How to get return value by C program in python

Question

I have a C(on windows) program which has a main function which returns a value.

#include <stdio.h>    
int testData()
{
  int testErr = 0;    
  // ....

  return(testErr);    
}

int main(void) {
  int mainErr = 0;

  mainErr = testData();  
  printf("mainerr = %d", mainErr);

  return mainErr;
}

This C code executable myTest.exe is run through python.

self.run_cmd = "myTest.exe "+cmd           # pass agruments to the C exe

    self.testRun = subprocess.run(self.run_cmd,
                                   stdout=subprocess.PIPE, stdin=None, stderr=subprocess.PIPE,
                                   bufsize=0, universal_newlines=True, timeout=100)
    print(">>>>", self.testRun.stdout)
    print(">>>>", self.testRun.stderr)

Using "print(">>>>", self.testRun.stdout)" I am not getting the C code's return value in python. In python how to get the return value returned by C main.

Does this answer your question? How to get exit code when using Python subprocess communicate method? — ChatterOne
– ChatterOne, Commented Jan 3, 2020 at 7:25
Thanks ChatterOne for the response. But it did not help. Trying other suggestions. — RKum
– RKum, Commented Jan 3, 2020 at 7:31

Some programmer dude · Accepted Answer · 2020-01-03 07:23:11Z

2

The run function returns a CompletedProcess object, which have a returncode property.

answered Jan 3, 2020 at 7:23

Some programmer dude

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Comments

Soumyajit · Accepted Answer · 2020-01-03 07:28:13Z

0

In python subprocess module monitors the return value of the command (i.e. 0 for success and 1 for failure). Since you are returning some value from your C program the program successfully exited with return code 0. What you can do here is you can use exit(1) in your C program to terminate the program with exit code 1 where you want to return 1. The exit code of the C program can be monitored using,

subproc = subprocess.run(<command to execute>, stdout=subprocess.PIPE, stderr=subprocess.PIPE,)
if (subproc.returncode == 0):
    logger.info('Exited successfully')

Other way is to write your desired output in some files or DB and read that from your python code.

edited Jan 3, 2020 at 7:28

answered Jan 3, 2020 at 7:24

Soumyajit

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2 Comments

nickelpro Over a year ago

The return code will be whatever is return'ed from main, calling exit when you're terminating the program by returning from main is unnecessary

Soumyajit Over a year ago

I agree, but for both return 0 and return 1 the subproc.returncode will always be 0 for a successful termination of the program.

Parv Sachdeva · Accepted Answer · 2020-01-03 07:30:45Z

0

The return value obtained from using subprocess will be bytes. After running :

self.testRun = subprocess.run(self.run_cmd, stdout=subprocess.PIPE, stdin=None, stderr=subprocess.PIPE,  bufsize=0, universal_newlines=True, timeout=100)

You can capture the output as :

result = self.testRun.stdout.decode('utf-8')

This result is a string and can be used further.

answered Jan 3, 2020 at 7:30

Parv Sachdeva

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1 Comment

RKum Over a year ago

Thanks Parv for the suggestion. Since stdout is already in utf format so no need of decoding it. stdout does give only the return value of the main but entire printf logging done by the C program and the python script.

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