Bitwise array error

Question

I am new in programming and even more in c programming. I am trying to read a binary file and then do bitwise process. Sample of my code and how I have written my program so far.

#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;
}

When I am compiling the code I get the error message:

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

I tried to declare another value as:

unsigned int memory_int[4];

When I compile the code I get the warning:

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);

Although that I changed the %" PRIu32" to %u I still get this warning.

I know so far that when I am using typedef struct I can call the content by using name.parameter, and the program seems to be operating correctly when I call first.size the value when need it. But I encounter a problem when I am trying to bitwise because I am asking the value to be broken in bits. Theoretically it should be possible since I have define it as 32 bit, but practically I am missing something extremely small. I tried to assign another value as uint32_t and also as unsigned int size[4] both times not successfully. Can some explain to me the reason and how to solve this problem?

Answer 1

There are some things to be modified in your code.

First, use " " in the name of the file to be read, as the fopen synax is

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

This way, the filename [path] will be const char *

Second, add the following lines to your code [after the existing line].

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

It will help you handle some unexpected conditions like "File does not exist"

Third, declare memory_int as an array of unsigned int containing 4 elements.

unsigned int memory_int[4] = {0};

Finally, altogether, the code looks like this.

#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;
}

Hope this works!!

Answer 2

#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" );
    fclose(file);
    return EXIT_FAILURE;
  }

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

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

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

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

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

When I am compiling the code I get the error message:

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

The reason that I was getting this error was that I was trying to compile a string like an array. I defined it as uint32_t size and I was calling it as first.size[3]. Of course it would never work, because one is a simple string and the second one is an array.

This is the testing code I create as a sample to test and learn the most in order to solve my problem. Since I just started to learn programming I have a lot of things to read understand and learn.

I hope this sample of code and help somebody else as it did for me.

Answer 3

In line printf ("This is first.size before sync_safe: %u\n", memory_int); : %u would be useful to print memory_int[0].

One way to print the four elements of memory_int would be to use %u %u %u %u.

I also changed some stuff in the bitwise operations. I prefered the %x (hexadecimal) specifier to display the output : It will be much easier to correct the output of bitwise operations.

#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;
}

Bye,

Francis