p6-21.c

UNIX程序设计教程

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
static uid_t euid, ruid; /* 保存有效和实际UIDS*/
/* 恢复有效UID 到原来的值 */
#define SCORES_FILE "/tmp/score"
void do_setuid(void)
{
    int status;
#ifdef _POSIX_SAVED_IDS
    status = setuid(euid);
#else
    status = setreuid(ruid, euid);
#endif
    if (status < 0) {
        fprintf(stderr, "Couldn't set uid.\n");
        exit(status);
    }
}
/* 设置有效UID到实际UID */
void undo_setuid(void)
{
    int status;
#ifdef _POSIX_SAVED_IDS
    status = setuid(ruid);
#else
    status = setreuid(euid, ruid);
#endif
    if (status < 0) {
       fprintf(stderr, "Couldn't set uid.\n");
       exit(status);
    }
}
/* 记录得分 */
int record_score (int score)
{
    FILE *stream;
    char *myname;
    /* 打开得分文件*/
    do_setuid();
    stream = fopen(SCORES_FILE, "a");
    undo_setuid();
    /* 写得分至分数文件 */
    if (stream) {
       myname = cuserid (NULL);
       if (score < 0)
          fprintf(stream, "%10s: Couldn't lift the caber.\n",myname);
       else
          fprintf (stream, "%10s: %d feet.\n", myname, score);
       fclose (stream);
       return 0;
    }
    else
       return -1;
}
int main (void)
{
    /* 保存实际和有效用户IDs */
    ruid = getuid ();
    euid = geteuid ();
    undo_setuid ();
    /* 进行游戏并记录得分 */
    /* ... */
}