fcntl不工作

Question

我有一个小程序，它疲倦地更改文件访问模式后，它已被打开。

int main(int argc, char* argv[])
{
    int fd;
    char *filename = argv[1];
    char data[1];
    int curval;                         //current flag value
    int newval;                         //new flag value

fd = open(filename, O_RDONLY);

while(read(fd, data, 1)>0)
{
    write(STDOUT_FILENO, data, 1);
}

lseek(fd, 0, SEEK_SET);

if((curval = fcntl(fd, F_GETFL, 0))<0)              
{
    perror("file flag get failed");
}


printf("%d\n", curval);
    newval = curval | O_WRONLY | O_APPEND;
printf("%d\n", newval);

    if(fcntl(fd, F_SETFL, newval)<0)
{
    perror("file flag set failed");
}

if(write(fd, argv[2], strlen(argv[2]))<0)   //appending more data to the file
{
    perror("write failed");
    }

lseek(fd, 0, SEEK_SET);

while(read(fd, data, 1)>0)
{
    write(STDOUT_FILENO, data, 1);
}
close (fd);
return 0;


}


Here is the output when i run this program with a text file as input.

$ cat input
this is the inital data
$ ./a.out input newdata
this is the inital data
0
1025
write failed: Bad file descriptor
this is the inital data


Why is the write in the program failing? Also I'm not able to find where the file status flag constants are defined. I checked in usr/include/

Answer 1

不允许您尝试执行的行为。从fcntl(2)手册页：

F\_SETFL (long) Set the file status flags to the value specified by arg. File access mode (O\_RDONLY, O\_WRONLY, O\_RDWR) and file creation flags (i.e., O\_CREAT, O\_EXCL, O\_NOCTTY, O\_TRUNC) in arg are ignored. On Linux this command can only change the O\_APPEND, O\_ASYNC, O\_DIRECT, O\_NOATIME, and O\_NONBLOCK flags.

Answer 2

如果您阅读Linux手册页，您将看到fcntl不能更改文件访问模式(例如，从只读到读写)。