execvp函数详解_如何在C / C ++中使用execvp（）函数

你的名字 2023-02-28 06:17 20阅读 0赞

execvp函数详解

In this article, we’ll take a look at using the execvp() function in C / C++.

在本文中，我们将介绍如何在C / C ++中使用execvp（）函数。

In UNIX, the execvp() function is very useful if you want to run another program using our C program.

在UNIX中，如果要使用我们的C程序运行另一个程序，execvp（）函数将非常有用。

**NOTE**: This function is applicable only to UNIX based Operating Systems. It doesn’t work on Windows

**注意** ：此功能仅适用于基于UNIX的操作系统。 在Windows上不起作用

Let’s take a look at executing UNIX commands from our program, using illustrative examples!

让我们使用示例说明一下从程序中执行UNIX命令的方法！

--------------------

## execvp（）的基本语法 **(****Basic Syntax of execvp()****)** ##

This function takes in the name of the UNIX command to run, as the first argument.

该函数将要运行的UNIX命令的名称作为第一个参数。

This is there in the `<unistd.h>` header file, so we must include it in our program.

该文件位于`<unistd.h>`头文件中，因此我们必须将其包含在程序中。

#include <unistd.h>
    
    int execvp(const char* command, char* argv[]);

Here, we refer to a “command” as any binary executable file that is a part of the `PATH` environment variable. So, if you want to run custom programs, make sure that you add it to your `PATH` variable!

在这里，我们将“命令”称为`PATH`环境变量一部分的任何二进制可执行文件。 因此，如果要运行自定义程序，请确保将其添加到`PATH`变量中！

The second argument (`argv`) represents the list of arguments to `command`. This is an array of `char*` strings.

第二个参数（ `argv` ）表示`command`的参数列表。 这是一个`char*`字符串数组。

Here, `argv` contains the complete command, along with it’s arguments.

在这里， `argv`包含完整的命令及其参数。

For example, the below array follows the format of `argv`.

例如，下面的数组遵循`argv`的格式。

char* argument_list[] = {"ls", "-l", NULL}; // NULL terminated array of char* strings
    
    // Ok! Will execute the command "ls -l"
    execvp("ls", argument_list);

This array **MUST** be `NULL` terminated, i.e, the last element of `argv` must be a `NULL` pointer.

该数组**必须**以`NULL`终止，即`argv`的最后一个元素必须是`NULL`指针。

### 现在我们的C程序会怎样？ **(****What happens to our C program now?****)** ###

This function will give the control of the current process (C program) to the command. So, the C program is instantly replaced with the actual command.

该功能将控制当前过程（C程序）到命令。 因此，C程序立即被实际命令替换。

So, anything that comes after `execvp()` will NOT execute, since our program is taken over completely!

因此，由于我们的程序已被完全接管，因此`execvp()`之后的所有内容均将`execvp()`执行！

However, if the command fails for some reason, `execvp()` will return -1.

但是，如果命令由于某种原因失败，则`execvp()`将返回-1。

So, whenever you use `execvp()`, if you want to maintain your C program, you generally use `fork()` to first spawn a new process, and then use `execvp()` on that new process.

因此，无论何时使用`execvp()` ，如果要维护C程序，通常都可以使用`fork()`首先产生一个新进程，然后在该新进程上使用`execvp()` 。

This is called the “fork-exec” model, and is the standard practice for running multiple processes using C.

这称为“ fork-exec”模型，并且是使用C运行多个进程的标准实践。

Let’s now look at some examples, to understand this function better. We’ll also be using `fork()` along with `execvp()`, so that we can still have our C program with us!

现在让我们看一些示例，以更好地理解此功能。 我们还将使用`fork()`和`execvp()` ，以便我们仍然可以使用C程序！

## 在C / C ++中使用execvp（）–一些示例 **(****Using execvp() in C / C++ – Some Examples****)** ##

If you want to see what exactly happens if you try to use `execvp()` without spawning a new process using `fork()`. the below program shows this.

如果您想查看如果尝试使用`execvp()`而没有使用`fork()`产生新进程时发生的情况，将会发生什么。 下面的程序显示了这一点。

We’ll be executing “ls -l” from our C program.

我们将在C程序中执行“ ls -l”。

Notice that the `printf()` statement after `execvp()` is NOT executed, since the other process has taken control!

注意， `execvp()`之后的`printf()`语句未执行，因为另一个进程已经控制了！

#include <stdio.h>
    #include <unistd.h>
    
    int main() {
        char* command = "ls";
        char* argument_list[] = {"ls", "-l", NULL};
    
        printf("Before calling execvp()\n");
    
        // Calling the execvp() system call
        int status_code = execvp(command, argument_list);
    
        if (status_code == -1) {
            printf("Process did not terminate correctly\n");
            exit(1);
        }
    
        printf("This line will not be printed if execvp() runs correctly\n");
    
        return 0;
    }

**Output**

**输出量**

Before calling execvp()
    total 3
    -rwxrwxrwx 1 user user 22088 May 30 16:37 a.out
    -rwxrwxrwx 1 user user 16760 May 30 16:37 sample
    -rw-rw-rw- 1 user user  1020 May 30 16:37 sample.c

As you can see, the part after `execvp()` does not execute at all, since “ls -l” took control of our process!

如您所见， `execvp()`之后的部分根本不执行，因为“ ls -l”控制了我们的过程！

Let’s re-write the same example, but let’s enclose the `execvp()` system call inside another process, using `fork()`.

让我们重写相同的示例，但是使用`fork()`将`execvp()`系统调用包含在另一个进程中。

Let’s see what happens now.

让我们看看现在会发生什么。

#include <stdio.h>
    #include <unistd.h>
    
    int main() {
        char* command = "ls";
        char* argument_list[] = {"ls", "-l", NULL};
    
        printf("Before calling execvp()\n");
    
        printf("Creating another process using fork()...\n");
    
        if (fork() == 0) {
            // Newly spawned child Process. This will be taken over by "ls -l"
            int status_code = execvp(command, argument_list);
    
            printf("ls -l has taken control of this child process. This won't execute unless it terminates abnormally!\n");
    
            if (status_code == -1) {
                printf("Terminated Incorrectly\n");
                return 1;
            }
        }
        else {
            // Old Parent process. The C program will come here
            printf("This line will be printed\n");
        }
    
        return 0;
    }

**Output**

**输出量**

Before calling execvp()
    Creating another process using fork()...
    This line will be printed
    user@shell:$ total 3
    -rwxrwxrwx 1 user user 22088 May 30 16:37 a.out
    -rwxrwxrwx 1 user user 16760 May 30 16:37 sample
    -rw-rw-rw- 1 user user  1020 May 30 16:37 sample.c

If you’re in a shell, the output may look weird, but that’s because multiple processes ran in parallel! Both the outputs were indeed printed, so we’ve been able to resolve our problem.

如果您在shell中，输出看起来可能很奇怪，但这是因为多个进程并行运行！ 这两个输出确实都已打印，因此我们已经能够解决问题。

--------------------

## 结论 **(****Conclusion****)** ##

We learned about using the execvp() function in C / C++, to execute other programs from our C program. However, note that this will give the other program complete control of our process.

我们了解了如何在C / C ++中使用execvp（）函数从C程序执行其他程序。 但是，请注意，这将使其他程序完全控制我们的过程。

Due to this, we need to enclose this under another process, using the `fork()` system call. Hopefully, this made sense to you, and you were able to run other programs, while still being able to have control over your C program!

因此，我们需要使用`fork()`系统调用将其包含在另一个进程中。 希望这对您有意义，您可以运行其他程序，同时仍然可以控制您的C程序！

## 参考资料 **(****References****)** ##

*  [Linux manual page][] on the execvp() function in C
    
    C语言中execvp（）函数上的[Linux手册页][Linux manual page]

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> 翻译自: [https://www.journaldev.com/40793/execvp-function-c-plus-plus][https_www.journaldev.com_40793_execvp-function-c-plus-plus]

execvp函数详解

[Linux manual page]: https://linux.die.net/man/3/execvp
[https_www.journaldev.com_40793_execvp-function-c-plus-plus]: https://www.journaldev.com/40793/execvp-function-c-plus-plus