How do I read the results of a system() call in C++?

Question

I'm using the following code to try to read the results of a df command in Linux using popen.

#include <iostream> // file and std I/O functions

int main(int argc, char** argv) {
    FILE* fp;
    char * buffer;
    long bufSize;
    size_t ret_code;

    fp = popen("df", "r");
    if(fp == NULL) { // head off errors reading the results
        std::cerr << "Could not execute command: df" << std::endl;
        exit(1);
    }

    // get the size of the results
    fseek(fp, 0, SEEK_END);
    bufSize = ftell(fp);
    rewind(fp);

    // allocate the memory to contain the results
    buffer = (char*)malloc( sizeof(char) * bufSize );
    if(buffer == NULL) {
        std::cerr << "Memory error." << std::endl;
        exit(2);
    }

    // read the results into the buffer
    ret_code = fread(buffer, 1, sizeof(buffer), fp);
    if(ret_code != bufSize) {
        std::cerr << "Error reading output." << std::endl;
        exit(3);
    }

    // print the results
    std::cout << buffer << std::endl;

    // clean up
    pclose(fp);
    free(buffer);
    return (EXIT_SUCCESS);
}

This code is giving me a "Memory error" with an exit status of '2', so I can see where it's failing, I just don't understand why.

I put this together from example code that I found on Ubuntu Forums and C++ Reference, so I'm not married to it. If anyone can suggest a better way to read the results of a system() call, I'm open to new ideas.

EDIT to the original: Okay, bufSize is coming up negative, and now I understand why. You can't randomly access a pipe, as I naively tried to do.

I can't be the first person to try to do this. Can someone give (or point me to) an example of how to read the results of a system() call into a variable in C++?

Answer 1

check your bufSize. ftell can return -1 on error, and this can lead to nonallocation by malloc with buffer having a NULL value.

The reason for the ftell to fail is, because of the popen. You cant search pipes.

Answer 2

First thing to check is the value of bufSize - if that happens to be <= 0, chances are that malloc returns a NULL as you're trying to allocate a buffer of size 0 at that point.

Another workaround would be to ask malloc to provide you with a buffer of the size (bufSize + n) with n >= 1, which should work around this particular problem.

That aside, the code you posted is pure C, not C++, so including is overdoing it a little.

Answer 3

I'm not sure you can fseek/ftell pipe streams like this.

Have you checked the value of bufSize ? One reason malloc be failing is for insanely sized buffers.

Answer 4

Why would malloc() fail?

The obvious reason is "because ftell() returned a negative signed number, which was then treated as a huge unsigned number".

According to the specification, ftell() returns -1 on failure. One obvious reason it would fail is that you cannot seek in a pipe or FIFO.

There is no escape; you cannot know the length of the command output without reading it, and you can only read it once. You have to read it in chunks, either growing your buffer as needed or parsing on the fly.

But, of course, you can simply avoid the whole issue by directly using the system call df probably uses to get its information: statvfs().

Answer 5

Pipes are not random access. They're sequential, which means that once you read a byte, the pipe is not going to send it to you again. Which means, obviously, you can't rewind it.

If you just want to output the data back to the user, you can just do something like:

// your file opening code

while (!feof(fp))
{
char c = getc(fp);
std::cout << c;
}

This will pull bytes out of the df pipe, one by one, and pump them straight into the output.

Now if you want to access the df output as a whole, you can either pipe it into a file and read that file, or concatenate the output into a construct such as a C++ String.

Answer 6

(A note on terminology: "system call" in Unix and Linux generally refers to calling a kernel function from user-space code. Referring to it as "the results of a system() call" or "the results of a system(3) call" would be clearer, but it would probably be better to just say "capturing the output of a process.")

Anyway, you can read a process's output just like you can read any other file. Specifically:

• You can start the process using pipe(), fork(), and exec(). This gives you a file descriptor, then you can use a loop to read() from the file descriptor into a buffer and close() the file descriptor once you're done. This is the lowest level option and gives you the most control.
• You can start the process using popen(), as you're doing. This gives you a file stream. In a loop, you can read using from the stream into a temporary variable or buffer using fread(), fgets(), or fgetc(), as Zarawesome's answer demonstrates, then process that buffer or append it to a C++ string.
• You can start the process using popen(), then use the nonstandard __gnu_cxx::stdio_filebuf to wrap that, then create an std::istream from the stdio_filebuf and treat it like any other C++ stream. This is the most C++-like approach. Here's part 1 and part 2 of an example of this approach.

Answer 7

To answer the question in the update:

char buffer[1024];
char * line = NULL;
while ((line = fgets(buffer, sizeof buffer, fp)) != NULL) {
    // parse one line of df's output here.
}

Would this be enough?

Answer 8

Thanks to everyone who took the time to answer. A co-worker pointed me to the ostringstream class. Here's some example code that does essentially what I was attempting to do in the original question.

#include <iostream> // cout
#include <sstream> // ostringstream

int main(int argc, char** argv) {
    FILE* stream = popen( "df", "r" );
    std::ostringstream output;

    while( !feof( stream ) && !ferror( stream ))
    {
        char buf[128];
        int bytesRead = fread( buf, 1, 128, stream );
        output.write( buf, bytesRead );
    }
    std::string result = output.str();
    std::cout << "<RESULT>" << std::endl << result << "</RESULT>" << std::endl;
    return (0);
}

Answer 9

You're making this all too hard. popen(3) returns a regular old FILE * for a standard pipe file, which is to say, newline terminated records. You can read it with very high efficiency by using fgets(3) like so in C:

#include <stdio.h>
char bfr[BUFSIZ] ;
FILE * fp;
// ...
if((fp=popen("/bin/df", "r")) ==NULL) {
   // error processing and return
}
// ...
while(fgets(bfr,BUFSIZ,fp) != NULL){
   // process a line
}

In C++ it's even easier --

#include <cstdio>
#include <iostream>
#include <string>

FILE * fp ;

if((fp= popen("/bin/df","r")) == NULL) {
    // error processing and exit
}

ifstream ins(fileno(fp)); // ifstream ctor using a file descriptor

string s;
while (! ins.eof()){
    getline(ins,s);
    // do something
}

There's some more error handling there, but that's the idea. The point is that you treat the FILE * from popen just like any FILE *, and read it line by line.