Running an External Program from C and Parsing its Output
As programmers, we often encounter situations where we need to interact with external programs from our own code. This can be for a variety of reasons, such as utilizing specialized tools or libraries, or simply for the sake of convenience. In this article, we will explore the process of running an external program from a C program and parsing its output.
To begin with, let's consider a simple scenario where we have a C program that needs to generate a report based on some data. Instead of writing the code to generate the report from scratch, we can utilize an existing tool that can generate reports in a specific format. This not only saves us time and effort, but also ensures consistency and accuracy in the report generation process.
So, how do we go about running an external program from our C code? The answer lies in the use of the `system()` function. This function allows us to execute a command in the terminal, just like how we would do it manually. In our case, we can use it to run the external program that generates the report for us.
Let's take a look at a sample code that uses the `system()` function to run an external program named "report_generator" and pass in some arguments:
```
#include <stdio.h>
#include <stdlib.h>
int main() {
char command[100];
sprintf(command, "./report_generator -f output.txt -d data.csv");
system(command);
return 0;
}
```
In the above code, we first declare a character array to hold the command we want to execute. Then, we use the `sprintf()` function to format the command string with the appropriate arguments. Finally, we pass this string to the `system()` function, which will then execute the command in the terminal.
But what about the output of the external program? How do we access it from our C code? This is where parsing comes into play. Parsing is the process of breaking down a string of data into smaller components for easier manipulation.
In our example, let's assume that the external program "report_generator" outputs the report in a comma-separated values (CSV) format. We can use the `popen()` function to capture the output of the external program and store it in a file or a variable. Let's see how we can do this in our code:
```
#include <stdio.h>
#include <stdlib.h>
int main() {
char command[100];
sprintf(command, "./report_generator -f output.txt -d data.csv");
// create a file pointer to store the output
FILE *fp;
char buffer[500];
// open the command in read mode
fp = popen(command, "r");
// read the output and store it in the buffer
while (fgets(buffer, sizeof(buffer), fp) != NULL) {
// do something with the output
}
// close the file pointer
pclose(fp);
return 0;
}
```
In the above code, we first create a file pointer to store the output of the external program. Then, we use the `popen()` function to open the command in read mode and store the output in the buffer. Finally, we can manipulate the output as per our requirements.
In conclusion, running an external program from our C code and parsing its output is a simple and efficient way to enhance our programs. It allows us to utilize existing tools and libraries, and also gives us more control over our code. So the next time you encounter a situation where you need to interact with an external program, give this approach a try and see the difference it makes. Happy coding!
