Write a file byte by byte in C using fwrite()

Question

I'm trying to read a file byte by byte and write it to another file. I have this code:

if((file_to_write = fopen(file_to_read, "ab+")) != NULL){

  for(i=0; i<int_file_size; i++){
    curr_char = fgetc(arch_file);

    fwrite(curr_char, 1, sizeof(curr_char), file_to_write);
  }
}

where int_file_size is the amount of bytes I want to read, arch_file is the file I'm reading from, and curr_char is a char pointer.

However this doesn't work. I get Segmentation fault (core dumped) error on the first iteration in the loop. I'm pretty sure there is something wrong with my fwrite() statement. Any help would be appreciated.

Answer 1

You should pass the address of curr_char, not the curr_char itself:

fwrite(&curr_char, 1, sizeof(curr_char), file_to_write);
//     ^------ Here

Answer 2

curr_char is a char pointer.

In that case,

curr_char = fgetc(arch_file);

is wrong. You're implicitly converting the int returned by fgetc to a char*, and then in fwrite, that value is interpreted as an address, from which the sizeof(char*) bytes are tried to be read and written to the file.

If curr_char points to memory allocated for a char,

*curr_char = fgetc(arch_file);
fwrite(curr_char, 1, sizeof *curr_char, file_to_write);

would be closer to correctness. But fgetc returns an int and not a char for a reason, it may fail, in which case it returns EOF. So you should have

int chr = fgetc(arch_file);
if (chr == EOF) {
    break;  // exit perhaps?
}
char c = chr;  // valid character, convert to `char` for writing
fwrite(&c, 1, sizeof c, file_to_write);

to react to file reading errors.