linux 环境变量函数getenv()和putenv()的使用

程序功能：编写一个程序来打印所选的任意环境变量的值；如果给程序传递第二个参数，还设置环境变量的值
//  1  The first few lines after the declaration of main ensure that the program,environ.c,has been called correctly.

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

/************************
argc:参数个数（包含程序名）
argv:代表参数自身的字符串数组；
argv[0]为程序名，argv[1]为第1个实际参数  argv[2]为第2个实际参数 
************************/

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

    if(argc == 1 || argc > 3) {    //确保实际参数只有1个(argc=2)或2个(argc=3)
        fprintf(stderr,"usage: environ var [value]n");
        exit(1);
    }

//  2  That done,we fetch the value of the variable from the environment,using getenv.

    var = argv[1]; //第一个实际参数的参数名
    value = getenv(var);  //第一个实际参数的参数值
    if(value) //判断参数值是否存在
        printf("Variable %s has value %sn",var,value);
    else
        printf("Variable %s has no valuen",var);

//  3  Next,we check whether the program was called with a second argument. If it was,we set the variable to the value of that argument by constructing a string of the form name=value and then calling putenv.

    if(argc == 3) { //如果第二个实际参数存在
        char *string; 
        value = argv[2]; //获取第2个参数的值
        string = malloc(strlen(var)+strlen(value)+2); //为第二个参数的“参数名=参数值”开辟空间（+2表示“=”和空格）
        if(!string) { //如果开辟空间失败
            fprintf(stderr,"out of memoryn");
            exit(1);
        }
        strcpy(string,var); 
        strcat(string,"=");
        strcat(string,value);
        printf("Calling putenv with: %sn",string);
        if(putenv(string) != 0) {  //putenv()成功返回0.若环境变量设置失败
            fprintf(stderr,"putenv failedn");
            free(string); //释放开辟的内存空间
            exit(1);
        }

//  4  Finally,we discover the new value of the variable by calling getenv once again.

        value = getenv(var);
        if(value)
            printf("New value of %s is %sn",value);
        else
            printf("New value of %s is null??n",var);
    }
    exit(0);
}