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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?

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  • What is the type of memory_int in your code? Commented Jan 3, 2014 at 7:29
  • It seems to be an unsigned int. However, you probably want it to be an unsigned int [4]. Commented Jan 3, 2014 at 7:30
  • I think he posted the code after he added memory_int, and before first.size was being used (and incorrectly he was attempting to subscript it when it is of course not a pointer, as the compiler message warned). Actually, on second readthrough the compiler messages are quoting lines that are not in this code sample, so I'm guessing he's tried different things but didn't post all of them. @Thanos, what compiler error do you get with this particular code sample? Commented Jan 3, 2014 at 7:35
  • The memory_int is defined as unsigned int memory_int[4]; but I still get this error: 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); I have also tried unint32_t memory_int[4]; and I get the same error message: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘uint32_t *’ [-Wformat=] printf ("This is memory_int before sync_safe: %u %u %u %u\n", memory_int_2); Commented Jan 4, 2014 at 2:23
  • I have noticed that the only way to make it operate correctly is whn I am using at the fread function (&first.size). Only then the printf function gives me the correct output. But I can not apply bitwise process with first.size. Any ideas how to do that? I keep getting this error: error: subscripted value is neither array nor pointer nor vector Commented Jan 4, 2014 at 2:52

3 Answers 3

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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!!

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5 Comments

Unfortunately it dose not work as expected. I keep getting this error message: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘unsigned int *’ [-Wformat=] printf ("This is memory_int before sync_safe: %u %u %u %u\n", memory_int_2);
I have noticed that the only way to make it operate correctly is whn I am using at the fread function (&first.size). Only then the printf function gives me the correct output. But I can not apply bitwise process with first.size. Any ideas how to do that? I keep getting this error: error: subscripted value is neither array nor pointer nor vector
@Thanos Firstly, there is no printf ("This is memory_int before sync_safe: %u %u %u %u\n", memory_int_2); code as you mentioned in my version. Secondly, for me, this code works perfectly. [sourav@localhost c_practice]# gcc -Wall -Werror -Wformat so_test1.c -o test [sourav@localhost c_practice]# ./test This is first.size before sync_safe: 1359741696 826626132 855638016 4278255616 This is first.size after sync_safe: 1376256 [sourav@localhost c_practice]#
@Thanos anyway, in context of this code memory_int_2 is not a valid variable name. Maybe is it memory_int[2]. Do you mind checking your version of code once again?
You are absolutely right. I was really confused why my code was not working. I was testing all the time my code on my primary program that has also other pieces of code that effect the result. By isolating the code and perform modification based on your assistance I manage to learn and understand a lot. Thanks a lot for your time and guidance I finally found my mistake. I will post my answer that maybe somebody else can benefit from it. Again thanks for your time and effort to assist me.
1

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

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4 Comments

Unfortunately it does not work, I get this warning: ` warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 2 has type ‘unsigned int *’ [-Wformat=] printf ("This is memory_int before sync_safe: %x %x %x %x\n", memory_int);`
I have noticed that the only way to make it operate correctly is whn I am using at the fread function (&first.size). Only then the printf function gives me the correct output. But I can not apply bitwise process with first.size. Any ideas how to do that? I keep getting this error: error: subscripted value is neither array nor pointer nor vector
@Thanos : In the code i pasted, printf ("This is blablabla before sync_safe: %x %x %x %x\n", memory_int[0],memory_int[1],memory_int[2],memory_int[3]); : As you noticed, printf expects values, fread expects a pointer to a place where the value will be set. So printf(...,first.size) but fread(...,&first.size). I added first.size = (first.size & 0x000F)| 0x1000; at the end of my code and it worked fine... I compiled this code using gcc main.c -o main and i did not get any warning.
You are right. I was testing all the time my code on my primary program that has also other pieces of code that affect the result. By isolating the code and perform modification based on your assistance I manage to learn and understand a lot. Thanks a lot for your time and guidance I finally found my mistake. I will post my answer that maybe somebody else can benefit from it. Again thanks for your time and effort to assist me.
0 
#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.

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