将 C ioctl/mmap 调用转换为 Python 时出错？

Question

我想将 Linux 驱动程序与 Python 一起使用，并且我正在转换我的 C 程序。

我的 C 程序工作正常，但我在 Python 中有一个错误：IOError: [Errno 14] Bad address

我正在使用：

Python 2.7.13（默认，2018 年 6 月 11 日，22:51:25） [GCC 7.2.0] 在 linux2

我的 OS 是 Petalinux。

这是C程序

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>

#define DUMMY_IOC_MAGIC 'V'
#define DUMMY_START_TX_CYCLIC  _IO(DUMMY_IOC_MAGIC, 0)
#define DUMMY_START_TX  _IO(DUMMY_IOC_MAGIC, 1)

int main()
{
  int fd, j, ret;
  int32_t *map_vptr;
  size_t number_of_samples = 64;
  size_t size_sample = 4;
  //////////////////////////////////////////////////////////////////////////////
  printf("Init tx.\n");
  fd = open("/dev/playback", O_RDWR | O_SYNC);
  if (fd < 0) {
    printf("Error opening device\n");
    return -1;
  }
  else{
    printf("Device is open...\n");
  }
  //////////////////////////////////////////////////////////////////////////////
  map_vptr = (int32_t*) mmap(0, size_sample * number_of_samples, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
  if (map_vptr == MAP_FAILED) {
    close(fd);
    printf("Error mmapping the file\n");
    return -1;
  }
  printf("Mapped.\n");
  //////////////////////////////////////////////////////////////////////////////
  int32_t *dst_ptr = map_vptr;
  int32_t counter = 0;
  for (j = 0; j < number_of_samples; ++j) {
    dst_ptr[j] = counter;
    counter = counter + 2;
  }
  //////////////////////////////////////////////////////////////////////////////
  // start TX transactions
  uint32_t tx_total_len = size_sample * number_of_samples;
  ret = ioctl(fd, DUMMY_START_TX, &tx_total_len);
  if (ret < 0)
  {
    printf("ioctl() failed, errno=%d\n", errno);
    return -1;
  }
  printf("End tx.\n");

  return 0;
}

这是Python程序

import mmap, os, fcntl

#define DUMMY_START_TX _IO('V', 1)
DUMMY_IOC_MAGIC = 'V'
DUMMY_START_TX = ord(DUMMY_IOC_MAGIC) << (4*2) | 1

NUMBER_OF_SAMPLES = 64;
SIZE_SAMPLE = 4
################################################################################
# Open device
print("Init tx.");
fd = os.open("/dev/playback", os.O_RDWR | os.O_SYNC)
if (fd < 0):
    print("Error opening device")
    exit(-1);
else:
    print("Device is open...")
################################################################################
map_vptr = mmap.mmap(length = SIZE_SAMPLE*NUMBER_OF_SAMPLES, prot = mmap.PROT_READ | mmap.PROT_WRITE,
            flags = mmap.MAP_SHARED, fileno = fd, offset = 0)
print("Mapped.");
################################################################################
for i in range(0,NUMBER_OF_SAMPLES):
    map_vptr.write_byte(chr(i))
################################################################################
# start TX transactions
tx_total_len = SIZE_SAMPLE * NUMBER_OF_SAMPLES
ret = fcntl.ioctl(fd, DUMMY_START_TX, tx_total_len);

fd.close()

print("End tx.");

输出为：

Init tx.
Device is open...
Mapped.
Traceback (most recent call last):
  File "tx_server.py", line 28, in <module>
    ret = fcntl.ioctl(fd, DUMMY_START_TX, tx_total_len);
IOError: [Errno 14] Bad address

Answer 1

来自 the Python doc:

fcntl.fcntl(fd, op[, arg])
The argument arg is optional, and defaults to the integer value 0. When present, it can either be an integer value, or a string. With the argument missing or an integer value, the return value of this function is the integer return value of the C fcntl() call. When the argument is a string it represents a binary structure, e.g. created by struct.pack(). The binary data is copied to a buffer whose address is passed to the C fcntl() call. The return value after a successful call is the contents of the buffer, converted to a string object.

以上对fcntl.ioctl()同样有效。在您的情况下，您应该使用 struct.pack() 来传递值，而不是直接传递整数，如下所示：

import struct

# ...

tx_total_len = SIZE_SAMPLE * NUMBER_OF_SAMPLES
ptr = struct.pack('I', tx_total_len)
#                  I == unsigned int of 4 bytes (uint32_t)

ret = fcntl.ioctl(fd, DUMMY_START_TX, ptr)

将 C ioctl/mmap 调用转换为 Python 时出错？

Error converting a C ioctl/mmap call to Python?

c

python

embedded

cpython