Open file or obtain information about open files
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Syntax
fileID = fopen(filename)
fileID = fopen(filename,permission)
fileID = fopen(filename,permission,machinefmt,encodingIn)
[fileID,errmsg] = fopen(___)
filename = fopen(fileID)
[filename,permission,machinefmt,encodingOut] = fopen(fileID)
Description
example
fileID = fopen(filename) opensthe file, filename, for binary read access, andreturns an integer file identifier equal to or greater than 3. MATLAB® reservesfile identifiers 0, 1, and 2 forstandard input, standard output (the screen), and standard error,respectively.
If fopen cannot open the file, then fileID is -1.
fileID = fopen(filename,permission) opensthe file with the type of access specified by permission.
example
fileID = fopen(filename,permission,machinefmt,encodingIn) additionallyspecifies the order for reading or writing bytes or bits in the fileusing the machinefmt argument. The optional encodingIn argumentspecifies the character encoding scheme associated with the file.
example
[fileID,errmsg] = fopen(___) additionallyreturns a system-dependent error message if fopen failsto open the file. Otherwise, errmsg is an emptycharacter vector. You can use this syntax with any of the input argumentsof the previous syntaxes.
filename = fopen(fileID) returns the file name that a previous call to fopen used when it opened the file specified by fileID. The output filename contains the full path only if the previous fopen call opened the file with r or r+ permissions. The fopen function does not read information from the file to determine the output value.
example
[filename,permission,machinefmt,encodingOut] = fopen(fileID) additionallyreturns the permission, machine format, and encoding that a previouscall to fopen used when it opened the specifiedfile. If the file was opened in binary mode, permission includesthe letter 'b'. The encodingOut outputis a standard encoding scheme name. fopen doesnot read information from the file to determine these output values.An invalid fileID returns empty character vectorsfor all output arguments.
Examples
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Open File and Pass Identifier to File I/O Function
Open a file and pass the file identifier tothe fgetl function to read data.
Open the file, tsunamis.txt, and obtain the file identifier.
fileID = fopen('tsunamis.txt');Pass the fileID to the fgetl function to read one line from the file.
tline = 'A global tsunami data set in xlsx format, comprising the following file:'
Close the file.
fclose(fileID)
Request Name of File to Open
Create a prompt to request the name of a file to open.If fopen cannot open the file, display the relevanterror message.
fileID = -1;errmsg = '';while fileID < 0 disp(errmsg); filename = input('Open file: ', 's'); [fileID,errmsg] = fopen(filename);end
Open File for Writing and Specify Access Type, Writing Order, Character Encoding
Open a file to write to a file using the Shift-JIScharacter encoding.
fileID = fopen('japanese_out.txt','w','n','Shift_JIS');
The 'w' input specifies write access, the 'n' inputspecifies native byte ordering, and 'Shift_JIS' specifiesthe character encoding scheme.
Get Information About Open Files
Suppose you previously opened a file using fopen.
fileID = fopen('tsunamis.txt');Get the file identifiers of all open files.
fIDs = openedFiles
fIDs = 3
Get the file name and character encoding for the openfile. Use ~ in place of output arguments you wantto omit.
[filename,~,~,encoding] = fopen(fileID)
filename = 'matlabroot\toolbox\matlab\demos\tsunamis.txt'encoding = 'windows-1252'
The output shown here is representative. Your results mightdiffer.
Input Arguments
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filename — Name of file to open
character vector or string scalar
Name of file to open, specified as a character vector or string scalar that includes the file extension.
On UNIX® systems, if filename begins with '~/' or '~, the username/'fopen function expands the path to the current or specified user's home directory, respectively.
Depending on the location of your file, filename can take on one of these forms.
| Current folder or folder on the MATLAB path | Specify the name of the file in If you open a file with read access and the file is not in the current folder, then If you open a file with write or append access and the file is not in the current folder, then Example: | ||||||||
Other folders | If the file is not in the current folder or in a folder on the MATLAB path, then specify the full or relative path name in Example: Example: | ||||||||
Remote Location | If the file is stored at a remote location, then
Based on your remote location,
If you are using a cloud file system, set environment variables to communicate with the remote file system. For more information, see Work with Remote Data. Files in a Hadoop Distributed File System (HDFS) volume cannot be opened in read-write mode. Example: |
Example: 'myFile.txt'
Data Types: char | string
permission — File access type
'r' (default) | 'w' | 'a' | 'r+' | 'w+' | 'a+' | 'A' | 'W' | ...
File access type, specified as a character vector or a stringscalar. You can open a file in binary mode or in text mode. On UNIX systems,both translation modes have the same effect. To open a file in binarymode, specify one of the following.
| Open file for reading. |
| Open or create new file for writing. Discard existingcontents, if any. |
| Open or create new file for writing. Append data to theend of the file. |
| Open file for reading and writing. |
| Open or create new file for reading and writing. Discardexisting contents, if any. |
| Open or create new file for reading and writing. Appenddata to the end of the file. |
| Open file for appending without automatic flushing ofthe current output buffer. |
| Open file for writing without automatic flushing of thecurrent output buffer. |
To open files in text mode, attach the letter 't' tothe permission argument, such as 'rt' or 'wt+'.
On Windows® systems, in text mode:
Read operations that encounter a carriage return followedby a newline character (
'\r\n') remove the carriagereturn from the input.Write operations insert a carriage return before anynewline character in the output.
Open or create a new file in text mode if you want to writeto it in MATLAB and then open it in Microsoft® Notepad, orany text editor that does not recognize '\n' asa newline sequence. When writing to the file, end each line with '\r\n'.For an example, see fprintf.Otherwise, open files in binary mode for better performance.
To read and write to the same file:
Open the file with a value for
permissionthatincludes a plus sign,'+'.Call
fseekorfrewindbetweenread and write operations. For example, do not callfreadfollowedbyfwrite, orfwritefollowedbyfread, unless you callfseekorfrewindbetweenthem.
Data Types: char | string
machinefmt — Order for reading or writing bytes or bits
'n' (default) | 'b' | 'l' | 's' | 'a' | ...
Order for reading or writing bytes or bits in the file, specifiedas one of the following character vectors or string scalars.
| Your system byte ordering (default) |
| Big-endian ordering |
| Little-endian ordering |
| Big-endian ordering, 64-bit long data type |
| Little-endian ordering, 64-bit long data type |
By default, all currently supportedplatforms use little-endian ordering for new files. Existing binaryfiles can use either big-endian or little-endian ordering.
Data Types: char | string
encodingIn — Character encoding
'UTF-8' | 'ISO-8859-1' | 'windows-1251' | 'windows-1252' | ...
Character encoding to use for subsequent read and write operations, including fscanf, fprintf, fgetl, fgets, fread, and fwrite, specified as a character vector or a string scalar. The character vector or string scalar must contain a standard character encoding scheme name such as the following.
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If you do not specify an encoding scheme when opening a file for reading, fopen uses auto character-set detection to determine the encoding. If you do not specify an encoding scheme when opening a file for writing, fopen defaults to using UTF-8 in order to provide interoperability between all platforms and locales without data loss or corruption. For more information, see Import Text Data Files with Low-Level I/O.
If you specify a value for encoding that is not in the list of supported values, MATLAB issues a warning. Specifying other encoding names sometimes (but not always) produces correct results.
Data Types: char | string
fileID — File identifier of an open file
integer
File identifier of an open file, specified as an integer.
Data Types: double
Tips
In most cases, it is not necessary to open a filein text mode. MATLAB import functions, all UNIX applications,and Microsoft Word and WordPad recognize
'\n'asa newline indicator.
Extended Capabilities
C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.
Usage notes and limitations:
Code generation does not support:
The input arguments
machinefmt,encodingIn,orfileID.The output argument
errmsg.The syntax
fopen('all').Opening a file in text mode. That is the
permissionargumentmust not contain the lettert. For example, thevalue cannot be'rt'.
The
permissionargument can containthree characters at most. The characters must be unique.If you disable extrinsic calls, then you cannot returnfile identifiers created with
fopento MATLAB functionsor extrinsic functions. Use these file identifiers only internally.When generating C/C++ executables, static libraries,or dynamic libraries, you can open up to 20 files.
The generated code does not report errors from invalidfile identifiers. Write your own file open error handling in your MATLAB code.Test whether
fopenreturns-1,which indicates that the file open failed. For example:...fid = fopen(filename, 'r');if fid == -1% fopen failedelse% fopen successful, okay to call freadA = fread(fid);...
The behavior of the generated code for
freadiscompiler-dependent when you:Open a file using
fopenwithapermissionofa+.Read the file using
freadbeforecalling an I/O function, such asfseekorfrewind,that sets the file position indicator.
Thread-Based Environment
Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.
Usage notes and limitations:
The
fopen('all')syntax is not supported in thread-based environments.
Version History
Introduced before R2006a
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R2024a: fopen("all") syntax will be removed
The fopen("all") syntax will be removed in a future release. To get the file identifiers of all open files, use the openedFiles function instead.
R2022b: Use function in thread-based environments
This function supports thread-based environments.
See Also
fclose | openedFiles | ferror | fseek | ftell | feof | fscanf | fprintf | fread | fwrite | frewind
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