#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);
	}
	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 %d is null ??\n",var);
	}
	exit(0);
}

执行结果