File-Handling System Calls

Opening Files

fd=open(path, flag, mode);

• Input Params
  • Path: relative/absolute
  • Flag: write/read/readwrite
  • Mode: Access rights
• Return value: file descriptor
  • File handling data structures (such as file pointer) and pointers to kernel functions
• There can be multiple file descriptors for the same file (eg someone reading and someone writing)

Creating Files

fd=creat(path, flag, mode);

Accessing Opened Files

newoffset=lseek(fd, offset, whence);

• Modifies where the file pointer points to
• Input Params
  • fd: File descriptor
  • offset: Address offset
  • whence: Specifies where the offset should be added, beginning of file, current file pointer, or offset from the end of the file
• Return Value: Returns the new file pointer

nread=read(fd, buf, count);

• Reads data and returns the number of bytes read
• Input Params:
  • fd: File descriptor
  • buf: Address of buffer where data will be transferred when read
  • count: Number of bytes to be read
• Return value: Number of bytes read

Closing a File

res=close(fd);

• Closes a file descriptor such that it does not refer to a file anymore
• Input Params: A file descriptor
• Return Value: 0 if successful -1 if not successful in closing

Renaming/Deleting a File

res=rename(oldpath, newpath);

• Changes the name of the file link and returns a flag
• Input Param: oldpath, newpath
• Return Value: 0 if successful -1 if not successful

res=unlink(pathname);

• Decreases the file link count and removes the corresponding directory entry in the file descriptor
• Input Param: Path name
• Return Value: 0 on success and -1 on error