fseek

Description

Moves the file pointer to a specified location.

#include <stdio.h>

int fseek( FILE *stream, long offset, int origin );

Remarks

The fseek function 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:

The fseek function can be used to reposition the pointer anywhere in a file. The pointer can also be positioned beyond the end of the file. However, an attempt to position the pointer in front of the beginning of the file causes an error.

The fseek function 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 has limited use because carriage-return–line-feed translations can cause fseek to produce unexpected results. The only fseek 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

Return Value

If successful, fseek returns 0. Otherwise, it returns a nonzero value. On devices incapable of seeking, the return value is undefined.

Compatibility

Standards:ANSI, UNIX

16-Bit:DOS, QWIN, WIN, WIN DLL

32-Bit:DOS32X

See Also

ftell, _lseek, rewind

Example

/* FSEEK.C: This program opens the file FSEEK.OUT and

* moves the pointer to the file's beginning.

*/

#include <stdio.h>

void main( void )

{

FILE *stream;

char line[81];

int result;

stream = fopen( "fseek.out", "w+" );

if( stream == NULL )

printf( "The file fseek.out was not opened\n" );

else

{

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 );

}

}

Output

File pointer is set to middle of first line.

This is the file 'fseek.out'.