FSEEK(3)

FSEEK(3) | Library Functions Manual | FSEEK(3)

# NAME

`fgetpos`, `fseek`, `fseeko`, `fsetpos`, `ftell`, `ftello`, `rewind` —
reposition a stream

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <stdio.h>`

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

int
`fseeko`(FILE *stream, off_t offset, int whence);

long int
`ftell`(FILE *stream);

off_t
`ftello`(FILE *stream);

void
`rewind`(FILE *stream);

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

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

# 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(3) function on the same stream.

The `fseeko`() function is identical to the `fseek`() function except that the offset argument is of type off_t.

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

The `ftello`() function is identical to the `ftell`() function except that the return value is of type off_t.

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 (see clearerr(3)).

In this implementations, “fpos_t” is a complex object that represents both the position and the parse state of the stream, making these routines the only way to portably reposition a text stream. The pos argument of `fsetpos`() must always be initialized by a call to `fgetpos`().

# RETURN VALUES

The `rewind`() function returns no value.

Upon successful completion, `fgetpos`(), `fseek`(), `fseeko`(), and `fsetpos`() return 0, whereas the functions `ftell`() and `ftello`() return the current offset. On failure, `fseek`(), `fseeko`(), `ftell`(), and `ftello`() return -1, while `fgetpos`() and `fsetpos`() return a nonzero value.

On error all functions set the global variable errno to indicate the error. Since the `rewind`() function does not return an error code, applications need to clear errno before calling it in order to detect errors.

# ERRORS

[`EBADF`]
  The stream specified is not a seekable stream.
[`EINVAL`]
  The whence argument to `fseek`() was not `SEEK_SET`, `SEEK_END`, or `SEEK_CUR`.
[`EOVERFLOW`]
  For `ftell`(), the current file offset cannot be represented correctly in an object of type long.

The function `fgetpos`(), `fseek`(), `fseeko`(), `fsetpos`(), `ftell`(), `ftello`(), and `rewind`() may also fail and set errno for any of the errors specified for the routines fflush(3), fstat(2), lseek(2), and malloc(3).

# SEE ALSO

lseek(2), clearerr(3), ungetc(3)

# STANDARDS

The `fgetpos`(), `fsetpos`(), `fseek`(), `ftell`(), and `rewind`() functions conform to ANSI X3.159-1989 (“ANSI C89”). The `fseeko`() and `ftello`() functions conform to X/Open System Interfaces and Headers Issue 5 (“XSH5”).

# BUGS

The `fgetpos`() and `fsetpos`() functions don't store/set shift states of the stream in this implementation.

September 11, 2021 | NetBSD 10.0