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Full Discussion: fork() fd
Top Forums Programming fork() fd Post 16199 by Herman on Tuesday 26th of February 2002 10:21:42 PM
Old 02-26-2002
fork() fd
I run this code, actually I want to both processes print the message from "data". But only one does. What happens? Anyone can help?


#include <stdio.h>
main(){

int fd, pid;
char x;


fd = open("data",0); /* open file "data" */
pid = fork();

if(pid != 0){
wait(0);
printf("PID = %d.\n", getpid());
printf("FD = %d.\n",fd);
while (read(fd, &x, 1)){
write(1, &x, 1);
}
}
else{
printf("PID = %d.\n", getpid());
printf("PD= %d.\n",fd);
while (read(fd, &x, 1))
write(1, &x, 1);
}
}

The out put from the code:

This is a test.
PID = 4985.
PD= 3.
PID = 4984.
FD = 3.
 

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LEARN ABOUT MOJAVE

dtruss

dtruss(1m)							   USER COMMANDS							dtruss(1m)

NAME

       dtruss - process syscall details. Uses DTrace.

SYNOPSIS

       dtruss [-acdeflhoLs] [-t syscall] { -p PID | -n name | command }

DESCRIPTION

       dtruss prints details on process system calls. It is like a DTrace version of truss, and has been designed to be less intrusive than truss.

       Of  particular  interest  is  the elapsed times and on cpu times, which can identify both system calls that are slow to complete, and those
       which are consuming CPU cycles.

       Since this uses DTrace, only users with root privileges can run this command.

OPTIONS

       -a     print all details

       -b bufsize
	      dynamic variable buffer size. Increase this if you notice dynamic variable drop errors. The default is "4m" for 4 megabytes per CPU.

       -c     print system call counts

       -d     print relative timestamps, us

       -e     print elapsed times, us

       -f     follow children as they are forked

       -l     force printing of pid/lwpid per line

       -L     don't print pid/lwpid per line

       -n name
	      examine processes with this name

       -W name
	      wait for a process matching this name

       -o     print on-cpu times, us

       -s     print stack backtraces

       -p PID examine this PID

       -t syscall
	      examine this syscall only

EXAMPLES

       run and examine the "df -h" command
	      # dtruss df -h

       examine PID 1871
	      # dtruss -p 1871

       examine all processes called "tar"
	      # dtruss -n tar

       run test.sh and follow children
	      # dtruss -f test.sh

       run the "date" command and print elapsed and on cpu times,
	      # dtruss -eo date

FIELDS

       PID/LWPID
	      Process ID / Lightweight Process ID

       RELATIVE
	      relative timestamps to the start of the thread, us (microseconds)

       ELAPSD elapsed time for this system call, us

       CPU    on-cpu time for this system call, us

       SYSCALL(args)
	      system call name, with arguments (some may be evaluated)

DOCUMENTATION

       See the DTraceToolkit for further documentation under the Docs directory. The DTraceToolkit docs may include full worked examples with ver-
       bose descriptions explaining the output.

EXIT

       dtruss will run forever until Ctrl-C is hit, or if a command was executed dtruss will finish when the command ends.

AUTHOR

       Brendan Gregg [Sydney, Australia]

SEE ALSO

       procsystime(1M), dtrace(1M), truss(1)

version 0.80							   Jun 17, 2005 							dtruss(1m)
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