execl()/fork() output

tadams · 02-03-2009, 07:38 PM

Code:

void run(char *bin, char *data)
{

if(fork())
{

     execl(bin, bin, data, 0);

}

else
{

int pid, status;

     pid = wait(&status);

}
}

.....

for(i = 0; i < 5; i++)
{

     printf("test %d\n", i);
     run(PATH, data);

}

output (with 'data' being different each time):

Code:

bash$ ./test
test 0
test 1
test 2
test 3
test 4
value 1 added
value 14 added
value 65 added
bash$

The purpose of my program is to run a program 5 different times with different arguments using execl() and fork() and be able to see the full output. As you can see in the above output, values for i=3 and i=4 cause the program I am running do a segmentation fault, which is what is supposed to happen, but it shows no output. I would like to see that output just like the previous outputs.

I think it is a problem with the code not fork()ing correctly, but I am unsure. Been hitting my head against a wall for days trying to figure this one out. A solution would be simply terrific.

Thanks so much,

Todd

MK27 · 02-03-2009, 07:45 PM

Quote:

As you can see in the above output, values for i=3 and i=4 cause the program I am running do a segmentation fault, which is what is supposed to happen, but it shows no output. I would like to see that output just like the previous outputs.

I have to call a real sleuth for this. Are you trying to say that you are surprised that you cannot find any output after the segfault?

tadams · 02-03-2009, 07:51 PM

Well, yes, since running the program manually using i=3 or i=4 shows the output then the next line "segmentation fault"., which is fine, its supposed to, but I want it to show that when I run it throw the parent process too.

MK27 · 02-03-2009, 08:09 PM

Try: SLEUTH

And if you want a spanking: the real meaning of sleuth

tadams · 02-03-2009, 08:33 PM

I honestly don't understand what your talking about or how 'sleuth' can help me.

MK27 · 02-03-2009, 09:10 PM

DITTO, or possibly this

tabstop · 02-03-2009, 09:15 PM

I'm guessing that if you want to know whether bad things happened to your execl program, you're going to have to check for it yourself (since your program is running the file, error messages aren't going to go to the terminal necessarily) via the return value of execl.

tadams · 02-03-2009, 11:49 PM

I hate to ask, but how exactly do I do that?

Codeplug · 02-04-2009, 07:47 AM

The parent has to detect the situation and do the printf() as to what happened to the child. Normally, the shell does this for you.

>> via the return value of execl.
You mean wait/waitpid - execl never returns.
http://www.opengroup.org/onlinepubs/...s/waitpid.html

gg

tadams · 02-04-2009, 10:44 AM

But I am using wait()... I don't understand, is there something wrong with my code?

tabstop · 02-04-2009, 10:49 AM

Only by omission -- you need to check status to see what happened to the process.

tadams · 02-04-2009, 11:02 AM

I know if I printf() the status it is going to just print any integer, I believe, so once again, thanks for your help, but I need a solution

Codeplug · 02-04-2009, 11:28 AM

>> you need to check status to see what happened to the process.
http://www.opengroup.org/onlinepubs/...s/waitpid.html

Let us know if the documentation is unclear.

gg

**Salem** · 02-04-2009, 11:42 AM

http://forums.devshed.com/c-programm...ut-587772.html
Your code still looks like some non-compilable pseudo-code you just typed out, rather than anything which you've run.

What's PATH?
Because execl has some rather strict rules, you can't just pass any old path to it.

Code:

if ( fork() == 0 ) {
  close(1);
  if ( open("stdout.txt",O_WRONLY) == -1 ) {
    perror("Can't redirect");
  }
  if ( execl( bin, bin, data, 0) == -1 ) {
    perror("Can't exec");
    _exit(1);
  }
} else {
  // wait
}

tadams · 02-04-2009, 12:18 PM

Code:

void run(char *bin, char *data)
{

if(fork() == 0)
{

     close(1);

if(open("stdout.txt", O_WRONLY) == -1)
{

     perror("open");

}

if(execl(bin, bin, data, 0) == -1)
{

     perror("execl");
     _exit(1);

}
}

else
{

int pid, status;

     pid = wait(&status);

}
}

Gives me: open: No such file or directory

Then when I touch stdout.txt, it doesn't give me that error anymore, but it doesnt write anything to stdout.txt either, and I still see output from the child.

Also two questions: I'm guessing close(1) closes stdout? and Why _exit(1) vs exit(1)?

Thanks.

02-03-2009, 07:38 PM	#1
tadams Registered User Join Date: Feb 2009 Posts: 9	execl()/fork() output Code: void run(char bin, char data) { if(fork()) { execl(bin, bin, data, 0); } else { int pid, status; pid = wait(&status); } } ..... for(i = 0; i < 5; i++) { printf("test %d\n", i); run(PATH, data); } output (with 'data' being different each time): Code: bash$ ./test test 0 test 1 test 2 test 3 test 4 value 1 added value 14 added value 65 added bash$ The purpose of my program is to run a program 5 different times with different arguments using execl() and fork() and be able to see the full output. As you can see in the above output, values for i=3 and i=4 cause the program I am running do a segmentation fault, which is what is supposed to happen, but it shows no output. I would like to see that output just like the previous outputs. I think it is a problem with the code not fork()ing correctly, but I am unsure. Been hitting my head against a wall for days trying to figure this one out. A solution would be simply terrific. Thanks so much, Todd
tadams is offline

02-03-2009, 07:51 PM	#3
tadams Registered User Join Date: Feb 2009 Posts: 9	Well, yes, since running the program manually using i=3 or i=4 shows the output then the next line "segmentation fault"., which is fine, its supposed to, but I want it to show that when I run it throw the parent process too.
tadams is offline

02-03-2009, 08:09 PM	#4
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	Try: SLEUTH And if you want a spanking: the real meaning of sleuth __________________ Accuracy and integrity mean nothing if you don't make it past the censors...PYTHAGORAS
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02-03-2009, 08:33 PM	#5
tadams Registered User Join Date: Feb 2009 Posts: 9	I honestly don't understand what your talking about or how 'sleuth' can help me.
tadams is offline

02-03-2009, 09:10 PM	#6
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	DITTO, or possibly this __________________ Accuracy and integrity mean nothing if you don't make it past the censors...PYTHAGORAS
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02-03-2009, 09:15 PM	#7
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	I'm guessing that if you want to know whether bad things happened to your execl program, you're going to have to check for it yourself (since your program is running the file, error messages aren't going to go to the terminal necessarily) via the return value of execl.
tabstop is offline

02-03-2009, 11:49 PM	#8
tadams Registered User Join Date: Feb 2009 Posts: 9	I hate to ask, but how exactly do I do that?
tadams is offline

02-04-2009, 07:47 AM	#9
Codeplug Registered User Join Date: Mar 2003 Posts: 3,844	The parent has to detect the situation and do the printf() as to what happened to the child. Normally, the shell does this for you. >> via the return value of execl. You mean wait/waitpid - execl never returns. http://www.opengroup.org/onlinepubs/...s/waitpid.html gg __________________ How to Ask Questions The Smart Way How to Report Bugs Effectively
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02-04-2009, 10:44 AM	#10
tadams Registered User Join Date: Feb 2009 Posts: 9	But I am using wait()... I don't understand, is there something wrong with my code?
tadams is offline

02-04-2009, 10:49 AM	#11
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	Only by omission -- you need to check status to see what happened to the process.
tabstop is offline

02-04-2009, 11:02 AM	#12
tadams Registered User Join Date: Feb 2009 Posts: 9	I know if I printf() the status it is going to just print any integer, I believe, so once again, thanks for your help, but I need a solution
tadams is offline

02-04-2009, 11:28 AM	#13
Codeplug Registered User Join Date: Mar 2003 Posts: 3,844	>> you need to check status to see what happened to the process. http://www.opengroup.org/onlinepubs/...s/waitpid.html Let us know if the documentation is unclear. gg __________________ How to Ask Questions The Smart Way How to Report Bugs Effectively
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02-04-2009, 11:42 AM	#14
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	http://forums.devshed.com/c-programm...ut-587772.html Your code still looks like some non-compilable pseudo-code you just typed out, rather than anything which you've run. What's PATH? Because execl has some rather strict rules, you can't just pass any old path to it. Code: if ( fork() == 0 ) { close(1); if ( open("stdout.txt",O_WRONLY) == -1 ) { perror("Can't redirect"); } if ( execl( bin, bin, data, 0) == -1 ) { perror("Can't exec"); _exit(1); } } else { // wait } __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
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02-04-2009, 12:18 PM	#15
tadams Registered User Join Date: Feb 2009 Posts: 9	Code: void run(char bin, char data) { if(fork() == 0) { close(1); if(open("stdout.txt", O_WRONLY) == -1) { perror("open"); } if(execl(bin, bin, data, 0) == -1) { perror("execl"); _exit(1); } } else { int pid, status; pid = wait(&status); } } Gives me: open: No such file or directory Then when I touch stdout.txt, it doesn't give me that error anymore, but it doesnt write anything to stdout.txt either, and I still see output from the child. Also two questions: I'm guessing close(1) closes stdout? and Why _exit(1) vs exit(1)? Thanks.
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