Function that copies parts of a file to another in C - c

Question

I have to make a function which reads a file line by line and copies a some of them to another file. The head of the function has to look like this: "int write_x(const char *input_path, const char *output_path, int x)" and can't be changed. For example if x = 3 every third line of the input txt-file should be copied to the output txt-file. Every line has a maximum of 80 signs. I hope someone can help me because i'm not programming very long yet.
This is what I have alreaday, but it doesn't work:
#include <stdlib.h>
#include <stdio.h>
int write_x(const char *input_path, const char *output_path, int x){
input_path = "in.txt";
output_path = "out.txt";
FILE *in
FILE *out
char text[100];
for(int i = 0; i < 100; i++){
fgets(text[i], 80, in);
if(i % x == 0) {
fprintf(out, "%s\n", text[i]);
}
}
return 0;
}

Answer 1

I didn't compile it (I'll leave that to you), but how about something like this?
Note that if your teacher penalizes for the use of 'goto', then you'll need to modify the error handling.
#include <stdio.h>
#include <stdlib.h>
int write_x(const char *input_path, const char *output_path, int x) {
int result = 0 ;
FILE * inputFile=NULL;
FILE * outputFile=NULL;
/* Open the input file for reading */
inputFile = fopen( input_path, "r" ) ;
if ( inputFile == NULL ) {
perror( "fopen(input_path)" ) ;
result = -1;
goto clean_up;
}
/* Open the output file for writing */
outputFile = fopen( output_path, "w") ;
if ( outputFile == NULL ) {
perror( "fopen(output_path)" ) ;
result = -1;
goto clean_up;
}
int lineNumber = 0 ;
char * lineBuffer = NULL ; // memory will be allocated by getline()... but you'll need to free it up afterwards
int lenBuffer ; // getline will set how bug the buffer is
/* Repeat for each line read */
while (getline( &lineBuffer, &lenBuffer, inputFile) != -1) {
/* Only at each 'x' line count */
if ( (++lineNumber % x ) == 0 ) {
/* Write the line to the output file */
if ( fwrite( lineBuffer, lenBuffer, 1, outputFile ) != lenBuffer ) {
perror("fwrite");
result = -1;
goto clean_up;
}
}
}
clean_up:
/* avoid memory leak */
if ( lineBuffer != NULL ) {
free( lineBuffer ) ;
}
/* Close the input file */
if ( inputFile != NULL ) {
fclose( inputFile ) ;
}
/* Close the output file */
if ( outputFile != NULL ) {
fclose( outputFile ) ;
}
/* All done */
return result ;
}

Answer 2

It seems like a really bad design to be passing paths to this function as arguments instead of FILE *'s, but you're under no obligation to read the file line by line. Just read one character at a time. Something like:
#include <stdlib.h>
#include <stdio.h>
FILE * xfopen(const char *path, const char *mode);
int
write_x(const char *input_path, const char *output_path, int x)
{
FILE *in = xfopen(input_path, "r");
FILE *out = xfopen(output_path, "w");
int c;
int count = 1;
while( (c = fgetc(in)) != EOF ){
if( count == x ){
fputc(c, out);
}
if( c == '\n' ){
count = ( count % x ) + 1;
}
}
return 0;
}
int
main(int argc, char **argv)
{
int step = argc > 1 ? strtol(argv[1], NULL, 10) : 1;
char *in = argc > 2 ? argv[2] : "in.txt";
char *out = argc > 3 ? argv[3] : "out.txt";
write_x(in, out, step);
return 0;
}
FILE *
xfopen(const char *path, const char *mode)
{
FILE *fp = path[0] != '-' || path[1] != '\0' ? fopen(path, mode) :
*mode == 'r' ? stdin : stdout;
if( fp == NULL ){
perror(path);
exit(EXIT_FAILURE);
}
return fp;
}