Is "argv[0] = name-of-executable" an accepted standard or just a common convention?

argv[0]: Standard or Convention? Unpacking the Program Name Argument

Explore whether argv[0] containing the executable's name is a formal standard or a widely adopted convention across C/C++ and other programming environments. Understand its implications and common behaviors.

When writing programs in C, C++, or other languages that expose the main function's arguments, developers often rely on argv[0] to provide the name of the executable. This seems intuitive and is almost universally observed. But is this behavior guaranteed by a formal standard, or is it merely a pervasive convention that we've come to expect? This article delves into the specifications, common practices, and potential variations of argv[0] across different operating systems and language runtimes.

The C Standard's Perspective on argv[0]

The C standard (ISO/IEC 9899) provides guidance on the main function's parameters, argc and argv. Specifically, section 5.1.2.2.1 'Program startup' states:

"If the value of argc is greater than zero, the string pointed to by argv[0] represents the program name; argv[0][0] shall be the null character if the program name is not available from the host environment."

This wording is crucial. It indicates that argv[0] represents the program name, but it doesn't strictly mandate that it must be the exact name used to invoke the program. It also allows for argv[0] to be an empty string (a null character at argv[0][0]) if the name isn't available. This means while it's a strong recommendation and common practice, the standard provides a degree of flexibility to the host environment.

#include <stdio.h>

int main(int argc, char *argv[]) {
    if (argc > 0) {
        printf("Program name (argv[0]): %s\n", argv[0]);
    } else {
        printf("argc is 0, program name not available.\n");
    }
    for (int i = 1; i < argc; i++) {
        printf("Argument %d: %s\n", i, argv[i]);
    }
    return 0;
}

A simple C program demonstrating access to argv[0] and other arguments.

Common Behavior and Deviations

Despite the C standard's flexibility, most modern operating systems and C/C++ runtimes adhere to the convention of populating argv[0] with the name used to invoke the program. This typically includes the path if the program was invoked with one (e.g., ./myprog, /usr/bin/myprog).

However, there are scenarios where this might not hold true:

1. Symbolic Links: If a program is invoked via a symbolic link, argv[0] usually reflects the name of the link, not the target executable.
2. exec Family Functions: When using functions like execve, the caller explicitly provides the argv array, including argv[0]. This allows for arbitrary strings to be passed as the 'program name'.
3. Embedded Systems/Minimal Environments: In highly constrained or specialized environments, the full program name might not be available or might be represented differently.
4. Windows Specifics: On Windows, argv[0] often contains the full path to the executable, even if invoked without one (e.g., myprog from PATH). The behavior can also be influenced by how the program is launched (e.g., via ShellExecute vs. CreateProcess).

These variations highlight that while the convention is strong, relying on argv[0] for absolute path resolution or security-critical identification might be problematic without additional checks.

flowchart TD
    A[Program Invocation] --> B{Host Environment Provides argv[0]?}
    B -- Yes --> C[argv[0] = Invocation Name/Path]
    B -- No --> D[argv[0] = Empty String (\0)]
    C --> E[C/C++ Runtime Receives argv]
    D --> E
    E --> F[Program Accesses argv[0]]
    F --> G{Is argv[0] reliable for path?}
    G -- Usually Yes --> H[Common Use Cases]
    G -- Sometimes No --> I[Edge Cases/Deviations]
    H[Displaying program name, relative path resolution]
    I[Symbolic links, execve, embedded systems]

Flowchart illustrating the typical process and potential deviations for argv[0].

Implications for Program Behavior

The content of argv[0] can influence a program's behavior in several ways:

• Self-identification: Programs often use argv[0] to print their own name in error messages or help output.
• Configuration Loading: Some programs might try to locate configuration files relative to their executable path, which they might derive from argv[0].
• Polymorphic Binaries: A single executable might behave differently based on the name it was invoked with (e.g., busybox on Linux).

While argv[0] is a powerful and generally reliable mechanism for these purposes, developers should be aware of its conventional nature rather than assuming a strict standard guarantee for all possible scenarios.