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Command Line Arguments 命令行参数

1. The Standard Prototype

To interact with the command line (Terminal/Shell), the main function is defined as:

int main(int argc, char const *argv[])

2. Parameters Explained

int argc (Argument Count)

  • Definition: The total count of string arguments passed to the program.

  • Key Detail: It includes the name of the program itself.

  • Example:

    • Command: ./demo hello world

    • Count: 3 (1: ./demo, 2: hello, 3: world)

char const *argv[] (Argument Vector)

  • Definition: An array of strings (pointers to characters) containing the actual arguments.

  • Structure:

    • argv[0]: The command used to start the program.

    • argv[1] to argv[argc-1]: The actual user inputs.

    • argv[argc]: Always NULL (marks the end of the array).

3. Deep Dive: The argv[0] Pitfall

The slide highlighted a specific behavior regarding Unix Symbolic Links.

  • Fact: argv[0] is not necessarily the original name of the compiled binary file.

  • Reality: argv[0] reflects how the program was invoked in the terminal.

  1. Original File: You compile a program named tool.

  2. Create Link: You create a symbolic link (shortcut) named linkname pointing to tool.

    • Command: ln -s tool linkname

  3. Execution:

    • If you run ./tool: argv[0] is "./tool".

    • If you run ./linkname: argv[0] is "./linkname".

Conclusion: Even though the underlying executable code is identical, the program can detect which "name" the user called it by.

4. Real-World Application: "Multi-Call Binaries"

This feature is widely used in Linux system utilities (e.g., BusyBox).

  • How it works:

    • There is only one real executable file (e.g., busybox).

    • Commands like ls, cp, rm are just symbolic links pointing to busybox.

  • Logic inside main():

    c if (strcmp(argv[0], "./ls") == 0) { run_ls_logic(); } else if (strcmp(argv[0], "./cp") == 0) { run_cp_logic(); }

  • Benefit: This saves disk space by bundling hundreds of tools into a single binary.