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显示环境变量
类型: Sample Code (HOWTO)	类别: Other
许可证: GNU General Public License	语言: C
描述: 显示环境变量

该代码片段的版本系列:

片段ID	下载版本	提交时间	提交人	删除
7	0.1	2001-07-24 10:59	bbsadmin

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最新版本的代码片段: 0.1

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

/* -------------------------------------------------------------- 

  environment 
  
  Part of the SourceLib Project at LinuxProgramming.com
  http://www.linuxprogramming.com/SourceLib

  This sample program has been released into the public domain.

  Purpose: Demonstrate the various ways to access a program's
  environment variables.

  Use the command "make" to build this program.

  ---------------------------------------------------------------

  First we use the global variable "environ" (declared in unistd.h)
  to count the number of strings in the environment and list
  them all.

  Then we demonstrate how to query an environment value with 
  getenv() (see the showEnvVar() function), as well as how to set it
  with the putenv() and setenv() functions.

  Note: putenv() is considered more portable because it's part of 
  the POSIX standard, while setenv() isn't.

  --------------------------------------------------------------- */

#define env_var1 "OSTYPE"
#define env_var2 "Fred"
#define env_var2_value "Gravel_pit_worker"
#define env_var_put "Fred=Flintstone"

/* --------------------------------------------------------------
*/

void showEnvVar(char *var)
{
  char *env_variable;

  if((env_variable = getenv(var)) != NULL)
    printf("The environment variable \"%s\" has the value \"%s\".\n\n",
      var,env_variable);
  else
    printf("The environment variable \"%s\" isn't set.\n\n",var);
}

/* --------------------------------------------------------------
*/

int main(int argc, char **argv)
{
  extern char **environ;
  int count, i;

  /* environ is an array of pointers to null-delimited C strings, 
     with the last pointer in the array being NULL.  We can 
     therefore step through them and stop counting at the first
     NULL pointer. */

  count = 0;
  while(environ[count] != NULL)
    count++;

  /* We know how many strings there are--tell the user, then 
     display the strings.  Notice that each is in the format
     VARIABLE_NAME=VALUE. */

  printf("Counted %d environment strings.\n\n",count);

  for(i = 0; i < count; i++)
    printf("env. var. #%3d: \"%s\"\n",i,environ[i]);

  printf("\nHit ENTER to continue... ");
  getchar();
  printf("\n");

  /* Display a commonly set environment variable. */

  printf("Quering an environment variable that should be set:\n");
  showEnvVar(env_var1);  

  /* Now display one that we made up. */

  printf("Quering an environment variable that should not be set:\n");
  showEnvVar(env_var2);  

  /* putenv() takes a single argument of the form VAR_NAME=VALUE,
     the same format as the strings pointed to by the environ array. 

     If the environment variable already exists, its value is replaced 
     with the new one. */ 

  if(putenv(env_var_put) != 0)
  {
    perror("Unable to putenv");
    exit(EXIT_FAILURE);
  }

  printf("We've set the variable, let's see it:\n");
  showEnvVar(env_var2);  

  /* setenv() uses different syntax than putenv().  It takes
     three parameters: the name of the environment variable, the value, 
     and a flag indicating whether it should be overwritten if the 
     variable already exists. 

     This first setenv() call should refuse to overwrite the variable
     because the flag is 0.  setenv() indicates success even when it
     doesn't change the environment variable because it already
     existed. */ 

  if(setenv(env_var2,env_var2_value,0) != 0)
  {
    perror("Unable to setenv");
    exit(EXIT_FAILURE);
  }

  printf("It should be the same since we didn't allow setenv() to overwrite it:\n");
  showEnvVar(env_var2);  

  /* This time setenv() should overwrite the variable because we're passing
     a flag of 1. */

  if(setenv(env_var2,env_var2_value,1) != 0)
  {
    perror("Unable to setenv");
    exit(EXIT_FAILURE);
  }

  printf("This time setenv() should have overwritten it:\n");
  showEnvVar(env_var2);  

  /* Indicate normal termination via the 
     EXIT_SUCCESS constant from stdlib.h */

  return EXIT_SUCCESS;
}

		

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