Comparision of C and C++ using std::string

main.cpp

#include <string>  
#include <stdlib.h>
#include <stdio.h>
#include "time.h"

void GetLen(FILE * fp, int &len);
void ChangeExtension(char *fname, char *newExt);
int StrReplace(char *target, const char *needle, const char *replacement, int len);
void CPPReplace(std::string &data);

int main(int c, char**v)
{
	FILE *fp = NULL;
	int len = 0;
	char *buf = NULL;
	char fname[256];
	time_t totalC = 0, totalCPP = 0;

	if (c < 2)
	{
		puts("I need filename(s), like this...");
		puts("fixcsv.exe file.txt");
		return 0;
	}

	for (int i = 1; i < c; i++)
	{
		fopen_s(&fp, v[i], "r");
		GetLen(fp, len);
		buf = (char*)calloc(1, len * 2);
		if (!buf)
		{
			puts("Wow! That file's too big for me. Out of memory!");
			return -1;
		}

		fread(buf, len, 1, fp);
		fclose(fp);
		std::string data = buf;
		time_t start, end;
		puts("Starting with std::string");
		start = clock();//predefined  function in c
		CPPReplace(data);
		end = clock();
		totalCPP += end - start;
		printf("CPP Runtime in mSecs: %lld\n", end - start);
		puts("Starting with C stdlib functions");
		start = clock();//predefined  function in c
		int result = StrReplace(buf, v[1], ",\n", len * 2);
		if (result)
		{
			puts("Something went wrong. I have to quit!");
			return -1;
		}
		end = clock();
		totalC += end - start;
		printf("C Runtime in mSecs: %lld\n", end - start);
 		strcpy(fname, v[i]);
		ChangeExtension(fname, "csv");
		fopen_s(&fp, fname, "w");
		fwrite(buf, strlen(buf), 1, fp);
		fclose(fp);
	}
	printf("Total Runtimes\nCPP with std::string: %f\nC with stdlib: %f\n", (float)totalCPP / CLOCKS_PER_SEC, (float)totalC / CLOCKS_PER_SEC);
	printf("In this example, C was %f times faster than C++\n", (float)(totalCPP / totalC));
	puts("Press return to continue");
	getchar();
	return 0;
}

void CPPReplace(std::string &data)
{
	const std::string s = ",";
	const std::string t = ",\n";

	std::string::size_type n = 0;
	while ((n = data.find(s, n)) != std::string::npos)
	{
		data.replace(n, s.size(), t);
		n += t.size();
	}
}

void GetLen(FILE * fp, int &len)
{
	fseek(fp, 0L, SEEK_END);
	len = ftell(fp);
	rewind(fp);
}

void ChangeExtension(char *fname, char *newExt)
{
#define EVAL_MAX_LEN (300)

	size_t nLen;
	char szOut[EVAL_MAX_LEN] = { 0 };
	char szPath[EVAL_MAX_LEN];
	strcpy(szPath, fname);
	nLen = strlen(szPath);
	if ((nLen > 0) && (nLen < EVAL_MAX_LEN))
	{

		while (nLen)
		{
			if (szPath[nLen] == '.')
			{
				szPath[nLen] = '\0';
				break;
			}
			nLen--;
		}
		sprintf(szOut, "%s.%s", szPath, newExt);
		strcpy(fname, szOut);
	}
	return;
}

int StrReplace(char *target, const char *needle, const char *replacement, int len)
{
	char *buffer = (char*)calloc(1, len);
	if (!buffer)
	{
		puts("Wow! That file's too big for me. Out of memory!");
		return -1;
	}

	char *insert_point = &buffer[0];
	const char *tmp = target;
	size_t needle_len = strlen(needle);
	size_t repl_len = strlen(replacement);

	while (1)
	{
		const char *p = strstr(tmp, needle);
		if (p == NULL)
		{
			strcpy(insert_point, tmp);
			break;
		}
		memcpy(insert_point, tmp, p - tmp);
		insert_point += p - tmp;
		memcpy(insert_point, replacement, repl_len);
		insert_point += repl_len;
		tmp = p + needle_len;
	}
	strcpy(target, buffer);
	delete(buffer);
	return 0;
}

Have you tried the simplest solution possible?
int c;
while( ( c = getchar( ) ) != EOF ) {
putchar( c );
if( c == ',' )
putchar( '\n' );
}