It's easy to find popular conventions for C-style I/O. What's more difficult is finding explanations as to why they are such. It's common to see a read with statements like:
fread(buffer, sizeof(buffer), 1, ptr);
How should a programmer think about using the parameters size and n of fread()?
For example, if my input file is 100 bytes, should I opt for a larger size with fewer n or read more objects of a smaller size?
If the size-to-be-read and n exceed the byte-size of an input file, what happens? Are the excess bytes that were read composed, colloquially speaking, of "junk values"?
size_t fread(void * restrict ptr, size_t size, size_t n, FILE * restrict stream);
How should a programmer think about using the parameters size and n of fread()?
When reading into an array:
size is the size of called pointer's de-referenced type.
n in the number of elements.
some_type destination[max_number_of_elements];
size_t num_read = fread(destination, sizeof *destination, max_number_of_elements, inf);
printf("Number of elements read %zu\n", num_read);
if (num_read == 0) {
if (feof(inf)) puts("Nothing read as at end-of-file");
else if (ferror(inf)) puts("Input error occurred");
else {
// since sizeof *destination and max_number_of_elements cannot be 0 here
// something strange has occurred (UB somewhere prior?)
}
For example, if my input file is 100 bytes, should I opt for a larger size with fewer n or read more objects of a smaller size?
In the case, the size of the data is 1, the max count 100.
#define MAX_FILE_SIZE 100
uint8_t destination[MAX_FILE_SIZE];
size_t num_read = fread(destination, sizeof *destination, MAX_FILE_SIZE, inf);
If the size-to-be-read and n exceed the byte-size of an input file, what happens?
The destination is not filled. Use the return value to determine.
Are the excess bytes that were read composed, colloquially speaking, of "junk values"?
No. There values before fread() remain the same, (as long as the return was not 0 and ferror() not set). If the destination was not initialized/assigned, then yes, it may be though of as junk.
Separate size, n allows fread() to function as desired even if size * n overflows size_t math. With current flat memory models, rarely is this needed.