec\u81ea\u52a8\u586b\u8865\u5bf9\u9f50\uff0c\u6211\u4eec\u81ea\u5df1\u52a0\u4e0a\u5b83\u53ea\u662f\u8d77\u5230\u663e\u5f0f\u7684\u63d0\u9192\u4f5c\u7528\u3002<\/p>\n
    \n
    \u4ee5\u4e0a\u5c31\u662f\u6211\u5728\u5b66\u4e60C\u8bed\u8a00\u5b57\u8282\u5bf9\u9f50\u65f6\u53c2\u8003\u7f51\u4e0a\u7684\u4e00\u4e9b\u6587\u7ae0\u4ee5\u53ca\u5b9e\u9645\u7f16\u5199\u6837\u4f8b\u8fdb\u884c\u6d4b\u8bd5\u603b\u7ed3\u7684\u5185\u5bb9\uff0c\u5728\u6b64\u8bb0\u5f55\u4e00\u4e0b\uff0c\u65b9\u4fbf\u4ee5\u540e\u53c2\u8003\u3002<\/p>\n
    \u53c2\u8003\u94fe\u63a5\uff1a<\/h5>\n
    C\/C++\u5b57\u8282\u5bf9\u9f50\u8be6\u89e3<\/strong><\/span>
    \nhttp:\/\/www.linuxsong.org\/2010\/09\/c-byte-alignment\/<\/a><\/p>\n
    C\u8bed\u8a00\u5b57\u8282\u5bf9\u9f50\u95ee\u9898\u8be6\u89e3<\/strong><\/span> #nice
    \n    http:\/\/www.cnblogs.com\/clover-toeic\/p\/3853132.html<\/a><\/p>\n
    C\u8bed\u8a00\u5b57\u8282\u5bf9\u9f50<\/strong>
    \n    http:\/\/blog.csdn.net\/21aspnet\/article\/details\/6729724<\/a><\/p>\n
    GCC\u4e2d\u7684aligned\u548cpacked\u5c5e\u6027
    \n    http:\/\/blog.shengbin.me\/posts\/gcc-attribute-aligned-and-packed<\/a><\/p>\n
    \u5931\u4f20\u7684C\u7ed3\u6784\u4f53\u6253\u5305\u6280\u827a
    \n    https:\/\/github.com\/ludx\/The-Lost-Art-of-C-Structure-Packing<\/a>
    \n    http:\/\/www.catb.org\/esr\/structure-packing\/<\/a><\/p>\n
    c\u8bed\u8a00\u4e2dstruct\u7684\u5185\u5b58\u5bf9\u9f50
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