Confusion regarding fwrite() and fread() parameters in C

Question

I am new to C and recently i have been working on a program whereby i am reading a stream of 512 bytes from a file using fread into a array and then after checking for certain conditions in that stream of bytes i am writing the stream of bytes into a new file using fwrite.
The code snippet for the same is

    unsigned char buffer[512];
    FILE *fp = fopen("file.name","r");

    if(fp == NULL)
    {
        perror("Error opening the file\n ");
        fclose(fp);
        return -1;
    }

    while(fread(&buffer, sizeof(buffer), 1 , fp) == 1) //my program works fine for both &buffer and only buffer
    {
          //Do something
    }

The definition of fread is:

    size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream)

Here ptr is the pointer to a block of memory where the read bytes will be stored.

I have defined buffer to be a character array and hence only passing buffer as the first parameter should have been enough as it is a pointer but the program works fine even when &buffer is being provided. The same is happening with fwrite as well. Now if buffer is the pointer then &buffer is the address of the pointer and should not have the same result as that of a pointer but it actually does, so why does the function work properly with both the different parameters?

`&buffer` is *not* the "address of the pointer". That statement doesn't even make sense. There is no "the pointer". — Kerrek SB, Jul 03 '14 at 22:02
Also note that your function will only read blocks of 512 bytes. If your file size isn't a multiple of that, you won't be able to read the remainder. — Kerrek SB, Jul 03 '14 at 22:03
Best answer from the link Jon cites: "That's because the array name (my_array) is different from a pointer to array. It is an alias to the address of an array, and its address is defined as the address of the array itself." — FoggyDay, Jul 03 '14 at 23:38

Deduplicator · Accepted Answer · 2014-07-03T22:06:00.980

In C, any data-pointer type is implicitly convertible to void*:

buffer designates the array. It is not a pointer. Arrays are in nearly all contexts converted to a pointer to their first elements though.
&buffer this is one of the exceptions, so here we have a pointer to said array instead.

Anyway, you probably want to do this instead:

size_t count;
while((count = fread(buffer, 1, sizeof(buffer), fp)) > 0)

Why?

Because you want to read all the data, and not miss the final partial block.
On an interactive device you want to act on the data as early as possible, not waiting until a full sizeof(buffer) block is done.

+1 Proper type and use of `size_t count` – chux - Reinstate Monica Jul 03 '14 at 23:25 — chux - Reinstate Monica, Jul 03 '14 at 23:25

Confusion regarding fwrite() and fread() parameters in C

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