環境變量（The Environment Variables)

/**                                                                                                                                                                    
 * print environment                                                                                                                                                   
 *                                                                                                                                                                     
 **/
#include <stdio.h>

extern char **environ;

int main()
{
        char **var;
        for (var = environ; *var != NULL; var++)
                printf("%s \n", *var);
        return 0;
}

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/**                                                                                                                                                                    
 * getenv                                                                                                                                                              
 *                                                                                                                                                                     
 **/

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

int main()
{
        char *server_name = getenv("SERVER_NAME");
        if (server_name == NULL)
                server_name = "default server name";
        printf("client accessing '%s' \n", server_name);
        return 0;
}

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

int main()
{
        char *envp[] = {
                "SERVER_NAME=my.server.name",
                NULL,};
        int ret = execle("client", "client", NULL, envp);
        return 0;
}