I would like to know how I can execute a Python (or Lua etc) script from my C code using execl (or similar)?
The following is some “parent / child” code showing how I am sending a STREAM of data to the child using PIPES. The code may not be perfect but you get the idea. Note the execl at the bottom:
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define STDIN_FILENO 0 /* Standard input. */
#define STDOUT_FILENO 1 /* Standard output. */
#define STDERR_FILENO 2 /* Standard error output. */
#define MAXLINE 4096
int main(void){
int n, parent_child_pipe[2], child_parent_pipe[2];
pid_t pid;
char line[MAXLINE];
if (pipe(parent_child_pipe) < 0 || pipe(child_parent_pipe) < 0)
puts("Error creating pipes...\n");
if ( (pid = fork()) < 0)
puts("Error forking...\n");
else if (pid > 0) { /* PARENT */
close(parent_child_pipe[0]);
close(child_parent_pipe[1]);
while (fgets(line, MAXLINE, stdin) != NULL) {
n = strlen(line);
if (write(parent_child_pipe[1], line, n) != n)
puts("write error to pipe...\n");
if ( (n = read(child_parent_pipe[0], line, MAXLINE)) < 0)
puts("read error from pipe...\n");
if (n == 0) {
puts("child closed pipe...\n");
break;
}
line[n] = 0; /* null terminate */
if (fputs(line, stdout) == EOF)
puts("fputs error...\n");
}
if (ferror(stdin))
puts("fgets error on stdin...\n");
exit(0);
} else { /* CHILD */
close(parent_child_pipe[1]);
close(child_parent_pipe[0]);
if (parent_child_pipe[0] != STDIN_FILENO) {
if (dup2(parent_child_pipe[0], STDIN_FILENO) != STDIN_FILENO)
puts("dup2 error to stdin...\n");
close(parent_child_pipe[0]);
}
if (child_parent_pipe[1] != STDOUT_FILENO) {
if (dup2(child_parent_pipe[1], STDOUT_FILENO) != STDOUT_FILENO)
puts("dup2 error to stdout...\n");
close(child_parent_pipe[1]);
}
**if (execl("./child", "child", (char *) 0) < 0)**
puts("execl error...\n");
}
}
Now the “child” program above is written in C and simply receives the stream via STDIN, manipulates the stream, and sends it back out using STDOUT.
Something like:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define STDIN_FILENO 0 /* Standard input. */
#define STDOUT_FILENO 1 /* Standard output. */
#define STDERR_FILENO 2 /* Standard error output. */
#define MAXLINE 4096
int main(void){
int n;
char line[MAXLINE];
while ( (n = read(STDIN_FILENO, line, MAXLINE)) > 0) {
line[n] = 0; /* null terminate */
n = strlen(line);
if (write(STDOUT_FILENO, line, n) != n)
puts("write error");
}
exit(0);
}
So that is working fine, but now I want to be able to write the CHILD in any scripting language like Python / Lua etc. How can I do this? I have tried stuff like:
if (execl(“path to python”, “test.py”, (char *) 0) < 0)
But it just seems to HANG waiting to receive input?
Can someone please help me on this? I assume PIPES can talk to anything like Lua / Python that can read from STDIN and send back to STDOUT?
UPDATE:
I have made some small changes now, here is the Python file: “NullFilter.py” that simply ECHOS what it is sent:
#!/usr/bin/python
import sys
class NullFilter:
def execute(self):
#Read data from STDIN...
data = sys.stdin.read()
#Write data to STDOUT...
sys.stdout.write(data)
exit(0)
if __name__ == '__main__':
nf = NullFilter()
nf.execute()
And now the C code calls it using:
...
if (execl("/usr/bin/python","./NullFilter.py","./NullFilter.py",NULL, (char *) 0) < 0)
puts("execl error...\n");
...
When I run it now I can enter text into STDIN, but have to hit CRTL-C to see what has happened: Here is the result:
debian@debian:~/Desktop/pipe example$ ./parent
hello
hello again
^CTraceback (most recent call last):
File "./NullFilter.py", line 17, in <module>
nf.execute()
File "./NullFilter.py", line 10, in execute
debian@debian:~/Desktop/pipe example$ data = sys.stdin.read()
KeyboardInterrupt
debian@debian:~/Desktop/pipe example$
UPDATE 2:
OK, so I have been playing around a little, and simply changed the Python code from “sys.stdin.read()” to “sys.stdin.readline()” and it seems to work to a certain degree, BUT NOT PERFECT at all…
#!/usr/bin/python
import sys
class NullFilter:
def execute(self):
#Read data from STDIN...
data = ""
for line in sys.stdin.readline():
data = data + line
#Write data to STDOUT...
sys.stdout.write(data)
exit(0)
if __name__ == '__main__':
nf = NullFilter()
nf.execute()
This is a workaround BUT not perfect at all. Any other ideas how I can get the STDIN to read the stream UNBUFFERED? I have looked at the SELECT module in Python:
http://docs.python.org/library/select.html
I have also tried passing “-u” to Python to make it “unbuffered” but still no luck ;-(
But surely this cannot be so difficult?
Lynton
After much playing around and trying to flush everything I realised I needed to CLOSE the STDOUT end of the PIPE going from the parent to the child (after writing to the pipe of course)…
So the code is now:
You can see the “close(parent_child_pipe[1]);” just after writing to the PIPE above, that is the crucial piece I had to do. That will force the stream to be flushed to the Python script, Lua script , C code etc….
In the above you will see I am executing a Python script “NullFilter.py”….
NOTE: If you run the code above it will work for one iteration of input / output as the Python script closes the pipe after the first test, but the essentials are there for you to build on…
Thanks for all the help though, I have learnt a lot from this exercise 😉
Lynton