C語言通過putenv和getenv函數來訪問環境變數。
#include<stdlib.h>
char *getenv(const char *name);
int putenv(const char *string);
環境是由一組格式為“名字=值”的字串組成。
getenv函數以給定的名字搜尋環境中的一個字串,並返回與該名字相關的值。如果請求不存在,就返回NULL。
由於getenv返回的字串儲存在靜態空間,所以想進一步利用它,就必須拷貝出來。
// 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