open系统调用

open()系统调用(2010-08-0123:22)分类：LinuxFilesystemopen()系统调用的服务例程为sys_open()函数，该函数接收的参数为：要打开的文件的路径名filename、访问模式的一些标志flags，以及如果该文件被创建所需要的许可位掩码mode。如果该系统调用成功，就返回一个文件描述符，也就是指向文件对象的指针数组current->files->fd_array或者current->files->fdtable.fd中新分配给文件的索引；否则，返回-1。 open()系统调用的所有标志---------------------------------------------------------------------include/asm-generic/fcntl.h#defineO_ACCMODE00000003#defineO_RDONLY 00000000  /*为只读而打开*/#defineO_WRONLY 00000001  /*为只写而打开*/#defineO_RDWR      00000002 /*为读和写而打开*/#ifndefO_CREAT/*如果文件不存在则创建它*/#defineO_CREAT     00000100  /*notfcntl*/#endif#ifndefO_EXCL/*对于O_CREAT标志，如果文件已经存在，则失败*/#defineO_EXCL      00000200  /*notfcntl*/#endif#ifndefO_NOCTTY/*从不把文件看作终端*/#defineO_NOCTTY 00000400  /*notfcntl*/#endif#ifndefO_TRUNC/*截断文件（删除所有的现有）*/#defineO_TRUNC     00001000  /*notfcntl*/#endif#ifndefO_APPEND/*总是在文件末尾写*/#defineO_APPEND 00002000#endif#ifndefO_NONBLOCK/*非阻塞打开*/#defineO_NONBLOCK  00004000#endif#ifndefO_DSYNC/*同步写（阻塞，直到物理写终止）*/#defineO_DSYNC     00010000  /*usedtobeO_SYNC,seebelow*/#endif#ifndefFASYNC/*通过信号发出I/O事件*/#defineFASYNC      00020000  /*fcntl,forBSDcompatibility*/#endif#ifndefO_DIRECT#defineO_DIRECT 00040000  /*directdiskaccesshint*/#endif/*大型文件（文件长度大于off_t所能示的范围但小于off64_t）*/#ifndefO_LARGEFILE#defineO_LARGEFILE 00100000#endif #ifndefO_DIRECTORY/*如果文件不是一个，则失败*/#defineO_DIRECTORY 00200000  /*mustbeadirectory*/#endif #ifndefO_NOFOLLOW/*不解析路径名尾部的符号链接*/#defineO_NOFOLLOW  00400000  /*don'tfollowlinks*/#endif#ifndefO_NOATIME/*不更新索引节点的访问时间。*/#defineO_NOATIME01000000#endif#ifndefO_CLOEXEC#defineO_CLOEXEC02000000  /*setclose_on_exec*/#endif---------------------------------------------------------------------有一些标志的定义是因体系结构而异的。 sys_open()定义如下：---------------------------------------------------------------------fs/open.cSYSCALL_DEFINE3(open,constchar__user*,filename,int,flags,int,mode){   longret;    if(force_o_largefile())      flags|=O_LARGEFILE;    ret=do_sys_open(AT_FDCWD,filename,flags,mode);   /*avoidREGPARMbreakageonx86:*/   asmlinkage_protect(3,ret,filename,flags,mode);   returnret;}---------------------------------------------------------------------这个函数的操作如下：首先，调用force_o_largefile()来判断是否支持大文件，若是，则设置标志的O_LARGEFILE位。force_o_largefile()其实是一个宏。这个宏也是因体系结构而异的。其次，调用do_sys_open(AT_FDCWD,filename,flags,mode)来完成实际的打开文件的任务。下面有更详细说明。最后，调用asmlinkage_protect()以使系统调用正确返回。它也是一个宏，为了防止编译器错误而设。其他平台为空，只有x86平台有定义，为：---------------------------------------------------------------------arch/x86/include/asm/linkage.h/* *Makesurethecompilerdoesn'tdoanythingstupidwiththe *argumentsonthestack-theyareownedbythe*caller*,not *thecallee.Thisjustfoolsgccintonotspillingintothem, *andkeepsitfromdoingtailcallrecursionand/orusingthe *stackslotsfortemporaries,sincetheyareliveand"used" *allthewaytotheendofthefunction. * *NOTE!Onx86-64,alltheargumentsareinregisters,sothis *onlymattersona32-bitkernel. */#defineasmlinkage_protect(n,ret,args...)\   __asmlinkage_protect##n(ret,##args)#define__asmlinkage_protect_n(ret,args...)\   __asm____volatile__("":"=r"(ret):"0"(ret),##args)#define__asmlinkage_protect0(ret)\   __asmlinkage_protect_n(ret)#define__asmlinkage_protect1(ret,arg1)\   __asmlinkage_protect_n(ret,"g"(arg1))#define__asmlinkage_protect2(ret,arg1,arg2)\   __asmlinkage_protect_n(ret,"g"(arg1),"g"(arg2))#define__asmlinkage_protect3(ret,arg1,arg2,arg3)\   __asmlinkage_protect_n(ret,"g"(arg1),"g"(arg2),"g"(arg3))--------------------------------------------------------------------- do_sys_open()函数定义如下：---------------------------------------------------------------------fs/open.clongdo_sys_open(intdfd,constchar__user*filename,intflags,intmode){   char*tmp=getname(filename);   intfd=PTR_ERR(tmp);    if(!IS_ERR(tmp)){      fd=get_unused_fd_flags(flags);      if(fd>=0){          structfile*f=do_filp_open(dfd,tmp,flags,mode,0);          if(IS_ERR(f)){             put_unused_fd(fd);             fd=PTR_ERR(f);          }else{             fsnotify_open(f->f_path.dentry);             fd_install(fd,f);          }      }      putname(tmp);   }   returnfd;}---------------------------------------------------------------------第一个参数是多么的眼熟啊，查找路径名的do_path_lookup()函数的第一个参数也是它，用于说明查找路径名的基目录。关于它，前面路径名查找已有说明了。其余参数则是sys_open()将传进来的参数传给了它。这个函数执行如下操作：1、调用getname(filename)从进程地址空间读取文件的路径名，将路径名的地址存放在局部变量tmp。getname(filename)本质上，首先从名为"names_cache"路径名slab缓存中分配内存区，然后将路径名从用户空间复制到该内存区中。 2、调用get_unused_fd_flags(flags)从当前进程的文件描述符表中找一个空位。其定义为：---------------------------------------------------------------------include/linux/file.h#defineget_unused_fd_flags(flags)alloc_fd(0,(flags))---------------------------------------------------------------------这是一个宏，仅仅是对alloc_fd()函数特殊参数下调用的别名，对alloc_fd()函数定义如下：---------------------------------------------------------------------fs/file.c/* *allocateafiledescriptor,markitbusy. */intalloc_fd(unsignedstart,unsignedflags){   structfiles_struct*files=current->files;   unsignedintfd;   interror;   structfdtable*fdt;    spin_lock(&files->file_lock);repeat:   fdt=files_fdtable(files);   fd=start;   if(fd<files->next_fd)                                  fd=files->next_fd;    if(fd<fdt->max_fds)      fd=find_next_zero_bit(fdt->open_fds->fds_bits,                   fdt->max_fds,fd);    error=expand_files(files,fd);   if(error<0)      gotoout;    /*    *Ifweneededtoexpandthefsarraywe    *mighthaveblocked-tryagain.    */   if(error)      gotorepeat;    if(start<=files->next_fd)      files->next_fd=fd+1;    FD_SET(fd,fdt->open_fds);   if(flags&O_CLOEXEC)      FD_SET(fd,fdt->close_on_exec);   else      FD_CLR(fd,fdt->close_on_exec);   error=fd;#if1   /*Sanitycheck*/   if(rcu_dereference_raw(fdt->fd[fd])!=NULL){      printk(KERN_WARNING"alloc_fd:slot%dnotNULL!\n",fd);      rcu_assign_pointer(fdt->fd[fd],NULL);   }#endif out:   spin_unlock(&files->file_lock);   returnerror;}---------------------------------------------------------------------这个函数执行如下操作：a.将current->files->next_fd字段赋值给局部变量。b.调用find_next_zero_bit(fdt->open_fds->fds_bits,fdt->max_fds,fd)来在文件描述符表中寻找下一个可以分配的文件描述符。这个函数也是因体系结构而异，系统中通用的函数定义为：---------------------------------------------------------------------lib/find_next_bit.cunsignedlongfind_next_zero_bit(constunsignedlong*addr,unsignedlongsize,  unsignedlongoffset){   constunsignedlong*p=addr+BITOP_WORD(offset);   unsignedlongresult=offset&~(BITS_PER_LONG-1);   unsignedlongtmp;    if(offset>=size)      returnsize;   size-=result;   offset%=BITS_PER_LONG;   if(offset){      tmp=*(p++);      tmp|=~0UL>>(BITS_PER_LONG-offset);      if(size<BITS_PER_LONG)          gotofound_first;      if(~tmp)          gotofound_middle;      size-=BITS_PER_LONG;      result+=BITS_PER_LONG;   }   while(size&~(BITS_PER_LONG-1)){      if(~(tmp=*(p++)))          gotofound_middle;      result+=BITS_PER_LONG;      size-=BITS_PER_LONG;   }   if(!size)      returnresult;   tmp=*p; found_first:   tmp|=~0UL<<size;   if(tmp==~0UL) /*Areanybitszero?*/      returnresult+size;   /*Nope.*/found_middle:   returnresult+ffz(tmp);}---------------------------------------------------------------------(1)、上面的BITOP_WORD(offset)清一色定义为：#defineBITOP_WORD(nr)     ((nr)/BITS_PER_LONG)首先求得第一个要查找的long的位置。(2)、offset&~(BITS_PER_LONG-1)等价于offset除以32再乘以32，以此来求得所要查找的第一个字第0位在表中的位置，只不过会比除法运算效率高很多。(3)、如果传递的起始查找位置甚至大于最大可能值，则返回最大可能值。(4)、查找第一个0位所在的long型值的位置。而局部变量tmp中会保存该long型量的值。(5)、返回查找到的第一个0位的位置或可能的最大值。 c.调用expand_files(files,fd)，来扩展文件描述符表，files_struct的fd_array数组成员可以在打开的文件较少时使用，但当打开的文件较多时，就会对文件描述符表进行扩展。 d.更新files->next_fd字段，将分配的文件描述符添加进fdt->open_fds，如果设置了O_CLOEXEC则将文件描述符添加进fdt->close_on_exec，若没有，则清除fdt->close_on_exec中的相应位。 e.返回文件描述符。 3、调用do_filp_open(dfd,tmp,flags,mode,0)函数，传递给它的参数依次为查找路径名的基目录、文件路径名、访问模式标志以及许可权位掩码、访问模式位。这个函数定义为：---------------------------------------------------------------------fs/namei.c1761/*1762 *Notethatthelowbitsofthepassedin"open_flag"1763 *arenotthesameasinthelocalvariable"flag".See1764 *open_to_namei_flags()formoredetails.1765 */1766structfile*do_filp_open(intdfd,constchar*pathname,1767                intopen_flag,intmode,intacc_mode)1768{1769        structfile*filp;1770        structnameidatand;1771        interror;1772        structpathpath;1773        intcount=0;1774        intflag=open_to_namei_flags(open_flag);1775        intforce_reval=0;17761777        if(!(open_flag&O_CREAT))1778                mode=0;17791780        /*1781         *O_SYNCisimplementedas__O_SYNC|O_DSYNC. Asmanyplacesonly1782         *checkforO_DSYNCiftheneedanysyncingatallweenforceit's1783         *alwayssetinsteadofhavingtodealwithpossiblyweirdbehaviour1784         *formaliciousapplicationssettingonly__O_SYNC.1785         */1786        if(open_flag&__O_SYNC)1787                open_flag|=O_DSYNC;17881789        if(!acc_mode)1790                acc_mode=MAY_OPEN|ACC_MODE(open_flag);17911792        /*O_TRUNCimpliesweneedaccesschecksforwritepermissions*/1793        if(open_flag&O_TRUNC)1794                acc_mode|=MAY_WRITE;17951796        /*AllowtheLSMpermissionhooktodistinguishappend1797           accessfromgeneralwriteaccess.*/1798        if(open_flag&O_APPEND)1799                acc_mode|=MAY_APPEND;18001801        /*findtheparent*/1802reval:1803        error=path_init(dfd,pathname,LOOKUP_PARENT,&nd);1804        if(error)1805                returnERR_PTR(error);1806        if(force_reval)1807                nd.flags|=LOOKUP_REVAL;18081809        current->total_link_count=0;1810        error=link_path_walk(pathname,&nd);1811        if(error){1812                filp=ERR_PTR(error);1813                gotoout;1814        }1815        if(unlikely(!audit_dummy_context())&&(open_flag&O_CREAT))1816                audit_inode(pathname,nd.path.dentry);18171818        /*1819         *Wehavetheparentandlastcomponent.1820         */18211822        error=-ENFILE;1823        filp=get_empty_filp();1824        if(filp==NULL)1825                gotoexit_parent;1826        nd.intent.open.file=filp;1827        filp->f_flags=open_flag;1828        nd.intent.open.flags=flag;1829        nd.intent.open.create_mode=mode;1830        nd.flags&=~LOOKUP_PARENT;1831        nd.flags|=LOOKUP_OPEN;1832        if(open_flag&O_CREAT){1833                nd.flags|=LOOKUP_CREATE;1834                if(open_flag&O_EXCL)1835                        nd.flags|=LOOKUP_EXCL;1836        }1837        if(open_flag&O_DIRECTORY)1838                nd.flags|=LOOKUP_DIRECTORY;1839        if(!(open_flag&O_NOFOLLOW))1840                nd.flags|=LOOKUP_FOLLOW;1841        filp=do_last(&nd,&path,open_flag,acc_mode,mode,pathname);1842        while(unlikely(!filp)){/*trailingsymlink*/1843                structpathholder;1844                structinode*inode=path.dentry->d_inode;1845                void*cookie;1846                error=-ELOOP;1847                /*S_ISDIRpartisatemporaryautomountkludge*/1848                if(!(nd.flags&LOOKUP_FOLLOW)&&!S_ISDIR(inode->i_mode))1849                        gotoexit_dput;1850                if(count++==32)1851                        gotoexit_dput;1852                /*1853                 *Thisissubtle.Insteadofcallingdo_follow_link()wedo1854                 *thethingbyhands.Thereasonisthatthiswaywehavezero1855                 *link_countandpath_walk()(calledfrom->follow_link)1856                 *honoringLOOKUP_PARENT. Afterthatwehavetheparentand1857                 *lastcomponent,i.e.weareinthesamesituationasafter1858                 *thefirstpath_walk(). Well,almost-ifthelastcomponent1859                 *isnormalwegetitscopystoredinnd->last.nameandwewill1860                 *havetoputname()itwhenwearedone.Procfs-likesymlinks1861              &nb