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fseek(), fseeko()

Change the current position of a stream

Synopsis:

#include <stdio.h>

int fseek( FILE* fp, 
           long offset, 
           int whence );

int fseeko( FILE* fp, 
            off_t offset, 
            int whence );

Arguments:

fp

A FILE pointer returned by fopen() or freopen().

offset

The position to seek to, relative to one of the positions specified by whence.

whence

The position from which to apply the offset; one of:

SEEK_SET

Compute the new file position relative to the start of the file. The value of offset must not be negative.

SEEK_CUR

Compute the new file position relative to the current file position. The value of offset may be positive, negative or zero. A SEEK_CUR with a 0 offset is necessary when you want to switch from reading to writing on a stream opened for updates.

SEEK_END

Compute the new file position relative to the end of the file.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The fseek() function changes the current position of the stream specified by fp. This position defines the character that will be read or written by the next I/O operation on the file.

The fseek() function clears the end-of-file indicator, and undoes any effects of the ungetc() function on the stream.

You can use ftell() to get the current position of the stream before changing it. You can restore the position by using the value returned by ftell() in a subsequent call to fseek() with the whence parameter set to SEEK_SET.

Returns:

0 for success, or nonzero if an error occurs.

Errors:

If an error occurs, errno is set to indicate the type of error.

Examples:

Determine the size of a file, by saving and restoring the current position of the file:

#include <stdio.h>
#include <stdlib.h>

long filesize( FILE *fp )
{
    long int save_pos;
    long size_of_file;

    /* Save the current position. */
    save_pos = ftell( fp );
    
    /* Jump to the end of the file. */
    fseek( fp, 0L, SEEK_END );
    
    /* Get the end position. */
    size_of_file = ftell( fp );
    
    /* Jump back to the original position. */
    fseek( fp, save_pos, SEEK_SET );

    return( size_of_file );
}

int main( void )
{
    FILE *fp;

    fp = fopen( "file", "r" );

    if( fp != NULL ) {
        printf( "File size=%ld\n", filesize( fp ) );
        fclose( fp );
        
        return EXIT_SUCCESS;
    }
    
    return EXIT_FAILURE;
}

Classification:

fseek() is ANSI, POSIX 1003.1; fseeko() is POSIX 1003.1

See also:

errno, fgetpos(), fopen(), fsetpos(), ftell()