#include <stdio.h>
#include <string.h>

int main(void)
{
	// String of what letters can be used for input.
        // The program doesn't need this hardcoded a-z, you could do
        // 0-9 and it ought to still work.
	unsigned char *charset=	"abcdefghijklmnopqrstuvwxyz";

	// What letters have already been used.
        // 0,0,0,0,0 means always pick the first letter, whatever it may be.
        // By removing it from the input set, we guarantee 0,0,... won't
        // actually pick duplicates.
	int chosen[16]={0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};

	int running=1;  	// Set this to 0 to stop the main loop

	// Might as well count combinations while we're at it
        // We use an unsigned long int since the count could be really huge.
	unsigned long int loopcount=0;

	// Remember the length of the set, so we don't have to count again
	const int maxpos=strlen(charset);

	// How long an output string do we want?
	// can be a maximum of 16 in this implementation)
	const int outlen=5;

	while(running) // Keep going until we're told to stop.
	{
		// Just a counter for loops.
		int pos;	
		// A blank C-string, the current set of allowed characters.
		char ltrs[64];
		// A blank C-string, the current output string.
		char out[17];

		// Fill the input set, ltrs, with a-z.
		strcpy(ltrs, charset);

		loopcount++;	// Count another loop.

                // Create the output string character by character by setting
                // out[0], out[1], ..., out[outlen-1].  Then finishing with
                // out[outlen]='\0' to NULL-terminate it as a proper C-string.
		for(pos=0; pos<outlen; pos++)
		{
			// Add the chosen letter to the output string.
			out[pos]=ltrs[ chosen[pos] ];

			// Remove the character from the input set.
			// We do this by replacing it by the
			// char at the end, and shrinking by one.
			ltrs[ chosen[pos] ] = ltrs[ maxpos-(pos+1) ];
			ltrs[ maxpos-(pos+1) ] = '\0';
		}

		out[outlen]='\0';// NULL-terminate it so we can print it

		printf("%s\n", out);

		// Advance to the next combination.
		for(pos=0; pos<outlen; pos++)
		{
			// Pick one letter ahead.
			chosen[pos]++;

			// Are we beyond the max?  letter 0 can choose
			// 0-25, letter 1 can choose 0-24, etc, etc.
			if(chosen[pos] >= (maxpos - pos))
			{
				// If we are, reset it to 0 and advance
				// the next chosen character.
				chosen[pos]=0;

				// Out of digits!  We are finished.
				if(pos == (outlen - 1))
				{
					running=0;
					break;
				}
				else
				{
					// We must advance the next
					// digit.  while loop keeps going
					continue;
				}
			}

			// This digit didn't overflow?  stop the loop, we're done.
			break;
		}
	}

        // print to stderr instead of stdout, so we can pipe or redirect stdout
        // and still see the combination count.
	fprintf(stderr, "%lu combinations\n", loopcount);
	return(0);
}

$ ./a.out | sort -u | wc -l
7893600 combinations
7893600
$