fseek

Name

fgetpos, fseek, fsetpos, ftell, rewind
- reposition a stream

Library

libc.lib

Synopsis

#include <stdio.h>

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

long ftell (FILE *stream);

void rewind (FILE *stream);

int fgetpos (FILE * restrict stream, fpos_t * restrict pos);

int fsetpos (FILE *stream, const fpos_t *pos);

Return values

The rewind function returns no value.

Upon successful completion, ftell return the current offset. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

Detailed description

The fseek function sets the file position indicator for the stream pointed to by stream. The new position, measured in bytes, is obtained by adding offset bytes to the position specified by whence. If whence is set to SEEK_SET, SEEK_CUR, or SEEK_END, the offset is relative to the start of the file, the current position indicator, or end-of-file, respectively. A successful call to the fseek function clears the end-of-file indicator for the stream and undoes any effects of the ungetc and ungetwc functions on the same stream.

The fseek function call does not allows the file offset to be set beyond the end of the existing end-of-file of the file.

The ftell function obtains the current value of the file position indicator for the stream pointed to by

stream.

The rewind function sets the file position indicator for the stream pointed to by stream to the beginning of the file. It is equivalent to:

     (void)fseek(stream, 0L, SEEK_SET)

except that the error indicator for the stream is also cleared. Since rewind does not return a value, an application wishing to detect errors should clear errno, then call rewind, and if errno is non-zero, assume an error has occurred. The fgetpos and fsetpos functions are alternate interfaces for retrieving and setting the current position in the file, similar to ftell and fseek, except that the current position is stored in an opaque object of type fpos_t pointed to by pos. These functions provide a portable way to seek to offsets larger than those that can be represented by a long int. They may also store additional state information in the fpos_t object to facilitate seeking within files containing multibyte characters with state-dependent encodings. Although fpos_t has traditionally been an integral type, applications cannot assume that it is; in particular, they must not perform arithmetic on objects of this type. If the stream is a wide character stream, the position specified by the combination of offset and whence must contain the first byte of a multibyte sequence.

Examples

/****************** this program shows setting file offset using fseek **************/
#include <stdio.h>
int main(void)
{       
        int retval;
        FILE* fp = fopen("c:\\input.txt", "w");
        fprintf(fp, "%s", "abcdefghijklmn");
        fprintf(fp, "%c", '\n');
        fprintf(fp, "%s", "fdsfdsafsdabcdefghijklmn");
        fclose(fp);
        fp = fopen("c:\\input.txt", "r");
        if (fp == NULL)
        {
        printf ("fopen failed\n");
        return -1;
        }
        retval = fseek(fp, 3, SEEK_SET); // seek to the 20th byte of the file
        if (retval)
        {
        printf ("fseek failed\n");
        return -1;
        }
        
        long pos = ftell(fp);
        if (pos ==3)
        {       
        printf("offset setting proper\n");
        }
        fclose(fp);
        return 0;
}

Output

offset setting proper

Errors

[`EBADF`]
	The stream argument is not a seekable stream.
[`EINVAL`]
	The whence argument is invalid or the resulting file-position indicator would be set to a negative value.
[`EOVERFLOW`]
	The resulting file offset would be a value which cannot be represented correctly in an object of type long for `fseek` and `ftell.`
[`ESPIPE`]
	The file descriptor underlying stream is associated with a pipe or FIFO or file-position indicator value is unspecified (see ungetc).

The functions fgetpos, fseek, fsetpos, and ftell, may also fail and set errno for any of the errors specified for the routines fflush, fstat, lseek, and malloc.

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