fseek

Move file position pointer to specified position in file

Syntax

fseek(fileID,offset,origin)

status = fseek(___)

Description

fseek(fileID,offset,origin) sets the file position indicator offset bytes from origin in the specified file.

status = fseek(___) returns 0 when the operation is successful. Otherwise, fseek returns -1. Use any of the previous input argument combinations.

Examples

Move to New Position in File

Open the following badpoem.txt file and perform read operations (which advance the position pointer) and then use seek to move to a new position in the file.

Use fopen to open the file. Then, use ftell to query the current position.

fid = fopen('badpoem.txt');
ftell(fid)

ans = 
0

Read the first three lines and query the position in the file after each read. Use fgetl to read and fseek to examine the current position after the read operation.

tline1 = fgetl(fid)  % read the first line 

tline1 = 
'Oranges and lemons,'

ftell(fid)

ans = 
20

Read the second line and examine the current position.

tline2 = fgetl(fid)  % read the second line 

tline2 = 
'Pineapples and tea.'

ftell(fid)

ans = 
40

Read the third line and examine the current position.

tline3 = fgetl(fid)  % read the third line 

tline3 = 
'Orangutans and monkeys,'

ftell(fid)

ans = 
64

To read line 2, set the position in the file to point to the beginning of line 2. Use fseek to set the position, and then perform a read operation.

fseek(fid,20,'bof');
fgetl(fid)

ans = 
'Pineapples and tea.'

Close the file.

fclose(fid); 

Input Arguments

fileID — File identifier

integer

File identifier of an open file, specified as an integer. Before using fseek, you must use fopen to open the file and obtain its identifier fileID.

Data Types: double

offset — Number of bytes

integer

Number of bytes to move from origin, specified as an integer. The value of offset can be positive, negative, or zero.

Data Types: double

origin — Starting location

integer | character vector | string

Starting location in the file, specified as a character vector, string scalar, or a scalar number.

'bof' or -1	Beginning of file
'cof' or 0	Current position in file
'eof' or 1	End of file

Data Types: double | char | string

Tips

• If a file has n bytes of data, then those n bytes are in positions 0 through n-1.

Alternatives

To move to the beginning of a file, call

frewind(fileID)

This call is identical to

fseek(fileID, 0, 'bof')

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• When the MATLAB^® behavior differs from the C compiler behavior, the generated code matches the C compiler behavior. Examples include:

  • Seeking past the end of a file.

  • Seeking away from the end of a file opened with append access.

• The offset is passed to the C run-time environment as a signed long data type. Therefore, the offset value must fit in the long data type on the target hardware.

Thread-Based Environment

Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.

This function fully supports thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

Version History

Introduced before R2006a

R2024b: Read data over HTTP and HTTPS using low-level file functions

You can read data from primary online sources by performing low-level file read operations over an internet URL.

R2022b: Use function in thread-based environments

This function supports thread-based environments.