Here is the code (valid C and C++)
#include <stdio.h>
int main() {
printf("asfd");
// LINE 1
return 0;
}
If in line 1 I put segfaulting expression the program would just crash without printing anything (as expected).
But why is the above code printing "asdf" and not exiting without buffer being flushed? What is under the hood and why does it work as expected?
stdout. flush() forces it to “flush” the buffer, meaning that it will write everything in the buffer to the terminal, even if normally it would wait before doing so.
Will buffer be automatically flushed to disk when a process exits? In general the answer is no.
A stream is flushed implicitly in the following situations: The predefined streams cout and clog are flushed when input is requested from the predefined input stream (cin). The predefined stream cerr is flushed after each output operation.
std::endl writes a newline to the output stream and flushes it. In most cases, the developer doesn't need to flush. Flushing operation is expensive, because it involves a system call. Instead of std::endl, you can simply write “\n”.
This is accomplished by these two sections in the C++ language specification:
[basic.start.main]
A return statement in
main
has the effect of leaving the main function and callingexit
with the return value as the argument.
and
[lib.support.start.term]
The function
exit
has additional behavior in this International Standard:
- ...
- Next, all open C streams with unwritten buffered data are flushed.
- ...
Generally, a return from main
is not the end of your program, nor is entry to main
the start.
Usually, the linker that creates the final executable for your program marks some location, perhaps named start
, as the place where execution is to begin. When the operating system loads your program and starts executing it, it starts execution at this place. There is code there that sets up an environment: Creates a stack, sets stream states, et cetera. Then this code calls main
.
When main
returns, it returns to this special code. That code then performs various clean-up work that is required at the end of a C or C++ program, as described in this answer.
If a program is terminated abruptly, this final code might not be executed.
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