linux - In C, what's the size of stdout buffer?

Question

Today I learned that stdout is line buffered when it's set to terminal and buffered in different cases. So, in normal situation, if I use printf() without the terminating ' ' it will be printed on the screen only when the buffer will be full. How to get a size of this buffer, how big is this?

Answer 1

The actual size is defined by the individual implementation; the standard doesn't mandate a minimum size (based on what I've been able to find, anyway). Don't have a clue on how you'd determine the size of the buffer.

Edit

Chapter and verse:

7.19.3 Files

...

3 When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block. When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is ?lled. When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered. Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is ?lled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment. Support for these characteristics is implementation-de?ned, and may be affected via the setbuf and setvbuf functions.

Emphasis added.

"Implementation-defined" is not a euphemism for "I don't know", it's simply a statement that the language standard explicitly leaves it up to the implementation to define the behavior.

And having said that, there is a non-programmatic way to find out; consult the documentation for your compiler. "Implementation-defined" also means that the implementation must document the behavior:

3.4.1

1

implementation-de?ned behavior
unspeci?ed behavior where each implementation documents how the choice is made

2 EXAMPLE An example of implementation-de?ned behavior is the propagation of the high-order bit when a signed integer is shifted right.