Linux练习（读取改变环境变量）

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