fseek
, _fseeki64
Moves the file pointer to a specified location.
Syntax
int fseek(
FILE *stream,
long offset,
int origin
);
int _fseeki64(
FILE *stream,
__int64 offset,
int origin
);
Parameters
stream
Pointer to FILE
structure.
offset
Number of bytes from origin
.
origin
Initial position.
Return value
If successful, fseek
and _fseeki64
returns 0. Otherwise, it returns a nonzero value. On devices incapable of seeking, the return value is undefined. If stream
is a null pointer, or if origin
isn't one of allowed values described below, fseek
and _fseeki64
invoke the invalid parameter handler, as described in Parameter validation. If execution is allowed to continue, these functions set errno
to EINVAL
and return -1.
Remarks
The fseek
and _fseeki64
functions moves the file pointer (if any) associated with stream
to a new location that is offset
bytes from origin
. The next operation on the stream takes place at the new location. On a stream open for update, the next operation can be either a read or a write. The argument origin
must be one of the following constants, defined in STDIO.H
:
origin value | Meaning |
---|---|
SEEK_CUR |
Current position of file pointer. |
SEEK_END |
End of file. |
SEEK_SET |
Beginning of file. |
You can use fseek
and _fseeki64
to reposition the pointer anywhere in a file. The pointer can also be positioned beyond the end of the file. fseek
and _fseeki64
clears the end-of-file indicator and negates the effect of any prior ungetc
calls against stream
.
When a file is opened for appending data, the current file position is determined by the last I/O operation, not by where the next write would occur. If no I/O operation has yet occurred on a file opened for appending, the file position is the start of the file.
For streams opened in text mode, fseek
and _fseeki64
have limited use, because carriage return-line feed translations can cause fseek
and _fseeki64
to produce unexpected results. The only fseek
and _fseeki64
operations guaranteed to work on streams opened in text mode are:
Seeking with an offset of 0 relative to any of the origin values.
Seeking from the beginning of the file with an offset value returned from a call to
ftell
when usingfseek
or_ftelli64
when using_fseeki64
.
Also in text mode, CTRL+Z is interpreted as an end-of-file character on input. In files opened for reading/writing, fopen
and all related routines check for a CTRL+Z at the end of the file and remove it if possible. It's removed because using the combination of fseek
and ftell
or _fseeki64
and _ftelli64
, to move within a file that ends with a CTRL+Z may cause fseek
or _fseeki64
to behave improperly near the end of the file.
When the CRT opens a file that begins with a Byte Order Mark (BOM), the file pointer is positioned after the BOM. (That is, it's positioned at the start of the file's actual content). If you have to fseek
to the beginning of the file, use ftell
to get the initial position, and then fseek
to that position rather than to position 0.
This function locks out other threads during execution and is therefore thread-safe. For a non-locking version, see _fseek_nolock
, _fseeki64_nolock
.
By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.
Requirements
Function | Required header |
---|---|
fseek |
<stdio.h> |
_fseeki64 |
<stdio.h> |
For more compatibility information, see Compatibility.
Example
// crt_fseek.c
// This program opens the file FSEEK.OUT and
// moves the pointer to the file's beginning.
#include <stdio.h>
int main( void )
{
FILE *stream;
char line[81];
int result;
if ( fopen_s( &stream, "fseek.out", "w+" ) != 0 )
{
printf( "The file fseek.out was not opened\n" );
return -1;
}
fprintf( stream, "The fseek begins here: "
"This is the file 'fseek.out'.\n" );
result = fseek( stream, 23L, SEEK_SET);
if( result )
perror( "Fseek failed" );
else
{
printf( "File pointer is set to middle of first line.\n" );
fgets( line, 80, stream );
printf( "%s", line );
}
fclose( stream );
}
File pointer is set to middle of first line.
This is the file 'fseek.out'.
See also
Stream I/O
fopen
, _wfopen
ftell
, _ftelli64
_lseek
, _lseeki64
rewind