Thread: return value

Im also interested about how i can use a return value of one program (for example hello world which returns 1) in another program?

Could anyone demonstrate me a simple example?

Originally Posted by Tool

And pass output of this program into another program, what will be the value passed to that program?

In this particular example, assuming the program is compiled with respect to the 1999 edition of the C standard, then the value is 0, since this is a special exception for the main function. However, more generally, if the caller of a function with a non-void return type attempts to use the value returned by the function, but control reaches the end of the called function without encountering a return statement, then the behaviour is undefined.

Originally Posted by Tool

And how can i for example use that return value somewhere else?

It depends on how you are invoking the program. If you are invoking that program using system() from another C program, typically the return value of system() would be that corresponding return value.

In theory, it always seems to me that system() should do this:

Originally Posted by man system

The value returned is -1 on error (e.g. fork(2) failed), and the
return status of the command otherwise.

In practice, this does not work (try it). However, if you use fork/exec/waitpid the "status pointer" (2nd arg) might work.

Here's a hack method that will do it:

Code:

!#/bin/bash

./test
echo $?

Here "test" is a local executable (eg, hello world), and the correct return value is reported. So you could run the process via such a script and grab the return value on stdout. Alternately you could export an environment variable and access that.

But that is definitely an unportable hack method. Curious to see if other people have the same experience with system:

Code:

#include <stdio.h>

int main(int argc, const char *argv[]) {
	int x = system("./test");

	printf("%d\n",x);
	return 0;
}

I do not get a correct value, wheras the bash script does.