文章目錄

• • • • 邊界對齊子命令使用
      • 源碼分析
      • • sgdisk.cc main函數入口
        • gptcl.cc DoOptions解析并執行具體命令函數
        • gpt.cc CreatePartition創建分區函數，設置起始扇區對齊
        • • gpt.cc Align分區對齊函數，設置起始扇區對齊

sgdisk命令是由 gdisk-0.8.6-4.el7.x86_64程序包安裝

sgdisk源碼路徑 https://github.com/Shihta/gdisk

邊界對齊子命令使用

本節主要描述sgdisk源碼中關于命令-a --set-alignment=value的邊界對齊邏輯描述,借此各位可以看到sgdisk命令集的其他參數源碼白編寫的邏輯
該參數使用方式一般是在創建分區過程中使用
sgdisk --set-alignment=8 --new=1:0:2G --mbrtogpt -- /dev/sdb
這個命令是在/dev/sdb上創建一個編號為1，大小為2G的gpt分區，且start_sector是4K對齊的。sgdisk底層做分區是以扇區為單位，同時設置邊界對齊時單位是扇區，默認以1M（2048個扇區）為單位進行對齊，最小只能設置8個扇區對齊

源碼分析

sgdisk.cc main函數入口

int main(int argc, char *argv[]) {GPTDataCL theGPT;#主要執行函數，對參數進行解析以及調用各個執行函數執行具體功能return theGPT.DoOptions(argc, argv);
} 

gptcl.cc DoOptions解析并執行具體命令函數

/*
該函數的返回值如下：
0 ：執行命令成功
1：執行參數過少
2：讀磁盤分區表錯誤
3：格式化磁盤分區格式為gpt時沒有-g參數
4：無法保存sgdisk對磁盤的操作
8：備份磁盤分區表失敗,-R參數failed
*/
int GPTDataCL::DoOptions(int argc, char* argv[]) {...#將參數長項和短項以及對應的描述封裝為結構體struct poptOption theOptions[] ={{"attributes", 'A', POPT_ARG_STRING, &attributeOperation, 'A', "operate on partition attributes","list|[partnum:show|or|nand|xor|=|set|clear|toggle|get[:bitnum|hexbitmask]]"},{"set-alignment", 'a', POPT_ARG_INT, &alignment, 'a', "set sector alignment", "value"},{"backup", 'b', POPT_ARG_STRING, &backupFile, 'b', "backup GPT to file", "file"},{"change-name", 'c', POPT_ARG_STRING, &partName, 'c', "change partition's name", "partnum:name"},{"recompute-chs", 'C', POPT_ARG_NONE, NULL, 'C', "recompute CHS values in protective/hybrid MBR", ""},{"delete", 'd', POPT_ARG_INT, &deletePartNum, 'd', "delete a partition", "partnum"},{"display-alignment", 'D', POPT_ARG_NONE, NULL, 'D', "show number of sectors per allocation block", ""},{"move-second-header", 'e', POPT_ARG_NONE, NULL, 'e', "move second header to end of disk", ""},{"end-of-largest", 'E', POPT_ARG_NONE, NULL, 'E', "show end of largest free block", ""},{"first-in-largest", 'f', POPT_ARG_NONE, NULL, 'f', "show start of the largest free block", ""},...}...// 確保最后一個參數一定是設備名稱（之前會做參數篩選)device = (char*) poptGetArg(poptCon);poptResetContext(poptCon);//如果設備名稱不為空的話開始進行具體的參數解析if (device != NULL) {JustLooking(); // reset as necessaryBeQuiet(); // Tell called functions to be less verbose & interactive//加載磁盤分區表if (LoadPartitions((string) device)) {...//開始進行具體的sgdisk參數解析while ((opt = poptGetNextOpt(poptCon)) > 0) {...//設置分區邊界對齊的短項參數  對應--set-alignment=valuecase 'a'://初始化分區邊界對齊參數 ，會為全局變量 sectorAlignment 賦值SetAlignment(alignment);break;...//創建分區參數短項，對應--new=num:start_sector:end_sectorcase 'n':JustLooking(0);//獲取分區編號newPartNum = (int) GetInt(newPartInfo, 1) - 1;//如果分區編號小于零則選擇第一個沒有被使用的編號（最小的沒有被使用的）if (newPartNum < 0)newPartNum = FindFirstFreePart();//從磁盤的剩余空間中尋找最大的未被分配的塊，取該塊中的第一個可用的扇區作為起始扇區配置low = FindFirstInLargest();/*將該扇區調整為512字節，因為不通磁盤有自己的扇區大小設置，這里會將這種類型的磁盤扇區大小設置為操作系統識別的512字節*/Align(&low);//依據該塊空間的起始扇區位置尋找終止扇區位置high = FindLastInFree(low);//這里做一個轉換，將我們創建分區在start和end處輸入的IEEE-1541-2002 value(K,M,G,T,P,E)轉換為扇區單位startSector = IeeeToInt(GetString(newPartInfo, 2), sSize, low, high, low);endSector = IeeeToInt(GetString(newPartInfo, 3), sSize, startSector, high, high);//創建分區：成功返回1，失敗返回0if (CreatePartition(newPartNum, startSector, endSector)) {saveData = 1;} else {cerr << "Could not create partition " << newPartNum + 1 << " from "<< startSector << " to " << endSector << "\n";neverSaveData = 1;} // if/else//釋放存放參數列表的char *空間free(newPartInfo);break;...}...}

gpt.cc CreatePartition創建分區函數，設置起始扇區對齊

uint32_t GPTData::CreatePartition(uint32_t partNum, uint64_t startSector, uint64_t endSector) {int retval = 1; // assume there'll be no problemsuint64_t origSector = startSector;if (IsFreePartNum(partNum)) {//確保起始扇區是能夠被我們設置的邊界對齊參數整除（全局變量sectorAlignment）/*Align(&startSector)函數操作如下 1. 如果startSector % sectorAlignment == 0那么直接返回成功2. 否則 假設startSector=2049,sectorAlignment=8該函數會定位兩個區間[2048,2049]和[2049-2056]，先從該第一個區間起始扇區++查看是否未被占用，如未被占用，則用作起始扇區否則從第二個區間2056扇區--查看是否未被占用，如未被占用，則用作起始扇區詳細可查看如下Align(&startSector)實現*/if (Align(&startSector)) {cout << "Information: Moved requested sector from " << origSector << " to "<< startSector << " in\norder to align on " << sectorAlignment<< "-sector boundaries.\n";} // ifif (IsFree(startSector) && (startSector <= endSector)) {if (FindLastInFree(startSector) >= endSector) {partitions[partNum].SetFirstLBA(startSector);partitions[partNum].SetLastLBA(endSector);partitions[partNum].SetType(DEFAULT_GPT_TYPE);partitions[partNum].RandomizeUniqueGUID();} else retval = 0; // if free space until endSector} else retval = 0; // if startSector is free} else retval = 0; // if legal partition numberreturn retval;
}

gpt.cc Align分區對齊函數，設置起始扇區對齊

int GPTData::Align(uint64_t* sector) {int retval = 0, sectorOK = 0;uint64_t earlier, later, testSector;if ((*sector % sectorAlignment) != 0) {earlier = (*sector / sectorAlignment) * sectorAlignment;later = earlier + (uint64_t) sectorAlignment;// Check to see that every sector between the earlier one and the// requested one is clear, and that it's not too early....if (earlier >= mainHeader.firstUsableLBA) {sectorOK = 1;testSector = earlier;do {sectorOK = IsFree(testSector++);} while ((sectorOK == 1) && (testSector < *sector));if (sectorOK == 1) {*sector = earlier;retval = 1;} // if} // if firstUsableLBA check// If couldn't move the sector earlier, try to move it later instead....if ((sectorOK != 1) && (later <= mainHeader.lastUsableLBA)) {sectorOK = 1;testSector = later;do {sectorOK = IsFree(testSector--);} while ((sectorOK == 1) && (testSector > *sector));if (sectorOK == 1) {*sector = later;retval = 1;} // if} // if} // ifreturn retval;
} // GPTData::Align()

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