environ.c

linux下编程入门及代码

//  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);
 }