Why doesn't printf() produce output when I expect it to?

For historical reasons, most Unix C libraries' stdio default to using line buffered streams, whereas most DOS and OS/2 C libraries' stdio default to using fully buffered streams. ANSI C species that standard output should be line buffered when connected to an interactive device, but not all libraries are ANSI compliant. You can control the buffering algorithm used for a particular stream with the setvbuf() function.

If you didn't understand that paragraph, read on.

printf() is part of the Standard I/O (stdio) library, which uses buffered streams for file IO. ANSI C specifies three algorithms for deciding when to flush the buffer (i.e. when to print buffered data to the file):

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not buffered. Data is flushed to the file as soon as possible, usually immediately after being received.

line buffered. Data is flushed to the file when a newline is received (and the newline is also flushed).

fully buffered. Data is flushed to the file when the buffer is full.

Buffered data is always flushed when the stream is closed or when fflush() is called. Since standard output is flushed when main() exits, all data printed with printf() will appear at that time, if it has not already. However, ANSI C does not require that a stream be flushed when scanf() is called on it. Therefore, if you print to a fully buffered stream and then request input on it, it is likely that the input will be read before the printed data appears.

You can control the buffering algorithm used for a particular stream with the setvbuf() function. For example, the statement setvbuf(stdout, NULL, _IOLBF, BUFSIZ) sets standard output to be line-buffered, which is what most Unix programmers expect. Any decent C reference will cover all of this material.

[Colin Jensen...]

After reading one too many bug reports about this phenomena, the gcc/2 maintainer changed its stdout to not-buffered whenever stdout is interactive. When stdout is sent to a pipe or file, stdout is still fully-buffered.