Why do I have different output between a terminal and a file when forking?
c, fork
Solution
When you output to a file, the stdio library automatically block-buffers the outbound bits.
When a program calls `exit(2)` or returns from `main(),` any remaining buffered bits are flushed.
In a program like this that doesn't generate much output, all of the I/O will occur after the return from `main(),` when the destination is not a tty. This will often change the pattern and order of I/O operations all by itself.
In this case, the result is further complicated by the series of `fork()` calls. This will duplicate the partially filled and as-yet-unflushed I/O buffers in each child image.
Before a program calls `fork(),` one might first flush I/O using fflush(3). If this flush is not done, then you may want all processes except one (typically: the children) to `_exit(2)` instead of `exit(3)` or return from `main(),` to prevent the same bits from being output more than once. (_exit(2) just does the exit system call.)
Problem
I'm learning to work with `fork()`, and I have some questions. Consider the following code: ``` #include <stdio.h> #include <unistd.h> int main() { int i; for(i = 0; i < 5; i++) { printf("%d", i); if((i%2)==0) if(fork()) fork(); } } ``` When I output to a terminal, I get the result I expect (i.e.: `0,1,1,1,2,2,2,...`). But when I output to a file, the result is completely different: Case 1: (output to terminal, e.g.: `./a.out`): Result is: `0,1,1,1,2,2,2,...` Case 2: (output to file, e.g.: `./a.out > output_file`) Result is: `0,1,2,3,4,0,1,2,3,4,0,1,2,3,4,...` Why it is like this?