Logging results of fork() in C to file to see results - c

Question

I'm trying to get the results of each run of the program (both the parent and child). The results print once on the screen and only once in a file. I can't seem to get two unique files created (one representing parent and one representing child). I'm not sure if getpid() is the effective way to separate parent and child identification. What could I be doing wrong?
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
static char *app1="/path/to/app1";
static char *app;
static pid_t forky=-1;
void otherfunction(){
int aflag=1;
//do something
if (aflag==1){
//run app 1
app=app1;
printf("Starting fork\n");
forky=fork();
}
}
int main(){
char dat[40000],test[10000];
sprintf(dat,"Exec start\nFORKY = %d\nPID = %d\nPPID = %d\n",forky,getpid(),getppid());
sprintf(test,"/TEST%d",getpid());
int h=open(test,O_WRONLY|O_CREAT);
write(1,dat,strlen(dat));
write(h,dat,strlen(dat));
close(h);
otherfunction();
return 0;
}

Answer 1

You're creating the file before you call fork. The fork is the last thing you do and then both processes just return 0.

Answer 2

As specified in fork's man page, the process created by calling fork is a copy of the parent process except for some specific differences, and this child process starts execution as if resuming from after the call to fork. So, it's kind of like you get two returns from fork, one for the parent and one for the child. So, it looks like you ask two questions here:
How to differentiate parent and child
Again, the man page mentions that fork will return the child's pid in the parent process and 0 for the child process so the following code sample will get you distinguished output from both:
#include <stdio.h>
#include <unistd.h>
int main(int argc, char **argv)
{
pid_t pid = fork();
if (pid == 0)
printf("Hello from child!!!\n");
else if(pid > 0)
printf("Hello from parent!!!\n");
else
printf("Wow, fork failed!!!");
return 0;
}
Obtaining separate files for each process
As mentioned above, both processes resume from after the call to fork, so the files must be created after calling to fork. In your example you are calling otherfunction last in main, so fork is pretty much the last call in both processes.
The following is an example that will give you different files with different content for each process, as well as print in stdout for each process. The usage of getpid here is just so you can actually check what the man page says.
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int main(int argc, char **argv)
{
pid_t pid = fork();
int h;
char *test;
char dat[100];
if (pid == 0)
test = "child";
else if(pid > 0)
test = "parent";
else
test = "failed";
h = open(test,O_WRONLY|O_CREAT);
sprintf(dat, "%s | fork returned = %d | my_pid = %d\n", test, pid, getpid());
write(1,dat,strlen(dat));
write(h,dat,strlen(dat));
close(h);
}