Interview Questions - UNIX File Management
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All UNIX files have its description stored in a structure called 'inode'. The inode contains info about the file-size, its location, time of last access, time of last modification, permission and so on. Directories are also represented as files and have an associated inode. In addition to descriptions about the file, the inode contains pointers to the data blocks of the file. If the file is large, inode has indirect pointer to a block of pointers to additional data blocks (this further aggregates for larger files). A block is typically 8k.Inode consists of the following fields:
- File owner identifier
- File type
- File access permissions
- File access times
- Number of links
- File size
- Location of the file data
- open(pathname,flag,mode) - open file
- creat(pathname,mode) - create file
- close(filedes) - close an open file
- read(filedes,buffer,bytes) - read data from an open file
- write(filedes,buffer,bytes) - write data to an open file
- lseek(filedes,offset,from) - position an open file
- dup(filedes) - duplicate an existing file descriptor
- dup2(oldfd,newfd) - duplicate to a desired file descriptor
- fcntl(filedes,cmd,arg) - change properties of an open file
- ioctl(filedes,request,arg) - change the behaviour of an open file
- The difference between fcntl anf ioctl is that the former is intended for any open file, while the latter is for device-specific operations.