i am trying to use fork and exec to print out hello, do an ls -a, then print world.

my program prints hello, does an ls -a twice on each file in pwd, then exits without printing world (output below).

how to make exec return and continue running my program.

thanks,
schneidz



schneidz@lq> uname -a -M -p
AIX lq 2 5 00yyyyyy4C00 powerpc IBM,7040-671
schneidz@lq> cat schneidz.c
#include "stdio.h"
#include "unistd.h"

main(int argc, char *argv[])
{
printf("\thello \n");
f_fork();
execlp("ls", "ls", "-a", 0);
printf("\tworld \n");
}
schneidz@lq> cc -g schneidz.c -o schneidz.x
schneidz@lq> schneidz.x
hello
. .. hello.tmp schneidz.c schneidz.x shh.txt
. .. hello.tmp schneidz.c schneidz.x shh.txt

execlp() is being called in the parent and the child. I have no idea what f_fork() is, but normally you'd do something like if(!fork()) { execlp(whatever); } else { printf("\tworld \n"); }

That way execlp() is only called in the parent. fork() returns the PID of the child process to the parent, but always returns 0 to the child.

1 members found this post helpful.

You cannot make execlp() "return". It only returns if there was an error executing the new program.

execlp() replaces the current process with the new program to execute. The program that the process was running is removed from memory, and does not come back.

The only way around this is to first create another process using fork(). Then call execlp(), but only in the child process. This child process is then running "ls" instead of your program. The "ls" program will of course not print "World", so you'll need to do that in the parent process, which should still be running the original program.

fork() duplicates the current program in a new process. So after fork() 2 processes are running concurrently. This means the last message ("World") printed by the parent process may appear in the middle of the output of "ls -a". If you do not want this to happen, you can wait for the child to finish, and then print "World". You can use wait() for a child process to finish. See "man 2 wait" for more info.

This "fork() a child proces, exec() another program, and wait for it to finish" is needed quite often. Therefore there is a library function that does all of this at once: system()

Here's an example how use system() to do what you are trying:
(if the teacher really wants you to use fork() and execlp() and not system(), check out their man pages and the hints above)

1 members found this post helpful.

thanks guys:

system() will work just good for me.

the aix man page says to use f_fork if an exec is called directly after. i assumed it was to help return to the calling program.

take care,
schneidz

Actually it's probably similar to the old vfork which would not copy the parent's pages and page tables to the child (since you're just going to overlay them with an exec call anyhow). Modern systems that implement proper copy on write and don't need to copy pages and page table entries don't need such hints.

Just some advice since I had to deal with this issue a few months ago. system() is generally inefficient and can be dependant on the specific installation. system() actually invokes a new shell to execute the given command. For something like ls -a this is overkill. It is better to use an exec() variant. The last time I used system() was to execute something like ls *.mp3 and I couldn't figure out how to invoke regexp expansion using exec, so I used system instead.

FYI: You can find more about wildcard expansion in "man 3 glob" and "man 3 wordexp".

Thank you. I did not know of those functions (although I guessed something like them existed). This seems to be a problem with a lot of people (including myself obviously). We need a function that does something common, but don't know what it's called or how to find it easily.

can someone tell me how to use a variable as a parameter to the system function? please see code and output below:
thx,

1 members found this post helpful.

thanks itsme,

worked like a charm