这里是在使用ps命令的简单实现下。
这当然不是最优雅的，但它应该适用于Unix和Cygwin：

# include <stdio.h>

# include <string.h>

int get_proc_uid(int pid, char *uid, int uid_size)
{
 FILE *fp;
 int pid_l, ret = -1;
 char uid_l[16];
 char cmd[64], line[128];

 snprintf(cmd, sizeof(cmd), "ps | grep %d", pid);
 fp = popen(cmd, "r");
 if(fp == NULL)
    return ret;

 while(fgets(line, sizeof(line), fp) != NULL)
    {
    if(strstr(line, "grep") == NULL)//filter grep dummy result
        {
        sscanf(line, "%d %s", &pid_l, uid_l);
        if(pid_l == pid)
            {
            strncpy(uid, uid_l, uid_size);
            ret = 0;
            break;
            }
        }
    }
 pclose(fp);

 return ret;
}