权限一直被拒绝，但据我所知，我的权限是宽打开的。
//snip
int outfile = creat("outFile.txt", O_RDWR | O_CREAT | S_IRWXU | S_IRWXG | S_IRWXO);
if (outfile < 0)
{
cout << "n" << "output file cannot be opened, error" << "n";
cout << strerror(errno) << "n";
exit(1);
}
int pagesize = getpagesize();
for (int i=0; i<pagesize; i++)
{
write(outfile, "-", 1);
}
//there is code here that outFile.txt has read and write permissions, and it always says read and write are OK
char* target = (char*)mmap(NULL, pagesize, PROT_READ | PROT_WRITE, MAP_SHARED, outfile, 0);
if (target == MAP_FAILED)
{
cout << "n" << "output mapping did not succeed" << "n";
cout << strerror(errno) << "n";
exit(1);
}
else
{
cout << "n" << "output mapping succeeded" << "n";
}
//snip
这是一个学校项目，我们得到了一个1GB的文件，4096页大小，并被告知我们不能只使用write((系统调用。
int outfile = creat("outFile.txt", O_RDWR | O_CREAT | S_IRWXU | S_IRWXG | S_IRWXO);
这里有两个问题：
creat在只写模式下创建一个文件，它的第二个参数是mode_t掩码。在这里传递O_标志，将其与S_模式位叠加在一起，会导致无意义的垃圾。mmap调用的参数需要O_RDWR文件描述符，如上所述，O_RDWR参数被解释为文件权限位，而不是文件打开模式。
这应该被open()取代，有三个参数：
int outfile = open("outFile.txt", O_CREAT | O_TRUNC | O_RDWR,
S_IRWXU | S_IRWXG | S_IRWXO);