int fseek(FILE *stream, long offset, int whence);
int fseeko(FILE *stream, off_t offset, int whence);
IEEE Std 1003.1-2017
The purpose is to reposition a file-position indicator in a stream. The
fseek() function shall set the file-position indicator for the stream pointed to by stream. If a read or write
error occurs, the error indicator for the stream shall be set and
The new position, measured in bytes from the beginning of the file, shall be obtained by adding offset to the position
specified by whence. The specified point is the beginning of the file for
SEEK_SET, the current value of the file-position
SEEK_CUR, or end-of-file for
If the stream is to be used with wide-character input/output functions, the application shall ensure that offset is
0 or a value returned by an earlier call to
ftell() on the same stream and
A successful call to
fseek() shall clear the end-of-file indicator for the stream and undo any effects of
ungetwc() on the same stream.
fseek() call, the next operation on an update stream may be either input or output.
If the most recent operation, other than
ftell(), on a given stream is
fflush(), the file offset in the underlying open file description shall be adjusted to
reflect the location specified by
fseek() function shall allow the file-position indicator to be set beyond the end of existing data in the file. If
data is later written at this point, subsequent reads of data in the gap shall return bytes with the value
0 until data is actually
written into the gap.
The behavior of
fseek() on devices which are incapable of seeking is implementation-defined. The value of the file offset
associated with such a device is undefined.
If the stream is writable and buffered data had not been written to the underlying file,
fseek() shall cause the
unwritten data to be written to the file and shall mark the last data modification and last file status change timestamps of the
file for update.
In a locale with state-dependent encoding, whether
fseek() restores the stream's shift state is
fseeko() function shall be equivalent to the
fseek() function except that the offset argument is of
fseeko() functions shall return
0 if they succeed.
Otherwise, they shall return
-1 and set
errno to indicate the error.
functions shall fail if, either the stream is unbuffered or the stream's buffer needed to be flushed, and
the call to
fseeko() causes an underlying
write() to be invoked, and:
O_NONBLOCKflag is set for the file descriptor and the thread would be delayed in the write operation.
EBADF- The file descriptor underlying the stream file is not open for writing or the stream's buffer needed to be flushed and the file is not open.
EFBIG- An attempt was made to write a file that exceeds the maximum file size.
EFBIG- An attempt was made to write a file that exceeds the file size limit of the process.
EFBIG- The file is a regular file and an attempt was made to write at or beyond the offset maximum associated with the corresponding stream.
EINTR- The write operation was terminated due to the receipt of a signal, and no data was transferred.
EINVAL- The whence argument is invalid. The resulting file-position indicator would be set to a negative value.
EIO- A physical I/O error has occurred, or the process is a member of a background process group attempting to perform a
write()to its controlling terminal,
TOSTOPis set, the calling thread is not blocking
SIGTTOU, the process is not ignoring
SIGTTOU, and the process group of the process is orphaned. This error may also be returned under implementation-defined conditions.
ENOSPC- There was no free space remaining on the device containing the file.
fseek(), the resulting file offset would be a value which cannot be represented correctly in an object of type
fseeko(), the resulting file offset would be a value which cannot be represented correctly in an object of type
EPIPE- An attempt was made to write to a pipe or FIFO that is not open for reading by any process; a
SIGPIPEsignal shall also be sent to the thread.
ESPIPE- The file descriptor underlying stream is associated with a pipe, FIFO, or socket.
functions may fail if:
ENXIO- A request was made of a nonexistent device, or the request was outside the capabilities of the device.