A simple program which sorts a text file alphabetically (one string per line), with the Shell sort algorithm

ShellSort.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <inttypes.h>
#include <errno.h>

#define BUFFERSIZE 64


void shellSort(const char *const);
void closeFile(FILE**);



int main(int argc, const char *argv[])
{
	(void)argc;
	(void)argv;

	if(argc == 2 && argv[1] != NULL)
	{
		shellSort(argv[1]);
	}

	else
	{
		fprintf(stderr, "ERROR, nothing to sort...\n");
	}

	return 0;
}



void shellSort(const char *const filePath)
{
	// Let's open the file with strings not alphabetically sorted, in reading mode.
	FILE *file = fopen(filePath, "r");

	if(file == NULL)
	{
		fprintf(stderr, "ERROR, Couldn\'t open \"%s\" as well: %s.\n", filePath, strerror(errno));
		return;
	}

	char buffer[BUFFERSIZE] = "";

	// Let's count the number of lines the file contained.
	uint16_t nbLines = 0;
	while(fgets(buffer, BUFFERSIZE, file))
	{
		nbLines++;
	}

	// Let's go back to the beginning !
	rewind(file);

	// Let's allocate a memory space for an array of strings.
	char **data = (char**)calloc(nbLines, sizeof(*data));

	if(data == NULL)
	{
		fprintf(stderr, "ERROR, Coulnd\'t allocate memory for data.\n");
		closeFile(&file);
		return;
	}

	for(uint16_t i = 0; i < nbLines; i++)
	{
		data[i] = (char*)calloc(BUFFERSIZE, sizeof(*data[i]));

		if(data[i] == NULL)
		{
			fprintf(stderr, "ERROR, Coulnd\'t allocate memory for data.\n");

			for(int16_t j = i - 1; j >= 0; j--)
			{
				free(data[j]);
				data[j] = NULL;
			}

			free(data);
			data = NULL;

			closeFile(&file);
			return;
		}
	}

	// Let's fill in the array with the strings from the file !
	for(uint16_t i = 0; i < nbLines; i++)
	{
		fgets(buffer, BUFFERSIZE, file);
		strncpy(data[i], buffer, BUFFERSIZE);
	}

	// Let's compute a 'gap' which fits the best with our case.
	uint8_t gap = 1;
	while(gap <= nbLines / 3)
	{
		gap = gap * 3 + 1;
	}

	uint16_t inner, outer;
	while(gap > 0)
	{
		for(outer = gap; outer < nbLines; outer++)
		{
			strncpy(buffer, data[outer], BUFFERSIZE);

			inner = outer;

			while(inner > gap - 1 && strcmp(data[inner - gap], buffer) > 0)
			{
				strncpy(data[inner], data[inner - gap], BUFFERSIZE);
				inner -= gap;
			}

			strncpy(data[inner], buffer, BUFFERSIZE);
		}

		gap = (gap - 1) / 3;
	}

	closeFile(&file);

	// Let's re-open this file in writing mode !
	file = fopen(filePath, "w");

	if(file == NULL)
	{
		fprintf(stderr, "ERROR, Coulnd\'t open the file as well: %s\n", strerror(errno));
		return;
	}

	// Writes down the string (alphabetically sorted) in the file !
	for(uint16_t i = 0; i < nbLines; i++)
	{
		fputs(data[i], file);
	}

	closeFile(&file);

	// Dis-allocate the array of strings.
	for(uint16_t i = 0; i < nbLines; i++)
	{
		free(data[i]);
		data[i] = NULL;
	}

	free(*data);
	*data = NULL;
}


void closeFile(FILE **file)
{
	if(fclose(*file) == EOF)
	{
		fprintf(stderr, "ERROR, Coulnd\'t close the file as well: %s\n", strerror(errno));
	}

	*file = NULL;
}