26.3 Reading from Files

To copy the contents of a file into a buffer, use the function insert-file-contents. (Don’t use the command insert-file in a Lisp program, as that sets the mark.)

Function: insert-file-contents filename &optional visit beg end replace

This function inserts the contents of file filename into the current buffer after point. It returns a list of the absolute file name and the length of the data inserted. An error is signaled if filename is not the name of a file that can be read.

This function checks the file contents against the defined file formats, and converts the file contents if appropriate and also calls the functions in the list after-insert-file-functions. See File Format Conversion. Normally, one of the functions in the after-insert-file-functions list determines the coding system (see Coding Systems) used for decoding the file’s contents, including end-of-line conversion. However, if the file contains null bytes, it is by default visited without any code conversions. See inhibit-null-byte-detection.

If visit is non-nil, this function additionally marks the buffer as unmodified and sets up various fields in the buffer so that it is visiting the file filename: these include the buffer’s visited file name and its last save file modtime. This feature is used by find-file-noselect and you probably should not use it yourself.

If beg and end are non-nil, they should be numbers that are byte offsets specifying the portion of the file to insert. In this case, visit must be nil. For example,

(insert-file-contents filename nil 0 500)

inserts the characters coded by the first 500 bytes of a file.

If beg or end happens to be in the middle of a character’s multibyte sequence, Emacs’s character code conversion will insert one or more eight-bit characters (a.k.a. “raw bytes”) (see Character Sets) into the buffer. If you want to read part of a file this way, we recommend to bind coding-system-for-read to a suitable value around the call to this function (see Specifying a Coding System for One Operation), and to write Lisp code which will check for raw bytes at the boundaries, read the entire sequence of these bytes, and convert them back to valid characters.

If the argument replace is non-nil, it means to replace the contents of the buffer (actually, just the accessible portion) with the contents of the file. This is better than simply deleting the buffer contents and inserting the whole file, because (1) it preserves some marker positions and (2) it puts less data in the undo list.

It is possible to read a special file (such as a FIFO or an I/O device) with insert-file-contents, as long as replace, and visit and beg are nil. However, you should normally use an end argument for these files to avoid inserting (potentially) unlimited data into the buffer (for instance, when inserting data from /dev/urandom).

Function: insert-file-contents-literally filename &optional visit beg end replace

This function works like insert-file-contents except that each byte in the file is handled separately, being converted into an eight-bit character if needed. It does not run after-insert-file-functions, and does not do format decoding, character code conversion, automatic uncompression, and so on.

If you want to pass a file name to another process so that another program can read the file, use the function file-local-copy; see Making Certain File Names “Magic”.