Question

我是编程新手，也是c编程的新手。我试图读取二进制文件然后按位进程。我的代码示例以及我到目前为止编写程序的方式。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <inttypes.h> /* Use C99 format specifiers: "%" PRIu8 "\n", "%" PRIu32 "\n" */

typedef struct{
  uint32_t size; /* Unsigned int 4 bytes (32 bits) */
}mp3_Header;

int main (int argc, char *argv[]) {

mp3_Header first;
unsigned int memory_int[4];


FILE *file = fopen( "song.mp3" , "rb" );

/* 1 Byte flags + revision_number 1 Byte + major_version 1 Byte + header_id 3 Bytes = 6 */
if ( fseek( file , 6 , SEEK_SET ) == -1 ) {
  fprintf( stderr, "Not able to fseek at possition 6" );
  return EXIT_FAILURE;
}

if ( fread( memory_int , sizeof(memory_int) , 1 , file ) != 1) {
  printf("Could not read first.size\n");
  exit (0);
}

printf ("This is first.size before sync_safe: %u\n", memory_int);

first.size = (memory_int[3] & 0xFF) |
     ((memory_int[2] & 0xFF) << 7 ) |
((memory_int[1] & 0xFF) << 14 ) |
((memory_int[0] & 0xFF) << 21 );

printf ("This is first.size after sync_safe: %" PRIu32"\n", first.size);

fclose(file);
return 0;
}

当我编译代码时，我收到错误消息：

error: subscripted value is neither array nor pointer nor vector first.size = (first.size[3] & 0xFF) |

我试图将另一个值声明为：

unsigned int memory_int[4];

当我编译代码时，我收到警告：

warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘unsigned int *’ [-Wformat=] printf ("This is memset memory_int: %u\n", memory_int);

虽然我将％“PRIu32”更改为％u但我仍然收到此警告。

到目前为止，我知道当我使用typedef struct时，我可以使用name.parameter调用内容，当我在需要时调用first.size值时程序似乎正常运行。但是当我尝试按位时遇到问题，因为我要求将值分解为位。理论上它应该是可能的，因为我已经将它定义为32位，但实际上我遗漏的东西非常小。我尝试将另一个值分配为uint32_t，并将无条件的int size [4]分配为两次都不成功。有人能解释一下原因以及如何解决这个问题吗？

Answer 1

您的代码中有一些要修改的内容。

首先，在要读取的文件名称中使用" "，因为fopen synax

FILE *fopen(const char *path, const char *mode);

这样，filename [path]将为const char *

第二次，在[现有行之后]的代码中添加以下行。

FILE *file = fopen("song.mp3" , "rb" );
if (!file)
        {
         printf("Unable to open the file for reading\n");
         exit (0);
        }

它可以帮助您处理一些意外情况，例如“文件不存在”

第三，将memory_int声明为包含4个元素的unsigned int数组。

unsigned int memory_int[4] = {0};

最后，代码看起来像这样。

#include <stdint.h>
#include <unistd.h>
#include <inttypes.h> /* Use C99 format specifiers: "%" PRIu8 "\n", "%" PRIu32 "\n" */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <inttypes.h> /* Use C99 format specifiers: "%" PRIu8 "\n", "%" PRIu32 "\n" */

typedef struct{
        uint32_t size; /* Unsigned int 4 bytes (32 bits) */
}mp3_Header;

int main (int argc, char *argv[]) {

        unsigned int memory_int[4] = {0};
        mp3_Header first;

        FILE *file = fopen("song.mp3" , "rb" );
        if (!file)
        {
                printf("Unable to open the file for reading\n");
                exit (0);
        }

        /* 1 Byte flags + revision_number 1 Byte + major_version 1 Byte + header_id 3 Bytes = 6 */
        if ( fseek( file , 6 , SEEK_SET ) == -1 ) {
                fprintf( stderr, "Not able to fseek at possition 6" );
                return EXIT_FAILURE;
        }

        if ( fread( memory_int , sizeof(memory_int) , 1 , file ) != 1) {
                printf("Could not read first.size\n");
                exit (0);
        }

        printf ("This is first.size before sync_safe: %u %u %u %u\n", memory_int[0],memory_int[1],memory_int[2],memory_int[3]);

        first.size = (memory_int[3] & 0xFF) |
                ((memory_int[2] & 0xFF) << 7 ) |
                ((memory_int[1] & 0xFF) << 14 ) |
                ((memory_int[0] & 0xFF) << 21 );

        printf ("This is first.size after sync_safe: %u\n", first.size);

        fclose(file);
        return 0;
}

希望这有效!!

Answer 2

排成一行printf ("This is first.size before sync_safe: %u\n", memory_int);：%u对于打印memory_int[0]非常有用。

打印memory_int的四个元素的一种方法是使用%u %u %u %u。

我也在按位操作中改变了一些东西。我更喜欢%x（十六进制）说明符来显示输出：校正按位运算的输出会更容易。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <inttypes.h> /* Use C99 format specifiers: "%" PRIu8 "\n", "%" PRIu32 "\n" */

typedef struct{
    uint32_t size; /* Unsigned int 4 bytes (32 bits) */
}mp3_Header;

int main (int argc, char *argv[]) {

    mp3_Header first;

    unsigned int memory_int[4];

    FILE *file = fopen( "song.dat" , "rb" );

    /* 1 Byte flags + revision_number 1 Byte + major_version 1 Byte + header_id 3 Bytes = 6 */
    if ( fseek( file , 6 , SEEK_SET ) == -1 ) {
        fprintf( stderr, "Not able to fseek at possition 6" );
        return EXIT_FAILURE;
    }
    //printf("%u\n",sizeof(memory_int));
    if ( fread( memory_int , sizeof(memory_int) , 1 , file ) != 1) {
        printf("Could not read first.size\n");
        exit (0);
    }

    printf ("This is first.size before sync_safe: %x %x %x %x\n", memory_int[0],memory_int[1],memory_int[2],memory_int[3]);

    memory_int[0]=0x4212;
    memory_int[1]=0x4213;
    memory_int[2]=0x4214;
    memory_int[3]=0x4215;

    first.size = (memory_int[3] & 0x000F) |
            ((memory_int[2] & 0x000F) << 8 ) |
            ((memory_int[1] & 0x000F) << 16 ) |
            ((memory_int[0] & 0x000F) << 24 );

    printf ("This is first.size after sync_safe: %x\n", first.size);

    fclose(file);
    return 0;
}

再见，

弗朗西斯

Answer 3

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <inttypes.h> /* Use C99 format specifiers: "%" PRIu8 "\n", "%" PRIu32 "\n" */

typedef struct{
  uint32_t size; /* Unsigned int 4 bytes (32 bits) */
}mp3_Header;

int main (int argc, char *argv[]) {

  mp3_Header first;
  unsigned int memory_int[4];
  uint32_t memory_int_2[4];
  char memory[4];
  unsigned char memory_2[4];


  FILE *file = fopen( "song.mp3" , "rb" );

  /* 1 Byte flags + revision_number 1 Byte + major_version 1 Byte + header_id 3 Bytes = 6 */
  if ( fseek( file , 6 , SEEK_SET ) == -1 ) {
    fprintf( stderr, "Not able to fseek at possition 6" );
    return EXIT_FAILURE;
  }

  if ( fread( memory_int , sizeof(memory_int) , 1 , file ) != 1) {
    printf("Could not read first.size\n");
    exit (0);
  }

  /* 1 Byte flags + revision_number 1 Byte + major_version 1 Byte + header_id 3 Bytes = 6 */
  if ( fseek( file , 6 , SEEK_SET ) == -1 ) {
    fprintf( stderr, "Not able to fseek at possition 6" );
    return EXIT_FAILURE;
  }

  if ( fread( &first.size , sizeof(first.size) , 1 , file ) != 1) {
    printf("Could not read first.size\n");
    exit (0);
  }

  /* 1 Byte flags + revision_number 1 Byte + major_version 1 Byte + header_id 3 Bytes = 6 */
  if ( fseek( file , 6 , SEEK_SET ) == -1 ) {
    fprintf( stderr, "Not able to fseek at possition 6" );
    return EXIT_FAILURE;
  }

  if ( fread( memory_int_2 , sizeof(memory_int_2) , 1 , file ) != 1) {
    printf("Could not read first.size\n");
    exit (0);
  }

  /* 1 Byte flags + revision_number 1 Byte + major_version 1 Byte + header_id 3 Bytes = 6 */
  if ( fseek( file , 6 , SEEK_SET ) == -1 ) {
    fprintf( stderr, "Not able to fseek at possition 6" );
    return EXIT_FAILURE;
  }

  if ( fread( memory , sizeof(memory) , 1 , file ) != 1) {
    printf("Could not read first.size\n");
    exit (0);
  }

 /* 1 Byte flags + revision_number 1 Byte + major_version 1 Byte + header_id 3 Bytes = 6 */
  if ( fseek( file , 6 , SEEK_SET ) == -1 ) {
    fprintf( stderr, "Not able to fseek at possition 6" );
    return EXIT_FAILURE;
  }

  if ( fread( memory_2 , sizeof(memory_2) , 1 , file ) != 1) {
    printf("Could not read first.size\n");
    exit (0);
  }

  printf ("This is memory_int[0] before sync_safe: %u %u %u %u\n", memory_int[0],memory_int[1],memory_int[2],memory_int[3]);
  printf ("This is first.size before sync_safe: %" PRIu32"\n", first.size);
  printf ("This is memory_int_2[0] before sync_safe with PRIu32: %" PRIu32" %" PRIu32" %" PRIu32" %" PRIu32"\n", memory_int_2[0],memory_int_2[1],memory_int_2[2],memory_int_2[3]);
  printf ("This is memory[0] before sync_safe: %d %d %d %d\n", memory[0],memory[1],memory[2],memory[3]);
  printf ("This is memory before sync_safe: %s\n", memory);
  printf ("This is memory_2[0] before sync_safe: %d %d %d %d\n", memory_2[0],memory_2[1],memory_2[2],memory_2[3]);
  printf ("This is memory_2 before sync_safe: %s\n", memory_2);

  first.size = (memory_int[3] & 0xFF) |
    ((memory_int[2] & 0xFF) << 7 ) |
    ((memory_int[1] & 0xFF) << 14 ) |
    ((memory_int[0] & 0xFF) << 21 );

  printf ("This is first.size after sync_safe: %" PRIu32"\n", first.size);

  first.size = (first.size & 0xFF);

  printf ("This is first.size after sync_safe: %" PRIu32"\n", first.size);

  first.size = (memory[3] & 0xFF) |
    ((memory[2] & 0xFF) << 7 ) |
    ((memory[1] & 0xFF) << 14 ) |
    ((memory[0] & 0xFF) << 21 );

  printf ("This is first.size after sync_safe: %" PRIu32"\n", first.size);

  first.size = (memory_2[3] & 0xFF) |
    ((memory_2[2] & 0xFF) << 7 ) |
    ((memory_2[1] & 0xFF) << 14 ) |
    ((memory_2[0] & 0xFF) << 21 );

  printf ("This is first.size after sync_safe: %" PRIu32"\n", first.size);

  fclose(file);
  return 0;
}

当我编译代码时，我收到错误消息：

错误：下标值既不是数组也不是指针也不是向量first.size =（first.size [3]＆amp; 0xFF）|

我收到此错误的原因是我试图编译像数组一样的字符串。我将其定义为uint32_t size，我将其称为first.size[3]。当然它永远不会工作，因为一个是简单的字符串，第二个是数组。

这是我创建的测试代码，作为测试和学习最多的示例，以解决我的问题。由于我刚开始学习编程，我有很多东西需要阅读才能理解和学习。

我希望这个代码示例能够为我帮助其他人。

按位数组错误

3 个答案: