Mysterious Fork() call?

Question

Comments

@Arjun Sir plz explain this.

Answer 1

The given answer is wrong. The output may be "HiHi" or "Hi". At the time of fork, printf might not have been executed and hence the code of printing might be copied to child as well. (printf is a non-blocking call). To avoid this behaviour, either we have to use fflush or use the return value of printf before calling fork.

Comments

sir,

if printf is after fork .does it give the same ouput.

what if the program like this(t1 is copied to all or printf will copied to all child)

t1=0,t2=0;

t2=fork();

Fork();

Printf("Hi");

Fork();

t1=Fork();

Printf("hi"); 

sir I don't know the buffer concept.please explain with tree method. How many child and how many time Hi printed.

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total 20 hi printed - first printf 4 times , second printf 16 times

total 16 process - 1 parent , 15 child.

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sir have alook at it once  can we relate this to our answer 

#include  <stdio.h>
#include  <string.h>
#include  <sys/types.h>

#define   MAX_COUNT  200
#define   BUF_SIZE   100

void  main(void)
{
     pid_t  pid;
     int    i;
     char   buf[BUF_SIZE];

     fork();
     pid = getpid();
     for (i = 1; i <= MAX_COUNT; i++) {
          sprintf(buf, "This line is from pid %d, value = %d\n", pid, i);
          write(1, buf, strlen(buf));
     }
}
What is the reason of using write rather than printf? It is because printf() is "buffered," meaning printf() will group the output of a process together. While buffering the output for the parent process, the child may also use printf to print out some information, which will also be buffered. As a result, since the output will not be send to screen immediately, you may not get the right order of the expected result. Worse, the output from the two processes may be mixed in strange ways. To overcome this problem, you may consider to use the "unbuffered" write

Answer 2

when fork() is executed, it creates two same copies of program (1 for parent and another for child) with different address space. hence both will print "Hi". hence output is :HiHi. No matter which "Hi" comes first.(order is unspecified)