Execing Processes in C++

Our example demonstrates how to replace the current process with a new one using the exec system call in C++. This is similar to the classic exec function in Unix-like operating systems.

#include <iostream>
#include <cstdlib>
#include <unistd.h>
#include <vector>

int main() {
    // For our example we'll exec 'ls'. We need the full path to the binary.
    const char* binary = "/bin/ls";

    // exec requires arguments in a vector of C-strings.
    // The first argument should be the program name.
    std::vector<char*> args = {
        const_cast<char*>("ls"),
        const_cast<char*>("-a"),
        const_cast<char*>("-l"),
        const_cast<char*>("-h"),
        nullptr  // The last element must be nullptr
    };

    // exec also needs the current environment.
    extern char** environ;

    // Here's the actual execve call. If this call is successful,
    // the execution of our process will end here and be replaced
    // by the '/bin/ls -a -l -h' process. If there is an error,
    // execve will return -1.
    if (execve(binary, args.data(), environ) == -1) {
        std::cerr << "execve failed: " << std::strerror(errno) << std::endl;
        return 1;
    }

    // This line will only be reached if execve fails
    return 0;
}

When we run our program, it is replaced by ls:

$ g++ -o exec_example exec_example.cpp
$ ./exec_example
total 24K
drwxr-xr-x  2 user user 4.0K Jun 15 10:00 .
drwxr-xr-x 10 user user 4.0K Jun 15 09:55 ..
-rwxr-xr-x  1 user user  17K Jun 15 10:00 exec_example
-rw-r--r--  1 user user  750 Jun 15 09:59 exec_example.cpp

Note that C++ doesn’t offer a direct equivalent to Go’s exec.LookPath. In this example, we’ve used the full path to ls. In a more robust implementation, you might want to search the PATH environment variable to find the executable.

Also, C++ doesn’t have built-in support for getting the current environment variables like Go’s os.Environ(). Instead, we use the environ external variable, which is a null-terminated array of strings representing the current environment.

Unlike Go, C++ (and C) do offer a classic Unix fork function. However, for many use cases, creating new processes or using threads can cover similar functionality.

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Execing Processes in C++