int main(void)
{
printf("Hello, world! \n");
fork();
return 0;
}
why it is print only 2 hello worlds? if evert time when the system excute the function fork() new procces is created, it need to print "Hello, world! \n" forever?
2 Answers
This Program should be printing Hello world once. still if it prints it twice it is because the line buffer is not cleared.
The line buffer should be cleared because there is \n in your printf.still its not cleared means this is about the platform you are using to execute the code.
You can verify this by adding fflush(stdout) after the printf().
int main(void)
{
printf("Hello, world! \n");
fflush(stdout);
fork();
return 0;
}
4 Comments
Christian Gibbons
I don't think the issue is the compiler so much as the system running it. The site OP used lets you choose different compilers and they exhibit the same issue (I tried it with both gcc and clang on the site).
LearningC
Its about the platform/site he is using. in system/other sites "\n" will clear the line buffer. ya it may not be in the compiler they are using. its somewhere in thier system probably.
LearningC
@ChristianGibbons anyway i cant tell its the compiler. edited the answer. thanks
Ingo Leonhardt
It's about stdout redirection, which changes buffering mode. I've just compiled in linux:
./a.out --> one print, ./a.out | more --> two prints. And of cours all online compilers have to redirect stdoutWhen you are executing your program, fork creates a new process and continues execution at the point that you call fork().
So when you reach fork(), the program has already called printf("Hello, world! \n");, and both the parent and child processes just return 0; and the program finished execution.
If you just want to print "Hello world" forever, just do:
while(true) {
printf("Hello, world! \n");
}
If you wanted to make a fork bomb (bad):
while(true) {
fork();
printf("Hello, world! \n");
}
I wouldn't recommend running this code, as its unsafe and will probably crash your terminal/computer.
3 Comments
Kevin
Depending on what system you're on you can run
ulimit -u N to limit yourself to N running processes. So even if you accidentally (or not) run a fork bomb it won't break the machine. It may still be difficult to kill all of the processes though (I suggest SIGSTOP followed by SIGKILL)
eqwert
@Kevin Ah, I didn't know about that
ulimit command. That could save quite a few accidental mistakes. Thanks!Kevin
It's saved my butt a few times :)
forksplits off a new process from the point at which theforkis called."Hello, world! "once, as you fork after the print.fflush(stdout)before forking, it prints correctly.