How to fprintf an int to binary in C? - c

Question

I'm trying to write the binary number of 16-bit signed integer to a file. I searched a lot and ofcourse I found many examples which converts integer variables to binary. But in my case these functions will not be efficient, because I need to convert 50e6 samples/s. Calling a function to convert each sample will need a lot of computing time.
So what I want to do is:
int array[] = {233, 431, 1024, ...}
for (i = 0; i < sizeof(array); i++){
fprintf(outfile, "%any_binary_format \n", array[i]);
}
result in the file should be:
0000000011101001
0000000110101111
0000010000000000

Answer 1

fprintf is intended for formatted output - the formatting being "human readable" text, it is therefore not the appropriate function to use if you want binary output. For that you should use fwrite():
for (i = 0; i < sizeof(array) / sizeof(*array); i++ )
{
fwrite (&array[i], sizeof(*array), 1, outfile ) ;
}
Note I have also fixed your loop termination to correctly iterate the number of elements in the array. But in fact the loop is unnecessary - the output is binary, the array is binary - you can just output the entire array thus:
fwrite( array, sizeof(array), 1, outfile ) ;
Your performance requirement of 50Msps will require write performance of around 95Mb/s sustained - that is a lot to ask, and unlikely to be achieved by writing one sample at a time. You may be better off using a memory mapped file, but unless you are using a real-time OS, there are no guarantees that you will sustain that output rate indefinitely - it only takes some other process to access the drive, and it may introduce an unacceptable delay.
Also note that the file must have been opened for binary output - especially on Windows to prevent translation of CR to CR+LF which will be disastrous for your sample data.

Answer 2

If you want to use printf you can use something like this:
#define BYTE_TO_BINARY_PATTERN "%c%c%c%c%c%c%c%c\n"
#define BYTE_TO_BINARY(byte) \
(byte & 0x80 ? '1' : '0'), \
(byte & 0x40 ? '1' : '0'), \
(byte & 0x20 ? '1' : '0'), \
(byte & 0x10 ? '1' : '0'), \
(byte & 0x08 ? '1' : '0'), \
(byte & 0x04 ? '1' : '0'), \
(byte & 0x02 ? '1' : '0'), \
(byte & 0x01 ? '1' : '0')
int main()
{
uint8_t value = 5;
printf(BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(value));
return 0;
}
Should print 00000101. I use this sometimes in embedded code when debugging to check register values. Just replace printf with fprintf if you want to write the ascii binary strings to file.

Answer 3

If your compiler supports inline you don't need to worry about the overhead of a small function, take a look at this.
Anyway you can simply implement the function as a macro.
If you want a faster approach you can use a larger buffer (the size for the faster runtime is machine-dependent) for example char str[1 << 16], writing the results to the buffer and using fwrite/write to the out stream.
Another approach is to map the process via mmap/msync.
Anyway you don't need to look at a faster function, but rather a deeper knowledge of the system you're working on.
#define SHORT_WIDTH 16
#define TEST 1
#define PADDING 1 /* set to 0 if you don't need the leading 0s */
char *ShortToBin(unsigned short x, char *buffer) {
#if PADDING
int i;
for(i = 0; i < SHORT_WIDTH; ++i)
buffer[SHORT_WIDTH - i - 1] = '0' + ((x >> i) & 1);
return buffer;
#else
char *ptr = buffer + SHORT_WIDTH;
do {
*(--ptr) = '0' + (x & 1);
x >>= 1;
} while(x);
return ptr;
#endif
}
#if TEST
#include <stdio.h>
int main() {
short n;
char str[SHORT_WIDTH+1]; str[SHORT_WIDTH]='\0';
while(scanf("%hd", &n) == 1)
puts(ShortToBin(n, str));
return 0;
}
#endif