在 C++ 中设置本地环境变量

Question

如何在 C++ 中设置环境变量？

• 它们不需要保留过去的程序执行
• 它们只需要在当前进程中可见
• 偏好独立于平台但对于我的问题只需要在 Win32/64 上工作

谢谢

Answer 1

我不确定环境变量是您所需要的，因为它们不会在程序运行之外使用。 无需参与操作系统。

您最好拥有一个单例类或一个包含所有这些值的命名空间，并在启动程序时初始化它们。

Answer 2

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

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

    char *var, *value;
    if (argc == 1 || argc > 3) {
        fprintf(stderr, "usage:environ variables \n");
        exit(0);
    }
    var = argv[1];
    value = getenv(var);
    //---------------------------------------
    if (value) {
        printf("variable %s has value %s \n", var, value);
    }
    else
        printf("variable %s has no value \n", var);
    //----------------------------------------
    if (argc == 3) {
        char* string;
        value = argv[2];
        string = malloc(strlen(var) + strlen(value) + 2);
        if (!string) {
            fprintf(stderr, "out of memory \n");
            exit(1);
        }
        strcpy(string, var);
        strcat(string, "=");
        strcat(string, value);
        printf("calling putenv with: %s \n", string);
        if (putenv(string) != 0) {
            fprintf(stderr, "putenv failed\n");
            free(string);
            exit(1);
        }
        value = getenv(var);
        if (value)
            printf("New value of %s is %s \n", var, value);
        else
            printf("New value of %s is null??\n", var);
    }
    exit(0);

} //----main

# commands to execure on linux   
# compile:
$ gcc -o myfile myfile.c

# run:
$./myfile xyz
$./myfile abc
$./myfile pqr

Answer 3

还有 setenv，它比 putenv 稍微灵活一些，因为 setenv 检查环境变量是否已经设置，并且不会设置覆盖它，如果您设置“覆盖”参数表明您不想覆盖它，并且名称和值是 setenv 的单独参数：

NAME
        setenv - change or add an environment variable
SYNOPSIS
       #include <stdlib.h>

       int setenv(const char *name, const char *value, int overwrite);

       int unsetenv(const char *name);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       setenv(), unsetenv():
           _POSIX_C_SOURCE >= 200112L
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE
DESCRIPTION
       The setenv() function adds the variable name to the environment with
       the value value, if name does not already exist.  If name does exist
       in the environment, then its value is changed to value if overwrite
       is nonzero; if overwrite is zero, then the value of name is not
       changed (and setenv() returns a success status).  This function makes
       copies of the strings pointed to by name and value (by contrast with
       putenv(3)).

       The unsetenv() function deletes the variable name from the
       environment.  If name does not exist in the environment, then the
       function succeeds, and the environment is unchanged.

我并不是说两者都是比对方更好或更差； 这仅取决于您的应用程序。

请参阅http://man7.org/linux/man-pages/man3/ setenv.3.html

Answer 4

NAME

       putenv - change or add an environment variable

SYNOPSIS

       #include <stdlib.h>

       int putenv(char *string);

DESCRIPTION
       The  putenv()  function adds or changes the value of environment
       variables.  The argument string is of the form name=value.  If name does
       not already exist in the environment, then string is added  to  the
       environment.   If name does exist, then the value of name in the
       environment is changed to value.  The string pointed to by string becomes
       part of the environment, so altering the string changes the environment.

我相信在 Win32 上它被称为 _putenv 。

如果您喜欢又长又难看的函数名称，请参阅 SetEnvironmentVariable 。