Reading strings from a file in reverse order
/*
Beginning C, Third Edition
By Ivor Horton
ISBN: 1-59059-253-0
Published: Apr 2004
Publisher: apress
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_STRING_LENGTH 1000
#define BUFFER_SIZE 50
/* Global variables */
FILE *pInFile = NULL; /* File pointer to input file */
FILE *pOutFile = NULL; /* File pointer to output file */
char *infilename = "C:\\myfile.txt"; /* Name of the file to be read */
char *outfilename = "C:\\outfile.txt"; /* Name of the file to be written */
char *buffer = NULL;
size_t buffer_size = BUFFER_SIZE;
void main()
{
size_t str_length = 0;
int str_count = 0;
fpos_t *positions = NULL;
int i = 0;
buffer = (char*)malloc(buffer_size); /* Create initial buffer */
if((pInFile = fopen(infilename, "r")) == NULL) /* Open the input file */
{
printf("Error opening %s for reading. Program terminated.", infilename);
abort();
}
/* Find out how many strings there are */
for(;;)
{
fread(&str_length, sizeof(size_t), 1, pInFile); /* Read the string length */
if(feof(pInFile)) /* If it is end of file */
break; /* We are finished */
/* Check buffer is large enough and increase if necessary */
if(str_length>buffer_size)
{
buffer_size = str_length+1;
free(buffer);
buffer = (char*)malloc(buffer_size);
}
fread(buffer, str_length, 1, pInFile); /* Read the string */
++str_count;
}
printf("\nThere are %d strings in the input file.", str_count);
/* Now get the position for the beginning of each record in the file */
/* The buffer is now large enough to hold the longest string */
rewind(pInFile);
positions = (fpos_t*)malloc(str_count*sizeof(fpos_t)); /* Array to store the positions */
for(i = 0 ; i<str_count ; i++)
{
fgetpos(pInFile, positions+i); /* Get the positions */
fread(&str_length, sizeof(size_t), 1, pInFile); /* Read the string length */
fread(buffer, str_length, 1, pInFile); /* Read the string */
}
/* Open the output file */
if((pOutFile = fopen(outfilename, "w")) == NULL)
{
printf("Error opening %s for reading. Program terminated.", outfilename);
abort();
}
/* Read the records in reverse order from the input file and write to the new file */
for(i = 0 ; i<str_count ; i++)
{
fsetpos(pInFile, positions+str_count-i-1); /* Set the file position */
fread(&str_length, sizeof(size_t), 1, pInFile); /* Read the string length */
fwrite(&str_length, sizeof(size_t), 1, pOutFile); /* Write to new file */
fread(buffer, str_length, 1, pInFile); /* Read the string */
fwrite(buffer, str_length, 1, pOutFile); /* Write to new file */
}
fclose(pInFile); /* Close input file */
fclose(pOutFile); /* Close output file */
printf("\nNew file write complete\n");
/* List contents of output file */
if((pOutFile = fopen(outfilename, "r")) == NULL) /* Open the new file to read it */
{
printf("Error opening %s for reading. Program terminated.", outfilename);
abort();
}
printf("\nThe strings in the new file are:");
for(i = 0 ; i<str_count ; i++)
{
fread(&str_length, sizeof(size_t), 1, pOutFile);
fread(buffer, str_length, 1, pOutFile);
buffer[str_length] = '\0';
printf("\n%s", buffer);
}
printf("\n");
fclose(pOutFile); /* Close file */
/* Free the memory we allocated */
if(buffer != NULL)
free(buffer);
if(positions != NULL)
free(positions);
}
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