#include 
     
      #include 
      
       #include 
       
        int main(int argc,char **argv){        char *var,*value;        if(argc==1||argc>3)        {                exit(1);        }        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 memoryy\n");                        exit(1);                }                strcpy(string,var);                strcat(string,"=");                strcat(string,value);                printf("Calling putenv with: %s\n",string);                if(putenv(string)!=0)                {                        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);}
       
      
     

函数原型：

#include <stdlib.h>

char *getenv(const char *name); 如果环境变量不存在，返回null.

int putenv(const char *string);如果添加环境变量失败返回-1