what is the point of using the setvbuf() function in c?

Question

Why would you want to set aside a block of memory in setvbuf()?

I have no clue why you would want to send your read/write stream to a buffer.

Answer 1

setvbuf is not intended to redirect the output to a buffer (if you want to perform IO on a buffer you use sprintf & co.), but to tightly control the buffering behavior of the given stream.

In facts, C IO functions don't immediately pass the data to be written to the operating system, but keep an intermediate buffer to avoid continuously performing (potentially expensive) system calls, waiting for the buffer to fill before actually performing the write.

The most basic case is to disable buffering altogether (useful e.g. if writing to a log file, where you want the data to go to disk immediately after each output operation) or, on the other hand, to enable block buffering on streams where it is disabled by default (or is set to line-buffering). This may be useful to enhance output performance.

Setting a specific buffer for output can be useful if you are working with a device that is known to work well with a specific buffer size; on the other side, you may want to have a small buffer to cut down on memory usage in memory-constrained environments, or to avoid losing much data in case of power loss without disabling buffering completely.