`fopen`

Opens a file and returns a file identifier.

Category: Data Import and Analysis > Data Import and Export

Syntax

fid = fopen(filename)

Opens the file for reading.

fid = fopen(filename, permission)

Opens with a mode such as 'w', 'a', 'r+'; add t for text mode.

[fid, msg] = fopen(...)

Also returns an error message when the open fails.

[name, perm, machinefmt, enc] = fopen(fid)

Queries the properties of an open file.

Description

fopen does not raise an error when a file is missing. It returns -1, so check the result before reading. The identifiers 0, 1 and 2 are reserved for standard input, standard output and standard error.

Every file opened should be closed with fclose.

Input arguments

filename: Name of the file.
permission: 'r' read (default), 'w' write and truncate, 'a' append, 'r+' read and write, 'w+' and 'a+' for update variants.

Output arguments

fid: Integer identifier, 3 or greater, to pass to other file functions; -1 if the file could not be opened.
msg: Text describing why opening failed.

Example

Open for reading, with a failing case

[fid, msg] = fopen('missing.txt')
fid = fopen('ok.txt', 'w');
fprintf(fid, 'hi');
fclose(fid);
fid = fopen('ok.txt');
[name, perm] = fopen(fid)
fclose(fid);

Output

fid = -1
msg = 'No such file or directory'
name = 'ok.txt'
perm = 'r'

fopen