C programming language Chapter 8 The UNIX System

C programming language Chapter 8 : The UNIX System Interface Presenters: Do Van Quyen, Le Thi Hien, Do Tien Thanh Internship in DASAN networks 21 Feb 2012

Contents 1. 2. 3. 4. 5. 6. 7. System Calls File Descriptors Low Level I/O - Read and Write Open, Creat, Close, Unlink Random Access – Lseek Listing Directories A Storage Allocator dovanquyen. vn@gmail. com 2/24

System Calls (Library calls) § Linux OS Root filesystem User space Linux OS System Call Interface Process Management Virtual file system Kernel. Network Stack Memory Management Arch Kernel space Device Driver Hardware Platform dovanquyen. vn@gmail. com 3/24

File Descriptors § A file descriptor is a low positive integer which indicates for accessing a file Integer value Name 0 Standard Input (stdin) 1 Standard Output (stdout) 2 Standard Error (stderr) § The user program refers to the file only by the file descriptor(Each file is referenced by a file descriptor ) dovanquyen. vn@gmail. com 4/24

Low Level I/O - Read and Write int n_read = read(int fd, char *buf, int n); int n_written = write(int fd, char *buf, int n); § The first argument is a file descriptor § The second argument is a character array in your program where the data is to go to or to come from § The third argument is the number of bytes to be transferred dovanquyen. vn@gmail. com 5/24

Open, Creat, Close, Unlink § Open () #include <fcntl. h> int fd; int open(char *name, int flags, int perms); fd = open(name, flags, perms); § The name argument is a character string containing the filename § The second argument, flags, is an int § O_RDONLY open for reading only § O_WRONLY open for writing only § O_RDWR open for both reading and writing § Perms is specify permissions if using O_CREAT § To open an existing file for reading: fd = open(name, O_RDONLY, 0); dovanquyen. vn@gmail. com 6/24

Open, Creat, Close, Unlink § Creat () #include <fcntl. h> int fd; int creat(char *name, int perms); fd = creat(name, perms); § Returns a file descriptor if it was able to create the file, and -1 if not § If the file already exists, creat will truncate it to zero length § If the file does not already exist, creates it with the permissions specified by the perms argument dovanquyen. vn@gmail. com 7/24

Open, Creat, Close, Unlink § Unlink() unlink(char *name) § The function unlink removes the file name from the file system. It corresponds to the standard library function remove dovanquyen. vn@gmail. com 8/24

Random Access – Lseek § The system call lseek provides a way to move around in a file without reading or writing any data long lseek(int fd, long offset, int origin); § Sets the current position in the file whose descriptor is fd to offset § Offset is taken relative to the location specified by origin § Origin can be 0, 1, or 2 to specify that offset is to be measured from the beginning, from the current position, or from the end of the file respectively dovanquyen. vn@gmail. com 9/24

Listing Directories § File is a collection of organized data, is managed by operating system. § Ex : a text file, a programing, … § Directory is a file which contains a list of filenames and some indication of where they are located. § Inode contains all file’s informations except filename. § A directory include : § Inode number #define NAME_MAX 14 typedef struct { § File name long ino; /* inode number */ char name[NAME_MAX+1]; /* name + '' terminator */ } Dirent; Lethihien. fet 8@gmail. com 10/24

Listing Directories § “dirent. h” typedef struct{ int fd; Dirent d; } DIR; Lethihien. fet 8@gmail. com Function Explain DIR *opendir(char Open directory *dirname); -Is directory : pointer to drectory information. -Fail or not directory : NULL Dirent *readdir(DIR *dfd); Read directory -Return a pointer point to next file in directory - No more file, return NULL void closedir(DIR *dfd); Close dirctory and free memory. 11/24

Listing Directories § “stat. h” § Calls system : § int stat(char*, struct stat *); § int fstat(int fd, struct stat *); Lethihien. fet 8@gmail. com 12/24

Listing Directories • void dirwalk(char *dir, void (*fcn)(char *)) { … dfd = opendir(dir); while (NULL !=(dp = readdir(dfd))) { if (0 ==strcmp(dp->name, ". ")|| strcmp(dp>name, ". . ")) continue; /* skip self and parent */ else { sprintf(name, "%s/%s", dir, dp->name); (*fcn)(name); } } closedir(dfd); } Lethihien. fet 8@gmail. com • { void fsize(char *name) struct stat stbuf; stat(name, &stbuf; if ((stbuf. st_mode & S_IFMT) == S_IFDIR) dirwalk(name, fsize); printf("%8 ld %sn", stbuf. st_size, name); } 13/24

A Storage Allocator § Memory map §. text area: it is read-only memory which has executing file. §. data area : this is area where global variable which are declared the value are allocated §. bss area( below stack section): this is area where global variable which are not declared the value are allocated § Heap area : is used to allocate dynamic variable (C use malloc() and free() to allocate and free memory) § Stack are : is used to allocate automatic (local) variable and parameter of each function when it is called § ENV area : is used to allocate environment variable Dotienthanh. bkhn@gmail. com 14/24

A Storage Allocator § Free list is a collection of free blocks. § 1 block contain 1 pointer to next free block, a size and free space of itself. (value of size fields = size of free space + size of header) § The blocks are kept in order of increasing storage address, and the last block has the highest addr § The last block has pointer to the first block Dotienthanh. bkhn@gmail. com 15/24

A Storage Allocator § Storage location base on Free list Dotienthanh. bkhn@gmail. com 16/24

A Storage Allocator § Case 1: Found 1 block having size = nbytes + header Dotienthanh. bkhn@gmail. com 17/24

A Storage Allocator § Case 2: Found 1 block having size > nbytes + header Dotienthanh. bkhn@gmail. com 18/24

A Storage Allocator § Case 3: require OS locates a new block § OS will try to locate 1 block. (Size of block >> nbytes is require) § New block is either between 2 exiting blocks or at the end/start of list § Block is needed to attach to free list § (if new block is adjacent to a exiting block, it will be coalesced into a single block) § Remaining work is the same case 2 Dotienthanh. bkhn@gmail. com 19/24

A Storage Allocator § Case 3: require OS locates a new block Dotienthanh. bkhn@gmail. com 20/24

A Storage Allocator § Case 3: require OS locates a new block § Coalesce new block into exiting block between 2 exiting blocks Dotienthanh. bkhn@gmail. com 21/24

A Storage Allocator § Case 3: require OS locates a new block § Coalesce new block into exiting block between 2 exiting blocks Dotienthanh. bkhn@gmail. com 22/24

Thank you! 23/24