p5-4.c

来自「UNIX程序设计教程」· C语言 代码 · 共 46 行

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main(int argc,char *argv[])
{
    char *var,*value;
    /* 检查调用参数是否正确*/
    if(argc==1||argc>3) {
        fprintf(stderr, "Usage: environ var [value]\n");
        exit(1);
    }
    /*  用getenv从环境中取变量之值  */
    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);
    /* 如果是三个参数的调用,构成"name=value"并调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);
        }
        /*  用getenv核实该变量新的值  */
        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);
}