int setvbuf(FILE *restrict stream, char *restrict buf,
The setvbuf() function may be used after the stream pointed to by stream is associated with an open file but before any other operation (other than an unsuccessful call to setvbuf()) is performed on the stream. The argument type determines how stream shall be buffered, as follows:
If buf is not a null pointer, the array it points to may be used instead of a buffer allocated by setvbuf() and the argument size specifies the size of the array; otherwise, size may determine the size of a buffer allocated by the setvbuf() function. The contents of the array at any time are unspecified.
For information about streams, see Standard I/O Streams .
Upon successful completion, setvbuf() shall return 0. Otherwise, it shall return a non-zero value if an invalid value is given for type or if the request cannot be honored, and may set errno to indicate the error.
The setvbuf() function may fail if:
The following sections are informative.
A common source of error is allocating buffer space as an "automatic" variable in a code block, and then failing to close the stream in the same block.
With setvbuf(), allocating a buffer of size bytes does not necessarily imply that all of size bytes are used for the buffer area.
Applications should note that many implementations only provide line buffering on input from terminal devices.
Standard I/O Streams , fopen() , setbuf() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>