c - Reliably counting bytes written by fwrite when disk is full -

does know reliable way count how many bytes have been written, using c standard i/o (fwrite), in disk-full situation?

i’ve had lot of trouble getting work. problem seems fwrite buffered, , thinks has written more bytes device can accept.

using small buffer, same size device blocks, fwrite report had written full buffer when had not, count ended being 1 block more correct. fixed testing error , adding total if there no error.

but then, larger buffer, fwrite write partial buffer, , fail count that. checked partial write, adding , breaking out of loop. ended following program (reduced mcve):

#include <stdio.h> #include <string.h>  //#define buf_size 4096 #define buf_size 8192 //#define buf_size 16384  int main(void) {     unsigned long long ct = 0;     size_t written;     unsigned char *buf;      buf = malloc(buf_size);     memset(buf, 0xff, buf_size);      while (1) {         written = fwrite(buf, 1, buf_size, stdout);         if (written < buf_size) {             ct += written;             break;         }         fflush(stdout);         if (ferror(stdout))             break;         ct += written;     }      fprintf(stderr, "%llu bytes written\n", ct);      return 0; } 

the device has 4k blocks, , either 68k or 72k free. tried buffer sizes of 4k, 8k, , 16k.

and darned thing still doesn’t work. when there’s 72k free , use 8k buffer, writes 72k, thinks wrote 4k , adds that.

i suppose use buffer size that’s equal block size. i’m not sure work reliably.

does know how make work in cases? i’m thinking might best bypass buffering issue entirely , use posix i/o instead (open , write).

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edit: nsilent22’s suggestion worked correctly , reduced loop 2 lines:

    setbuf(stdout, null);     ...     while ((written = fwrite(buf, 1, buf_size, stdout)) > 0)         ct += written; 

consider using setbuf function null parameter buffer. turn off stream buffering.