Running an External Program from C and Parsing Its Output

If you’ve been working with C programming, you might have encountered a scenario where you want to execute an external utility and process its output. For instance, you may have a utility that lists files required for a game, and you need to grab that output within your C program to perform further actions. This guide will explain how to do just that in a clear and concise manner.

The Challenge: Executing a Utility and Capturing Its Output

You want to run a utility that generates a list of strings (like file names) and obtain that output. The goal is to make sure this works across different operating systems: Windows, Mac, and Linux. Here, we will explore a simple yet effective solution using a specific function in C.

Solution Overview: Using popen()

For Unix-like environments, the function that can be used is popen(). This function enables you to run a program and read its output directly in your C code. Here’s a basic understanding of how popen() works:

What is popen()?

popen() stands for “pipe open.” It creates a pipe, forks a new process, and invokes the shell to execute the command you specify. This allows you to capture the output as if you were reading from a file.

Basic Usage of popen()

The basic syntax to use popen() is as follows:

FILE *popen(const char *command, const char *type);

• command is a string containing the command you want to run.
• type specifies how you want to access the output (“r” for reading and “w” for writing). For your use case, you should use “r”.

Example Code

Here’s a simple example of how you can implement a call to an external utility and read its output:

#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE *fp;
    char buffer[128];

    // Run the command
    fp = popen("your_utility_command_here", "r");
    if (fp == NULL) {
        perror("popen error");
        return -1;
    }

    // Read output a line at a time - output it.
    while (fgets(buffer, sizeof(buffer), fp) != NULL) {
        printf("%s", buffer); // Process the output as needed
    }

    // Close the process
    pclose(fp);
    return 0;
}

Key Steps in the Code

1. Open a Pipe: Call popen() to execute the command and obtain a file pointer fp.
2. Process Output: Use fgets() in a loop to read each line of output from the command until nothing is left.
3. Output Handling: You can print the output or store it for further processing.
4. Close the Pipe: Always use pclose() to free up resources after you’re done.

Considerations for Different Platforms

• The popen() function is widely available in UNIX-like environments (Linux, MacOS), but support on Windows is limited. It’s essential to test your implementation based on the specific operating systems you plan to target.
• For more complex needs, especially in Windows, exploring inter-process communication might be necessary. This often involves more advanced techniques for sending and receiving data between processes.

Conclusion

In summary, capturing the output of an external program within a C application can be efficiently achieved using the popen() function. This method is particularly powerful for executing utilities that generate text output. By following the steps outlined, you can easily integrate external commands into your applications, regardless of whether you’re operating on Mac, Windows, or Linux.

Feel free to extend your learning by experimenting with different commands and handling their outputs in diverse ways that suit your application’s needs!