Unix C/C++ Input and Output Function Reference

by digipine posted Nov 01, 2017


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Unix, C, and C++
Function Reference
Unix Input and Output
  int open(char * filename, int flags)
  int open(char * filename, int flags, int mode)
      include: <fcntl.h>
      flags = bitwise | or of any of the following:
             O_RDONLY        Only read operations permitted
             O_WRONLY        Only write operations permitted
             O_RDWR          Read and Write operations both permitted
             O_NONBLOCK      Non-blocking, applies to open operation only
             O_APPEND        All writes go to end of file
             O_CREAT         Create file if it doesn't already exist
             O_TRUNC         Delete existing contents of file
             O_EXCL          Open fails if file already exists
             O_SHLOCK        Get a "shared lock" on the file
             O_EXLOCK        Get an "exclusive lock" on the file
             O_DIRECT        Try to avoid all caching of operations
             O_FSYNC         All writes immediately effective, no buffering
             O_NOFOLLOW      If file is symbolic link, open it, don't follow it
      mode required if file is created, ignored otherwise.
           mode specifies the protection bits, e.g. 0644 = rw-r--r--
      returns <0 for error, or integer file descriptor.
  int close(int fd)
      include: <unistd.h>
      fd = file descriptor as returned by open
      returns <0 for error, 0 for success.
  int read(int fd, void * ptr, int numbytes)
      include: <unistd.h>
      fd = file descriptor as returned by open
      ptr = address in memory where data is to be stored;
            may be a pointer of any type.
      numbytes = number of bytes to attempt to read
      returns <0 for error, 0 for end-of-file,
              or number of bytes successfully read.
              (for a non-blocking interactive file, that may be zero).
  int write(int fd, void * ptr, int numbytes)
      include: <unistd.h>
      fd = file descriptor as returned by open
      ptr = address in memory where data already is;
            may be a pointer of any type.
      numbytes = number of bytes to attempt to write
      returns <=0 for error,
              or number of bytes successfully written.
              (for a non-blocking file, that may be less than the number attempted
               without any errors having occurred).
  int lseek(int fd, int position, int startpoint)
      include: <unistd.h>
      Sets the file position effective for the next read or write operation.
      fd = file descriptor as returned by open
      position = position within file:
            number of bytes between startpoint and the desired position, may be negative.
      startpoint = location in file that position is relative to, one of:
             SEEK_SET        Position is number of bytes from beginning of file
             SEEK_END        Position is number of bytes after end of file
             SEEK_CUR        Position is number of bytes after current position
      returns <0 for error,
              or resulting file position relative to beginning of file.
      Moving beyond the end of the file is permitted; the file is extended in length
      only if a write occurs beyond the current end of file. Moveing before the
      beginning of a file is not permitted. A write operation after a move only
      overwrites the number of bytes actually written.
  int stat(char * file, struct stat * info)
      include: <sys/types.h>
      include: <sys/stat.h>
      Finds information about a file.
      file = The name (or path) for the file.
      info = A pointer to an uninitialised stat structure.
            Even in C++, the type must be "struct stat *" because of the name clash.
            Information about the file is stored in the info object.
      returns <0 for error (including file does not exist),
              or 0  for success.
      The useful fields of a stat object are as follows. All have types that behave like
      ints, although they may be 16, 32, or 64 bits long.
             st_size              size in bytes
             st_ino               inode number
             st_mode              protection or mode (see below)
             st_nlink             number of hard links
             st_uid               file's owner's identification number
             st_gid               file's group identification number
             st_birthtime         date and time of file creation (see below)
             st_mtime             date and time of last modification (see below)
             st_atime             date and time of last access (see below)
      All times are stored as time_t values, as described here.
      Mode and Protection. st_mode is the logical-or of a number of bits each representing
      different properties. The named constants for these bits, and their values in octal are:
             S_ISUID   0004000    set user id on execution
             S_IRUSR   0000400    protection: readable by owner
             S_IWUSR   0000200                writable by owner
             S_IXUSR   0000100                executable by owner
             S_IRGRP   0000040                readable by group
             S_IWGRP   0000020                writable by group
             S_IXGRP   0000010                executable by group
             S_IROTH   0000004                readable by all
             S_IWOTH   0000002                writable by all
             S_IXOTH   0000001                executable by all
      Four bits of the mode give the file type. The mask for type is S_IFMT = 0170000
          if (mode & S_IFMT) is S_IFREG = 0100000,  type = perfectly ordinary file
          if (mode & S_IFMT) is S_IFDIR = 0040000,  type = directory
          if (mode & S_IFMT) is S_IFLNK = 0120000,  type = symbolic link
          if (mode & S_IFMT) is S_IFIFO = 0010000,  type = named pipe
          if (mode & S_IFMT) is S_IFSOCK = 0140000,  type = named socket