Writing bits to a file in C

Question

I have this string: \"101\" I want to write it to a file, in C, not as text: \"101\" and so 8 bits x char. but directly use the string as bits: the bit \"1\", the bit \"0\" and

Answer 1

All filesystems¹ deal with files in terms of bytes (and allocate storage space with a much larger granularity, 512 bytes at a time minimum). There's no way you are going to get a file that is 3 bits long.

The best you can do is use up one whole byte but ignore 5 of its bits. To do that (assuming that the number is always going to fit into a byte), convert the input string to an integral type:

long l = strtol(c, 0, 2);

Then get its least significant byte:

unsigned char b = l & 0xffl;

And write it to the file:

fwrite(&b, 1, 1, binFile);

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¹ Well, maybe not all. There might be some researchers somewhere that experiment with bit-sized filesystems. I wouldn't know.