Chapter 26. Input and Output
In input/output operations, filehandle may be a filehandle as opened by the open operator, a predefined filehandle (e.g., STDOUT), or a scalar variable that evaluates to a reference to or the name of a filehandle to be used.
<filehandle>In scalar context, reads a single record, usually a line, from the file opened on filehandle. In list context, reads the rest of the file.
<><ARGV>Reads from the input stream formed by the files specified in
@ARGV,or standard input if no arguments were supplied.- binmode filehandle [
,layers ] Arranges for the file opened on filehandle to be read or written using the specified I/O layers (default:
:raw). For a list of Standard I/O Layers.- close [ filehandle ]
Closes the filehandle. Resets
$.if it was an input file. If filehandle is omitted, closes the currently selected filehandle.- dbmclose %hash
Closes the file associated with the hash. Superseded by untie.
- dbmopen
%hash, dbmname, mode Opens a dbm file and associates it with the hash. Superseded by tie.
- eof filehandle
Returns true if the next read will return EOF (end of file) or if the file is not open.
- eof
Returns the EOF status for the last file read.
- eof
() Indicates EOF on the pseudo-file formed of the files listed on the command line.
- fcntl filehandle
,function, $var Calls system-dependent file control functions.
- fileno filehandle
Returns the file descriptor for a given (open) file.
- flock filehandle
,operation Calls a system-dependent locking routine on the file. operation is formed by adding one or more values or
LOCK_constants from the table.- getc [ filehandle ]
Returns the next character from the file, or an empty string on end of file. If filehandle is omitted, reads from
STDIN.- ioctl filehandle
,function, $var Calls system-dependent I/O control functions.
- open filehandle [
,modeandname ] - open filehandle
,mode,name [,. . . ] Opens a file and associates it with filehandle. If filehandle is an uninitialized scalar variable, a new, unique filehandle is automatically created.
modeandname must contain the name of the file, prefixed with the mode with which to open it. If modeandname is not provided, a global (package) variable with the same name as filehandle must provide the mode and name.
See general Open Modes.
In modeandname,
-may be used to designate standard input or output. Whitespace is allowed, as a consequence this form of open cannot easily be used to open files with names that start or end with whitespace.The form with three or more arguments allows more control over the open mode and file name.
If name is undef, an anonymous temporary file is opened. If name is a reference to a scalar, the contents of the scalar are read from or written to.
mode may have a list of I/O layers appended that will be applied to the handle. For a list of Standard I/O Layers.
- pipe readhandle
,writehandle Creates a pair of connected pipes. If either handle is an uninitialized scalar variable, a new, unique filehandle is automatically created.
- print [ filehandle ] list†
Prints the elements of list, converting them to strings if needed. If filehandle is omitted, prints to the currently selected output handle.
- printf [ filehandle ] list†
Equivalent to print filehandle sprintf list.
- read filehandle
, $var,length [,offset] Reads length characters from the file into the variable at offset. Returns the number of characters actually read, 0 on EOF, and undef on failure.
- readline expr
Internal function that implements the
<>operator.- readpipe scalar expr
Internal function that implements the qx operator. expr is executed as a system command.
- seek filehandle
,position,whence Arbitrarily positions the file on a byte position. whence can be one of the values or
SEEK_constants from the table.- select [ filehandle ]
Sets the current default filehandle for output operations if filehandle is supplied. Returns the currently selected filehandle.
- select rbits
,wbits,nbits,timeout Performs a select syscall with the same parameters.
- sprintf format
,list Returns a string resulting from formatting a (possibly empty) list of values. See the section Chapter 27 for a complete list of format conversions. See the section Chapter 28 for an alternative way to obtain formatted output.
- sysopen filehandle
,path,mode [,perms ] Performs an open syscall. The possible values and flag bits of mode and perms are system-dependent; they are available via the standard module
Fcntl.If filehandle is an uninitialized scalar variable, a new, unique filehandle is automatically created.mode is formed by adding one or more values or
0_constants from the table.- sysread filehandle
, $var,length [,offset ] Reads length characters into
$var at offset. Returns the number of characters actually read, 0 on EOF, and undef on failure.- sysseek filehandle
,position,whence Arbitrarily positions the file on a byte position, for use with sysread and syswrite. whence can be one of the values or
SEEK_constants from the table.- syswrite filehandle
,scalar [,length [,offset ] ] Writes length characters from scalar at offset. Returns the number of characters actually written, or undef if there was an error.
- tell [ filehandle ]
Returns the current byte position for the file. If filehandle is omitted, assumes the file last read.
Open Modes
The following modes are valid for all forms of open:
| Input only. This is the default when the mode is empty. |
| Output only. The file is created or truncated if necessary. |
| Open the file in append mode. The file is created if necessary. |
| Read/write update access. |
| Write/read update access. |
| Read/append access. |
These modes may be followed by & to duplicate an already opened filehandle or, if numeric, file descriptor. Use &= with a numeric argument to create an alias to the already opened file descriptor.
Modes for the two-argument open include:
| Opens a pipe to read from or write to a command. |
| Forks, with the file connected to the standard input of the child. |
| Forks, with the file connected to the standard output of the child. |
Modes for the three-argument open include:
| Opens a pipe to write to a command. |
| Opens a pipe to read from a command. |
Common constants
Several input/output related constants can be imported from the standard module Fcntl.
Constants related to open and sysopen are imported by default. For some constants, the widely accepted values are shown in octal.
Description | ||
|
| Read-only access. |
|
| Write-only access. |
|
| Read and write access. |
|
| Create the file if nonexistent. |
|
| Fail if the file already exists. |
|
| Append data to the end of the file. |
|
| Truncate the file. |
| Nonblocking input/output. | |
| Same as | |
| Synchronous input/output. | |
| Lock exclusive. | |
| Lock shared. | |
| File must be a directory. | |
| Do not follow symlinks. | |
| Use binary mode for input/output. | |
| Allow file to be larger than 4 GB. | |
| Terminal will not become the controlling tty. |
Constants related to seek and sysseek must be imported explictly by specifying :seek in the import list of Fcntl.
Value | Name | Description |
|
| Seek position. |
|
| Seek offset from current position. |
|
| Seek offset from end of file. |
Constants related to flock must be imported explictly by specifying :flock in the import list of Fcntl.
Standard I/O Layers
:bytesUse 8-bit bytes, as opposed to
:utf8.:crlfDo CR/LF to newline translation, and vice versa.
:encoding(enc)Select a specific encoding.
:perlioUse Perl’s “PerlIO” implementation.
:rawUse low-level I/O.
:stdioUse the system’s “standard I/O” implementation.
:unixUse Unix-style low-level I/O.
:utf8Use Perl’s internal encoding of Unicode.
:Via(module)Use the specified module to handle the I/O.
:win32Use native I/O (Microsoft Windows platforms only).