Question

I find fwrite fails when I am trying to write somewhat big data as in the following code.

#include <stdio.h>
#include <unistd.h>

int main(int argc, char* argv[])
{
    int size = atoi(argv[1]);
    printf("%d\n", size);
    FILE* fp = fopen("test", "wb");
    char* c = "";
    int i = fwrite(c, size, 1, fp);
    fclose(fp);
    printf("%d\n", i);
    return 0;
}

The code is compiled into binary tw
When I try ./tw 10000 it works well. But when I try something like ./tw 12000 it fails.(fwrite() returns 0 instead of 1)
What’s the reason of that? In what way can I avoid this?

EDIT: When I do fwrite(c, 1, size, fp) it returns 8192 instead of larger size I give.

2nd EDIT: When I write a loop that runs for size times, and fwrite(c, 1, 1, fp) each time, it work perfectly OK.
It seems when size is too large(as in the first EDIT) it only writes about 8192 bytes.
I guess something has limited fwrite write up to fixed size bytes at a time.

3rd EDIT: The above is not clear.
The following fails for space - w_result != 0 when space is large, where space is determined by me and w_result is object written in total.

w_result = 0;
char* empty = malloc(BLOCKSIZE * size(char));
w_result = fwrite(empty, BLOCKSIZE, space, fp);
printf("%d lost\n", space - w_result); 

While this works OK.

w_result = 0;
char* empty = malloc(BLOCKSIZE * sizeof(char)); 
for(i = 0; i < space; i ++)
    w_result += fwrite(empty, BLOCKSIZE, 1, fp);
printf("%d lost\n", space - w_result);

(every variable has been declared.)

I corrected some errors the answers memtioned. But the first one should work according to you.

Answer 1

With fwrite(c, size, 1, fp); you state that fwrite should write 1 item that is size big , big out of the buffer c.

c is just a pointer to an empty string. It has a size of 1. When you tell fwrite to go look for more data than 1 byte in c , you get undefined behavior. You cannot fwrite more than 1 byte from c.

(undefined behavior means anything could happen, it could appear to work fine when you try with a size of 10000 and not with a size of 12000. The implementation dependent reason for that is likely that there is some memory available, perhaps the stack, starting at c and 10000 bytes forward, but at e.g. 11000 there is no memory and you get a segfault)