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About c : How-can-I-run-an-external-program-from-C-and-parse-its-output

Question Detail

I’ve got a utility that outputs a list of files required by a game. How can I run that utility within a C program and grab its output so I can act on it within the same program?

UPDATE: Good call on the lack of information. The utility spits out a series of strings, and this is supposed to be portable across Mac/Windows/Linux. Please note, I’m looking for a programmatic way to execute the utility and retain its output (which goes to stdout).

Question Answer

As others have pointed out, popen() is the most standard way. And since no answer provided an example using this method, here it goes:

#include <stdio.h>

#define BUFSIZE 128

int parse_output(void) {
    char *cmd = "ls -l";    
    
    char buf[BUFSIZE];
    FILE *fp;

    if ((fp = popen(cmd, "r")) == NULL) {
        printf("Error opening pipe!\n");
        return -1;
    }

    while (fgets(buf, BUFSIZE, fp) != NULL) {
        // Do whatever you want here...
        printf("OUTPUT: %s", buf);
    }

    if (pclose(fp)) {
        printf("Command not found or exited with error status\n");
        return -1;
    }

    return 0;
}

Sample output:

OUTPUT: total 16
OUTPUT: -rwxr-xr-x 1 14077 14077 8832 Oct 19 04:32 a.out
OUTPUT: -rw-r--r-- 1 14077 14077 1549 Oct 19 04:32 main.c

For simple problems in Unix-ish environments try popen().

From the man page:

The popen() function opens a process by creating a pipe, forking and invoking the shell.

If you use the read mode this is exactly what you asked for. I don’t know if it is implemented in Windows.

For more complicated problems you want to look up inter-process communication.

popen is supported on Windows, see here:

http://msdn.microsoft.com/en-us/library/96ayss4b.aspx

If you want it to be cross-platform, popen is the way to go.

Well, assuming you’re on a command line in a windows environment, you can use pipes or command line redirects. For instance,

commandThatOutputs.exe > someFileToStoreResults.txt

or

commandThatOutputs.exe | yourProgramToProcessInput.exe

Within your program, you could use the C standard input functions to read the other programs output (scanf, etc.): http://irc.essex.ac.uk/www.iota-six.co.uk/c/c1_standard_input_and_output.asp . You could also use the file example and use fscanf. This should also work in Unix/Linux.

This is a very generic question, you may want to include more details, like what type of output it is (just text, or a binary file?) and how you want to process it.

Edit: Hooray clarification!

Redirecting STDOUT looks to be troublesome, I’ve had to do it in .NET, and it gave me all sorts of headaches. It looks like the proper C way is to spawn a child process, get a file pointer, and all of a sudden my head hurts.

So heres a hack that uses temporary files. It’s simple, but it should work. This will work well if speed isn’t an issue (hitting the disk is slow), or if it’s throw-away. If you’re building an enterprise program, looking into the STDOUT redirection is probably best, using what other people recommended.

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

int main(int argc, char* argv[])
{
    FILE * fptr;                    // file holder
    char c;                         // char buffer


    system("dir >> temp.txt");      // call dir and put it's contents in a temp using redirects.
    fptr = fopen("temp.txt", "r");  // open said file for reading.
                                    // oh, and check for fptr being NULL.
    while(1){
        c = fgetc(fptr);
        if(c!= EOF)
            printf("%c", c);        // do what you need to.
        else
            break;                  // exit when you hit the end of the file.
    }
    fclose(fptr);                   // don't call this is fptr is NULL.  
    remove("temp.txt");             // clean up

    getchar();                      // stop so I can see if it worked.
}

Make sure to check your file permissions: right now this will simply throw the file in the same directory as an exe. You might want to look into using /tmp in nix, or C:\Users\username\Local Settings\Temp in Vista, or C:\Documents and Settings\username\Local Settings\Temp in 2K/XP. I think the /tmp will work in OSX, but I’ve never used one.

In Linux and OS X, popen() really is your best bet, as dmckee pointed out, since both OSs support that call. In Windows, this should help: http://msdn.microsoft.com/en-us/library/ms682499.aspx

MSDN documentation says
If used in a Windows program, the _popen function returns an invalid file pointer that causes the program to stop responding indefinitely. _popen works properly in a console application. To create a Windows application that redirects input and output, see Creating a Child Process with Redirected Input and Output in the Windows SDK.

You can use system() as in:

system("ls song > song.txt");

where ls is the command name for listing the contents of the folder song and song is a folder in the current directory. Resulting file song.txt will be created in the current directory.

//execute external process and read exactly binary or text output
//can read image from Zip file for example
string run(const char* cmd){
    FILE* pipe = popen(cmd, "r");
    if (!pipe) return "ERROR";
    char buffer[262144];
    string data;
    string result;
    int dist=0;
    int size;
    //TIME_START
    while(!feof(pipe)) {
        size=(int)fread(buffer,1,262144, pipe); //cout<<buffer<<" size="<<size<<endl;
        data.resize(data.size()+size);
        memcpy(&data[dist],buffer,size);
        dist+=size;
    }
    //TIME_PRINT_
    pclose(pipe);
    return data;
}

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