fread reads size*nmemb bytes from the current position of a stream into memory. On success, it advances that position and returns the number of bytes read; a smaller return indicates an error or incomplete read. section 14.6
size_t fread(void* restrict ptr, size_t size, size_t nmemb,
FILE* restrict stream);
size_t fwrite(void const*restrict ptr, size_t size, size_t nmemb,
FILE* restrict stream);
int fseek(FILE* stream, long int offset, int whence);
long int ftell(FILE* stream);
The declaration exposes fread's four arguments: destination memory, object size, object count, and input stream. section 14.6
Reading a file byte-for-byte
FILE* inp = fopen(__FILE__, "rb"); if (inp) { size_t ibytes = fread(there, 1, sizeof there, inp); cmp = memcmp(here, there, sizeof here); fclose(inp); } Here, size is 1, so fread requests sizeof there bytes. The file is opened with "rb": binary mode prevents text-stream translation. The result is recorded in ibytes and compared with the expected byte array. section 14.6
TAKEAWAY rules:
- Open streams in binary mode when reading binary data portably; otherwise, platform-specific end-of-line translation can alter the data. section 14.6
- Check the return value against the requested byte count; a smaller value means the complete request was not satisfied. section 14.6
- Combine fread with fseek and ftell when navigation through a file stream requires moving forward or backward. section 14.6
