学步园

C语言通过putenv和getenv函数来访问环境变量。

#include<stdlib.h>

char *getenv(const char *name);

int putenv(const char *string);

环境是由一组格式为“名字=值”的字符串组成。

getenv函数以给定的名字搜索环境中的一个字符串，并返回与该名字相关的值。如果请求不存在，就返回NULL。

由于getenv返回的字符串存储在静态空间，所以想进一步利用它，就必须拷贝出来。

//  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>

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

    if(argc == 1 || 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];
    value = getenv(var);
    if(value)
        printf("Variable %s has value %s\n", var, value);
    else
        printf("Variable %s has no value\n", 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) {
        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);
        }

//  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 %s\n", var, value);
        else
            printf("New value of %s is null??\n", var);
    }
    exit(0);
}

./environ HOME

Variable HOME has value /home/zq

./environ qq

Varliable qq has no value

./environ zq qq

Varliable zq has no value

Calling putenv with: zq=qq

New value of zq is qq

./environ zq

Varliable zq has no value