Problem with the first character of the binary file while reading it in a C file

Question

I am trying to read a binary file and its content.

/*aoObj.fb is the pointer of the file (e.x. FILE *fp)*/

char ch;
aoObj.fp = fopen(aoObj.f_name, "rb");

if (aoObj.fp == NULL)

      {
        perror("Error while opening the file.\n");
        exit(EXIT_FAILURE);
      }

        /* /\*list all strings *\/ */
        printf("\n\nThe content of the file: \n");

        while ((ch = fgetc(aoObj.fp)) != EOF)
          printf("%c", ch);

        fclose(aoObj.fp);
        (void) opt_free(&aoObj);
        return 0;
}

But I am facing issues when I print the content of thi file because only the first character of the input isn't printed ok, as follows:

May I know why this is happening ?

EDIT: All the variables which are being read are declared as STRINGS

Change char ch to int ch and I guess you could use getc() too. See if it solves. — Rohan Bari
– Rohan Bari, Commented Jun 7, 2020 at 15:09
Nope, neither getc() nor the int char didn't solve the issue — asha
– asha, Commented Jun 7, 2020 at 15:12
the posted code does not compile! Please post a minimal reproducible example so we can reproduce the problem and help you debug it. — user3629249
– user3629249, Commented Jun 7, 2020 at 17:20

user3629249 · Accepted Answer · 2020-06-07 20:08:16Z

2

The OP states the file contents are 'binary' not 'text' Therefore, accessing the file should be via the I/O operators made for binary files,

Suggest:

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

Since the data read from the 'binary' file is NOT ascii characters, it is an 'error' to try to print those 'binary' characters with the 'output format conversion' specifier: %c.

Suggest:

printf( "%02x\n", ch );

Note: the %02x so a leading nibble of 0x0 will be printed rather than suppressed.

When the code is corrected to use: fread() rather than fgetc() the declaration of ch can/should be unsigned char ch; so no need to change that to int ch;

The following proposed code:

cleanly compiles
performs the desired functionality
is missing a main() function and the passing of the parameter: f_name so does not link
properly checks for an error when opening the input file
uses the returned value from fread() to 'assume' EOF, however, it may be instructive (and for robust code) to check the value of errno just to assure there was no other error.
documents why each header file is included

Note: the proposed code is not very efficient as it only reads a single byte at a time rather than a whole buffer full of bytes

Note: the proposed code will output one byte contents (in hex) on a single line. You might want to modify that to output several bytes contents (in hex) before moving to a new line.

and now, the proposed code:

#include <stdio.h>    // FILE, fopen(), perror(), printf(), fclose()
                      // fread()
#include <stdlib.h>   // exit(), EXIT_FAILURE

void myfunc( char *f_name )
{
    unsigned char ch;
    FILE *fp = fopen( f_name, "rb");
    if (fp == NULL)
    {
        perror("Error while opening the file.\n");
        exit(EXIT_FAILURE);
    }

    /* /\*list all strings *\/ */
    printf("\n\nThe content of the file: \n");

    size_t bytesRead;
    while ( ( bytesRead = fread( &ch, 1, 1, fp ) ) == 1 )
    {
        printf("%02x\n", ch);
    }

    fclose(fp);
}

edited Jun 7, 2020 at 20:08

answered Jun 7, 2020 at 17:54

user3629249

16.6k1 gold badge18 silver badges17 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

chux Over a year ago

Note that printf("%02x\n", ch); may print FFFFFFFF when ch = -1; Suggest char ch; --> unsigned char ch;

chux Over a year ago

I do mean -1. When ch is signed., it typically has values in the [-128 ... 127] range. If ch had the value of 0x81 or 129, is is likely an unsigned char and pose no trouble here.

user3629249 Over a year ago

@chux-ReinstateMonica, As usual, your correct. BTW: I modified the code to have char ch = (char)0x81 and it output ffffff81, so any negative value would be a problem. I'll fix the answer.

Collectives™ on Stack Overflow

Problem with the first character of the binary file while reading it in a C file

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related