Format a quantity in bytes into a human readable string (C)

human-size.c

#include <stdio.h> 
#include <stdlib.h> // atoll
#include <stdint.h> // uint64_t
#include <inttypes.h> // PRIu64

static const char *humanSize(uint64_t bytes)
{
	char *suffix[] = {"B", "KB", "MB", "GB", "TB"};
	char length = sizeof(suffix) / sizeof(suffix[0]);

	int i = 0;
	double dblBytes = bytes;

	if (bytes > 1024) {
		for (i = 0; (bytes / 1024) > 0 && i<length-1; i++, bytes /= 1024)
			dblBytes = bytes / 1024.0;
	}

	static char output[200];
	sprintf(output, "%.02lf %s", dblBytes, suffix[i]);
	return output;
}

int main(int argc, char **argv)
{
	if (argc == 1) {
		fprintf(stderr, "Usage: %s <bytes>\n", *argv);
		return 1;
	}

	uint64_t bytes = atoll(argv[1]);

	printf("%" PRIu64 " Bytes: %s\n", bytes, humanSize(bytes));

	return 0;
}

Examples:

$ ./main 1500 
1500 Bytes: 1.46 KB 
$ ./main $((1024*1024*1024)) 
1073741824 Bytes: 1.00 GB 
$ ./main $((1024*1024*1024*1024*1024)) 
1125899906842624 Bytes: 1024.00 TB

Thanks for this example.
Wouldn't it be better if you allocated a char* variable and send its address to function to assign the result to it directly?

cool, when 1024 it prints "1024 B". maybe 1K would be good.
And the static buffer is error-prone,,,

Thanks for this code _ / \ _

Another variation:

char *humanSize(uint64_t bytes, char *hrbytes)
{
    char   *suffix[] = { "B", "KB", "MB", "GB", "TB" };
    char    length = sizeof(suffix) / sizeof(suffix[0]);
    int     i;

    for(i = 0; i < length; i++) {
        if(bytes < 1024)
            break;

        bytes >>= 10;
    }

    snprintf(hrbytes, BUFSIZ, "%lu%s", bytes, suffix[i]);
    return(hrbytes);
}

How does this work? how can you return a pointer to a stack array, wouldnt the pointer point to garbage??

I'm sorry, I should have explained this more. I prefer not to return pointers to static data. Here's an example of usage:

int main(int argc, char **argv)
{
    char   *humanSize(uint64_t, char *);
    char    hrbytes[BUFSIZ];

    printf("bytes = %s\n", humanSize(12345, hrbytes));
}