即时将程序输出重定向到文件，而无需从批处理文件缓冲

Question

I have C program where the messages are printed with printf command. From batch file I call my program with

call myProgram.exe parms > myFile.txt

I need that myFile be updated every time (or almost every time) when myProgram print a message. Now, I cannot see anything in file until the program finish its execution, probably because some buffering. Is this buffering problem from my C program, from my batch file or from operating system, and how can I resolve it?

Answer 1

stdio.h functions are buffering. See setvbuf . The buffering may be operating system specific, and may change when redirecting (eg because internally isatty(3) could be used inside printf etc...).

^{Notice that this buffering is actually useful, and generally speeds up output a lot.}

The simplest way is to call fflush(3) from time to time. Otherwise call setvbuf with _IOFBF on stdout .

Using setvbuf near the start of the program is simpler. However, calling fflush at appropriate places might be more efficient (because using a larger buffer should be faster).