How to fprintf an int to binary in 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

Such format does not exist. You will have to write your own function. — Christian Gibbons, Sep 24 '18 at 16:17
There is no standard specifier to achieve this. You have to write your own. — Osiris, Sep 24 '18 at 16:18
When you say you want to write the 16 bits `0000000011101001`, do you want to write them as 16 bytes, 4 bytes, or 2 bytes? (I'm guessing 2, but the preceding three comments all presume 16.) — Steve Summit, Sep 24 '18 at 16:22
It is not clear that you understand the difference between a binary file and a string of ASCII `'1'` and `'0'` digits. Your "_result in the file should be_" is ambiguous. If performance the issue you need binary - i.e. 2 bytes per sample, not 16 ASCII digits and a newline. That is probably what you meant but as @SteveSummit has pointed out, others have not interpreted it in that way. — Clifford, Sep 24 '18 at 16:26
In your loop you are most likely accessing `array` out of bounds. — Osiris, Sep 24 '18 at 16:27
I want my program to output a converted value but I don't want it to do any conversion. — Paul Childs, Nov 24 '21 at 21:54

Clifford · Accepted Answer · 2018-09-24T17:07:51.433

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.

Marius · Answer 2 · 2018-09-24T16:54:08.857

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.

While this may appear to be what he asked for, it seems unlikely that is what he intended given the performance requirement. It also does not result in a _binary_ file; rather a text file containing only 0x31 and 0x30 character values. — Clifford, Sep 24 '18 at 17:11

blablablabla · Answer 3 · 2021-09-06T20:28:04.190

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

How to fprintf an int to binary in C?

3 Answers3