void setbuf (FILE stream, char buf);
int setvbuf (FILE stream, char buf, int type, size_t size);
setbuf can also be used after a stream has been opened and written to if the buffer has been flushed with fflush prior to calling setbuf. [See fopen(3S)]. This will prevent loss of data in the existing buffer.
To use setbuf after a read, make sure that the application has read all of the characters in one buffer and make sure that it will not receive any more characters until after calling setbuf.
While there is no limitation on the size of the buffer, the constant BUFSIZ, defined in the stdio.h header file, is typically a good buffer size:
setvbuf may be used after a stream has been opened but before it is read or written. type determines how stream will be buffered. Valid values for type (defined in stdio.h) are:
If buf is not the NULL pointer, the array it points to is used for buffering, instead of an automatically allocated buffer. size specifies the size of the buffer to be used. If input/output is unbuffered, buf and size are ignored.
For a further discussion of buffering, see Intro(3S).
Parts of buf are used for internal bookkeeping of the stream and, therefore, buf contains less than size bytes when full. It is recommended that the automatically allocated buffer is used when using setvbuf.