If stream points to an output stream or an update stream in which the most recent operation was not input, fflush() causes any unwritten data for that stream to be written to the file, and the st_ctime and st_mtime fields of the underlying file are marked for update.

If stream is a null pointer, fflush() performs this flushing action on all streams for which the behavior is defined above. Additionally, an input stream or an update stream into which the most recent operation was input is also flushed if it is seekable and is not already at end-of-file. Flushing an input stream discards any buffered input and adjusts the file pointer such that the next input operation accesses the byte after the last one read. A stream is seekable if the underlying file is not a pipe, FIFO, socket, or TTY device. An input stream, seekable or non-seekable, can be flushed by explicitly calling fflush() with a non-null argument specifying that stream.

Upon successful completion, fflush() returns 0. Otherwise, it returns EOF and sets errno to indicate the error.

The fflush() function will fail if:

EAGAIN

The O_NONBLOCK flag is set for the file descriptor underlying stream and the process would be delayed in the write operation.

EBADF

The file descriptor underlying stream is not valid.

EFBIG

An attempt was made to write a file that exceeds the maximum file size or the process's file size limit; or 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 fflush() function was interrupted by a signal.

EIO

The process is a member of a background process group attempting to write to its controlling terminal, TOSTOP is set, the process is neither ignoring nor blocking SIGTTOU, and the process group of the process is orphaned.

ENOSPC

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

EPIPE

An attempt is made to write to a pipe or FIFO that is not open for reading by any process. A SIGPIPE signal will also be sent to the process.

The fflush() function may fail if:

ENXIO

A request was made of a non-existent device, or the request was beyond the limits of the device.

See attributes(5) for descriptions of the following attributes:

getrlimit(2), ulimit(2), attributes(5)