Read binary file, save in buffer, print out content of buffer

Question

I have a big problem that need's to be solved before I can continue with my program.

I have to open a binary file, read it's content, save the content into a buffer, allocate space on the heap with malloc, close the file and finally printf( the content of the .bin file). I came this far (closing file is not implemented yet):

void executeFile(char *path){
    FILE *fp; /*filepointer*/
    size_t size; /*filesize*/
    unsigned int buffer []; /*buffer*/

    fp = fopen(path,"rb"); /*open file*/
    fseek(fp, 0, SEEK_END); 
    size = ftell(fp);         /*calc the size needed*/
    fseek(fp, 0, SEEK_SET); 
    buffer = malloc(size);  /*allocalte space on heap*/

    if (fp == NULL){ /*ERROR detection if file == empty*/
        printf("Error: There was an Error reading the file %s \n", path);           
        exit(1);
    }
    else if (fread(&buffer, sizeof(unsigned int), size, fp) != size){ /* if count of read bytes != calculated size of .bin file -> ERROR*/
        printf("Error: There was an Error reading the file %s - %d\n", path, r);
        exit(1);
    }else{int i;
        for(i=0; i<size;i++){       
            printf("%x", buffer[i]);
        }
    }
}

I think I messed up the buffer and I am not really sure if I read the .bin file correctly because I can't print it with printf("%x", buffer[i])

Hope you guys can help

Greetings from germany :)

Answer 1

Recommended changes:

1. Change your buffer to a char (byte), as the ftell() will report the size in bytes (char) and malloc() use the byte size also.

   unsigned int buffer []; /buffer/

to

unsigned char *buffer; /*buffer*/

2. [Edit] 2021: Omit cast
   2) This is OK, size is bytes and buffer points to bytes, but could be explicitly cast

   buffer = malloc(size); /allocate space on heap/

to

buffer = (unsigned char *) malloc(size);  /*allocate space on heap*/
/* or for those who recommend no casting on malloc() */
buffer = malloc(size);  /*allocate space on heap*/

3. change 2nd parameter from sizeof(unsigned int) to sizeof *buffer, which is 1.

   else if (fread(buffer, sizeof(unsigned int), size, fp) != size){

to

else if (fread(buffer, sizeof *buffer, size, fp) != size){ 

4. Change "%x" to "%02x" else single digit hexadecimal numbers will confuse the output. E. g. is "1234" four bytes or two?

   printf("%x", buffer[i]);

to

printf("%02x", buffer[i]);

5. Your clean-up at the end of the function may include

   fclose(fp); free(buffer);