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&ECL; implements all stream types described in &ANSI;. Additionally, when configured with option --enable-clos-streams, &ECL; includes a version of Gray streams where any object that implements the appropiate methods (stream-input-p, stream-read-char, etc) is a valid argument for the functions that expect streams, such as read, print, etc.

&ECL; distinguishes between two kinds of streams: character streams and byte streams. Character streams only accept and produce characters, written or read one by one, with write-char or read-char, or in chunks, with write-sequence or any of the Lisp printer functions. Character operations are conditioned by the external format, as described in ANSI Common Lisp also supports binary streams. Here input and output is performed in chunks of bits. Binary streams are created with the function open passing as argument a subtype of integer and the implementation is free to round up that integer type to the closest size it supports. In particular &ECL; rounds up the size to a multiple of a byte. For example, the form (open "foo.bin" :direction :output :element-type '(unsigned-byte 13)), will open the file foo.bin for writing, using 16-bit words as the element type.

An external format is an encoding for characters that maps character codes to a sequence of bytes, in a one-to-one or one-to-many fashion. External formats are also known as "character encodings" in the programming world and are an essential ingredient to be able to read and write text in different languages and alphabets. &ECL; has one of the most complete supports for external formats, covering all of the usual codepages from the Windows and Unix world, up to the more recent UTF-8, UCS-2 and UCS-4 formats, all of them with big and small endian variants, and considering different encodings for the newline character. However, the set of supported external formats depends on the size of the space of character codes. When &ECL; is built with Unicode support (the default option), it can represent all known characters from all known codepages, and thus all external formats are supported. However, when &ECL; is built with the restricted character set, it can only use one codepage (the one provided by the C library), with a few variants for the representation of end-of-line characters. In &ECL;, an external format designator is defined recursively as either a symbol or a list of symbols. The grammar is as follows external-format-designator := symbol | ( {symbol}+ ) and the table of known symbols is shown below. Note how some symbols (:cr, :little-endian, etc) just modify other external formats. Symbols Codepage or encoding Unicode required :cr #\Newline is Carriage Return No :crlf #\Newline is Carriage Return followed by Linefeed No :lf #\Newline is Linefeed No :little-endian Modify UCS to use little endian encoding. No :big-endian Modify UCS to use big endian encoding. No :utf-8 ext:utf8 Unicode UTF-8 Yes :ucs-2 ext:ucs2 ext:utf-16 ext:utf16 ext:unicode UCS-2 encoding with BOM. Yes :ucs-2le ext:ucs2le ext:utf-16le UCS-2 with big-endian encoding Yes :ucs-2be ext:ucs2be ext:utf-16be UCS-2 with big-endian encoding Yes :ucs-4 ext:ucs4 ext:utf-32 ext:utf32 UCS-4 encoding with BOM. Yes :ucs-4le ext:ucs4le ext:utf-32le UCS-4 with big-endian encoding Yes :ucs-4be ext:ucs4be ext:utf-32be UCS-4 with big-endian encoding Yes ext:iso-8859-1 ext:iso8859-1 ext:latin-1 ext:cp819 ext:ibm819 Latin-1 encoding Yes ext:iso-8859-2 ext:iso8859-2 ext:latin-2 ext:latin2 Latin-2 encoding Yes ext:iso-8859-3 ext:iso8859-3 ext:latin-3 ext:latin3 Latin-3 encoding Yes ext:iso-8859-4 ext:iso8859-4 ext:latin-4 ext:latin4 Latin-4 encoding Yes ext:iso-8859-5 ext:cyrillic Latin-5 encoding Yes ext:iso-8859-6 ext:arabic ext:asmo-708 ext:ecma-114 Latin-6 encoding Yes ext:iso-8859-7 ext:greek8 ext:greek ext:ecma-118 Greek encoding Yes ext:iso-8859-8 ext:hebrew Hebrew encoding Yes ext:iso-8859-9 ext:latin-5 ext:latin5 Latin-5 encoding Yes ext:iso-8859-10 ext:iso8859-10 ext:latin-6 ext:latin6 Latin-6 encoding Yes ext:iso-8859-13 ext:iso8859-13 ext:latin-7 ext:latin7 Latin-7 encoding Yes ext:iso-8859-14 ext:iso8859-14 ext:latin-8 ext:latin8 Latin-8 encoding Yes ext:iso-8859-15 ext:iso8859-15 ext:latin-9 ext:latin9 Latin-7 encoding Yes ext:dos-cp437 ext:ibm-437 IBM CP 437 Yes ext:dos-cp850 ext:ibm-850 ext:cp850 Windows CP 850 Yes ext:dos-cp852 ext:ibm-852 IBM CP 852 Yes ext:dos-cp855 ext:ibm-855 IBM CP 855 Yes ext:dos-cp860 ext:ibm-860 IBM CP 860 Yes ext:dos-cp861 ext:ibm-861 IBM CP 861 Yes ext:dos-cp862 ext:ibm-862 ext:cp862 Windows CP 862 Yes ext:dos-cp863 ext:ibm-863 IBM CP 863 Yes ext:dos-cp864 ext:ibm-864 IBM CP 864 Yes ext:dos-cp865 ext:ibm-865 IBM CP 865 Yes ext:dos-cp866 ext:ibm-866 ext:cp866 Windows CP 866 Yes ext:dos-cp869 ext:ibm-869 IBM CP 869 Yes ext:windows-cp932 ext:windows-932 ext:cp932 Windows CP 932 Yes ext:windows-cp936 ext:windows-936 ext:cp936 Windows CP 936 Yes ext:windows-cp949 ext:windows-949 ext:cp949 Windows CP 949 Yes ext:windows-cp950 ext:windows-950 ext:cp950 Windows CP 950 Yes ext:windows-cp1250 ext:windows-1250 ext:ms-ee Windows CP 1250 Yes ext:windows-cp1251 ext:windows-1251 ext:ms-cyrl Windows CP 1251 Yes ext:windows-cp1252 ext:windows-1252 ext:ms-ansi Windows CP 1252 Yes ext:windows-cp1253 ext:windows-1253 ext:ms-greek Windows CP 1253 Yes ext:windows-cp1254 ext:windows-1254 ext:ms-turk Windows CP 1254 Yes ext:windows-cp1255 ext:windows-1255 ext:ms-hebr Windows CP 1255 Yes ext:windows-cp1256 ext:windows-1256 ext:ms-arab Windows CP 1256 Yes ext:windows-cp1257 ext:windows-1257 ext:winbaltrim Windows CP 1257 Yes ext:windows-cp1258 ext:windows-1258 Windows CP 1258 Yes