If the stream is writable and...

If the stream has an underlying file description and is writable, and...

If the stream was created by fmemopen(), open_memstream(), or open_wmemstream() and the stream is writable, and if the stream is buffered and data in the stream's buffer has not been written to the underlying memory buffer, fseek() shall cause the unwritten data to be written to the underlying memory buffer.

[EINVAL]

The whence argument is invalid. The resulting file-position indicator would be set to a negative value.

[EINVAL]

The whence argument is invalid, the resulting file-position indicator would be set to a negative value, or the stream was created by fmemopen() and the resulting file-position indicator would be beyond the end of the underlying memory buffer.

[ENOSPC]

There was no free space remaining on the device containing the file.

[ENOMEM]

The stream was created by open_memstream() or open_wmemstream() and insufficient memory is available.

[ENOSPC]

There was no free space remaining on the device containing the file or in the buffer used by the fmemopen() function.

None.

When the stream was created by fmemopen(), fseek() fails if an attempt is made to seek beyond the end of the underlying memory buffer. This is different than fseek() on a file when a file size limit is in effect because the size specified to fmemopen() is a fixed, absolute limit whereas a file size limit is artificial and can be changed. With a file size limit, it is possible to seek past the limit, then raise the limit and successfully write at the new position; there is no equivalent possibility with the buffer size specified to fmemopen().

[CX]If the stream has an underlying file description and is writable, and buffered data has not been written to the underlying file, fsetpos() 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.

If the stream was created by fmemopen(), open_memstream(), or open_wmemstream() and the stream is writable, and if the stream is buffered and data in the stream's buffer has not been written to the underlying memory buffer, fsetpos() shall cause the unwritten data to be written to the underlying memory buffer.[/CX]

[ENOSPC]

There was no free space remaining on the device containing the file.

[ENOMEM]

The stream was created by open_memstream() or open_wmemstream() and insufficient memory is available.

[ENOSPC]

There was no free space remaining on the device containing the file or in the buffer used by the fmemopen() function.

None.

The ERRORS section does not include an [EINVAL] error equivalent to the one for fseek() because applications are required to obtain the fpos_t value using fgetpos(), in which case the file position to be set will always be valid. Directly manipulating the fpos_t object to set a position results in undefined behavior.  However, if an implementation detects that the requested file position would be a negative value, or would be beyond the end of the underlying memory buffer of a stream that was created by fmemopen(), it is recommended that fsetpos() returns a non-zero value and sets errno to [EINVAL].