A file is a one-dimensional array of bytes with at least one hard link (file name). Files can contain ASCII or binary information. Files contain data, shell scripts, or programs. File names are also used to represent abstract objects such as sockets, pipes, and device drivers.
The kernel does not distinguish record boundaries in regular files. Programs can establish their own boundary markers if desired. For example, many programs use line-feed characters to mark the end of lines. "Working with Files" contains a list of the subroutines used to control files.
Files are represented in the journaled file system (JFS) by disk index nodes (i-node). Information about the file (such as ownership, access modes, access time, data addresses, and modification time) is stored in the i-node. For more information about the internal structure of files, see "Working with JFS i-nodes".
The journaled file system supports the following file types:
| File Types Supported By Journaled File System | ||
| Type of File | Macro Name Used in mode.h | Description |
| Regular | S_ISREG | A sequence of bytes with one or more names. Regular files can contain ASCII or binary data. These files can be randomly accessed (read from or written to) from any byte in the file. |
| Directory | S_ISDIR | Contains directory entries (file name and i-number pairs). Directory formats are determined by the file system. Processes read directories as they do ordinary files, but the kernel reserves the right to write to a directory. Special sets of subroutines control directory entries. |
| Block Special | S_ISBLK | Associates a structured device driver with a file name. |
| Character Special | S_ISCHR | Associates an unstructured device driver with a file name. |
| Pipes | S_ISFIFO | Designates an interprocess communication channel (IPC). The mkfifo subroutine creates named pipes. The pipe subroutine creates unnamed pipes. |
| Symbolic Links | S_ISLNK | A file that contains either an absolute or relative path name to another file name. |
| Sockets | S_ISSOCK | An IPC mechanism that allows applications to exchange data. The socket subroutine creates sockets, and the bind subroutine allows sockets to be named. |
The maximum size of a regular file in a file system enabled for large files (available beginning in Version 4.2) is slightly less than 64 gigabytes (68589453312). All nonregular files in a file system enabled for large files and all files in other JFS file system types have a maximum file size of 2 gigabytes minus 1 (2147483647). The maximum length of a file name is 255 characters, and the maximum length of a path name is 1023 bytes. For more information, see "File Space Allocation".
The operating system offers many subroutines that manipulate files. Brief descriptions of the most common file-control subroutines are provided in two categories:
| creat | Creates a new, empty, regular file. |
| open | Creates a new, empty file if the O_CREAT flag is set. |
| mkfifo | Creates a named pipe. |
| mkdir | Creates a directory. |
| mknod | Creates a file that defines a device. |
| socket | Creates a socket. |
| pipe | Creates an IPC. |
| link | Creates an additional name (directory entry) for an existing file. |
| open | Returns a file descriptor used by other subroutines to reference the opened file. The open operation takes a regular file name and a permission mode that indicates whether the file is to be read from, written to, or both. |
| read | Removes data from an open file if the appropriate permissions (O_RDONLY or O_RDWR) were set by the open subroutine. |
| write | Puts data into an open file if the appropriate permissions (O_WRONLY or O_RDWR) were set by the open subroutine. |
| lseek or llseek | Moves the I/O pointer position in an open file. |
| close | Closes open file descriptors (including sockets). |
| rmdir | Removes directories from the file system. |
| chown | Changes ownership of a file. |
| chmod | Changes the access modes of a file. |
| stat | Reports the status of a file including the owner and access modes. |
| access | Determines the accessibility of a file. |
| rename | Changes the name of a file. |
| truncate | Changes the length of a file. |
| ioctl | Controls functions associated with open file descriptors, including special files, sockets, and generic device support like the termio general terminal interface. |
| fclear | Creates space in file. |
| fsync | Writes changes in a file to permanent storage. |
| fcntl, dup, or dup2 | Controls open file descriptors. |
| lockf or flock | Controls open file descriptors. |
For more information on types and characteristics of file systems, see "File Systems Overview" in AIX Version 4.3 System Management Guide: Operating System and Devices.
Files, Directories, and File Systems for Programmers lists other information on these topics.
Working with JFS i-nodes introduces the structure of the i-node and its contents.
File Space Allocation explains how the file system allocates data blocks.