fwrite cuts off character string - c

Question

I am trying to output the statistics from my program into a file. I first create some empty strings, then amend them using sprintf, this is because I need to turn floats into chars. I then write them to a file.
I sort of works, but the output .txt file only returns 4 digits of precision regardless what I specify in sprintf.
CODE:
METRIC.RESP_TIME =(( (long int )(tval_after.tv_sec*1000000 + tval_after.tv_usec) - (long int )(tval_before.tv_sec*1000000 + tval_before.tv_usec)));
METRIC.RESP_TIME = (float) METRIC.RESP_TIME/1000000;
float ave_resp_time = METRIC.RESP_TIME/R;
float ave_through = METRIC.BYTES_RECEIVED/METRIC.RESP_TIME;
FILE *fp;
char size_str [30]; //malloc(((int)strlen(DOWNLOAD_FILE)+ (int)strlen(Q[LOCAL_QUEUE_COUNT].CHORE_NAME)))];
char rate_str [30];
char through_put_str [30];
sprintf(size_str,"TOTAL BYTES RECIEVED [B]: %5.0d ", METRIC.BYTES_RECEIVED);
sprintf(rate_str,"TOTAL TIME REQUIRED [s]: %2.8f ", ave_resp_time);
sprintf(through_put_str,"AVERAGE THROUGHPUT [B/s]: %2.8f ", ave_through);
fprintf(stdout,"%d\n",METRIC.BYTES_RECEIVED);
fp = fopen( METRICS_FILE, "w");
if(NULL == fp){
printf("Could not make metrics file: error %d ",errno);
return 0;
}
fwrite(size_str,(size_t)sizeof(size_str),1,fp);
fwrite(rate_str,sizeof(rate_str),1,fp);
fwrite(through_put_str,sizeof(through_put_str),1,fp);
fclose(fp);
return 0;
OUTPUT:
TOTAL BYTES RECIEVED [B]: 5526TOTAL TIME REQUIRED [s]: 0.001AVERAGE THROUGHPUT [B/s]: 2992
Hoping to make it look like:
TOTAL BYTES RECIEVED [B]: 55264892
TOTAL TIME REQUIRED [s]: 0.0019634
AVERAGE THROUGHPUT [B/s]: 29929054

Answer 1

You need an explicit new line character; fwrite() does not add one, and also you can directly use fprintf() instead of what you have.
To fix your code, do this
sprintf(size_str, "TOTAL BYTES RECIEVED [B]: %5.0d\n", METRIC.BYTES_RECEIVED);
/* ^ this will break the line */
the strcat(size_str, "\0"); is not needed.
You don't have to do all this, because you can just
fprintf(fp, "TOTAL BYTES RECIEVED [B]: %5.0d\n", METRIC.BYTES_RECEIVED);
/* ^ this will break the line */

Answer 2

As mentioned by #Jonathan Leffler, use well sized buffers rather than hoping that 30 is ample.
One method that has worked well is to size the buffer per the sprintf()
// char size_str [30];
// sprintf(size_str,"TOTAL BYTES RECIEVED [B]: %5.0d ", METRIC.BYTES_RECEIVED);
#define INT_MAX_PRT (sizeof(int) * CHAR_BIT/3 + 3)
const char rcv_fmt[] = "TOTAL BYTES RECEIVED [B]: %5.0d\n";
char size_str [sizeof rcv_fmt + INT_MAX_PRT];
sprintf(size_str, rcv_fmt, METRIC.BYTES_RECEIVED);
This approach is a bit more challenging with floating point as the "%f" width could be so large.
// char rate_str [30];
// sprintf(rate_str,"TOTAL TIME REQUIRED [s]: %2.8f ", ave_resp_time);
#define FLT_MAX_PRT (1 /* sign */ + FLT_MAX_10_EXP + 1)
const char *time_fmt[] = "TOTAL TIME REQUIRED [s]: %2.8f\n";
char rate_str[sizeof time_fmt + FLT_MAX_PRT + 8];
sprintf(rate_str, time_fmt, ave_resp_time);
Still, since it is possible to mistake the needed buffer size, code could also use snprintf() to minimize the harm. But in the end, a proper size buffer is needed.
Note: added '\n' to the formats.

Answer 3

First things first, you should get out of the habit of using sizeof on any array. This will get you into trouble faster than you realize.
Second, you seem to be doing double-duty here. There is no need to use sprintf + fwrite at all. You should just fprintf and pass you fp as the first arg.