I'm writing a wrapper to help me in my future projects (I finished C book), and want to copy a file without using fgetc. Here's my code, it doesn't work:
int CopyFile(char* filename, char* dest)
{
FILE* fp, *fout;
fp = fopen(filename,"rb");
//fout = fopen(dest, "wb");
if(fp == NULL)
{
return -1;
}
/*while((c = fgetc(fp)) != EOF)
{
fputc(c,fout);
}*/
long size = GetFileSize(fp);
printf("%lu", size);
char* file = malloc(size);
fread(file, size, 1, fp);
//fclose(fp);
fout = fopen(dest, "wb");
fwrite(file, size, 1, fout);
fclose(fp);
fclose(fout);
return 0;
}
I even open the files with a hexeditor and they aren't similar. What am I doing wrong?
The problem is in the GetFileSize function, where you move the file-pointer to the end, but you never rewind to the beginning again.
That means your fread function call will not actually read anything, as the pointer already is at the end of the file. So what's written is the contents of the allocated memory, which is indeterminate (and will be seemingly random).
If you checked what fread returned, you would have seen this immediately.
Let this be a lesson on always checking the return values of functions which can fail in one way or the other.
Related
Okay, so I have tried to read a whole file with fread(), and I can do it successfully, but the longer the file, the more the excess characters I get on the output.
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main() {
FILE* fpointer = fopen("test.txt", "r");
char* wholeFile;
long int fileSize;
if (fpointer == NULL) return 0;
fseek(fpointer, 0, SEEK_END);
fileSize = ftell(fpointer);
rewind(fpointer);
printf("fileSize == %ld\n", fileSize);
wholeFile = (char*)malloc(fileSize+1, sizeof(char));
if (wholeFile == NULL) return 1;
fread(wholeFile, sizeof(char), fileSize, fpointer);
fclose(fpointer);
wholeFile[fileSize] = '\0';
printf("This is whole file:\n\n%s", wholeFile);
free(wholeFile);
return 0;
}
If the file looks like this:
This is cool file.
I get this as output:
This is cool file.²²²²
And if the file is like this:
This
is
cool
file.
I get this as the output:
This
is
cool
file.═══²²²²
Any idea where I'm wrong?
EDIT: Edited code according to comments.
You need to allocate one more than the size of the file and set the last position in the buffer to 0.
C expects character arrays to be null terminated.
Use "rb" to open the file in binary mode. This will ensure you get a reliable count of bytes in the file from Windows.
FILE* fpointer = fopen("test.txt", "rb");
wholeFile = (char*)malloc(fileSize + 1);
wholeFile[fileSize] = '\0';
FILE* inFile = fopen(inF, "rb");
if (inFile == NULL) {
printf("Invalid input!\n");
exit(EXIT_FAILURE);
}
char* bigBuffer;
char* nextChar = (char*) malloc(sizeof(char));
unsigned long i = 0;
unsigned long j;
while ((j = fread(nextChar, sizeof(char), 1, inFile)) == 1) {
i += j;
}
bigBuffer = malloc(i * sizeof(char));
fread(bigBuffer, sizeof(char), i, inFile);
fclose(inFile);
printf("%s\n", outF);
FILE* outFile = fopen(outF, "wb");
//if (outFile == NULL)
//printf("null\n");
j = fwrite(bigBuffer, sizeof(char), i, outFile);
printf("%lu\n", j);
fclose(outFile);
free (bigBuffer);
free (nextChar);
I'm trying to write a binary file using fopen in wb mode. After running my program, a file of the proper name is made in the proper place, but I just can't open it or read it. When I try to open it, a message pops up saying "Can't open...." In addition, the name of the file itself isn't formatted properly in Finder (I'm on a Mac). The name is elevated and cut off a little. It definitely looks like something is wrong with the file. I tried just making a regular file using fopen in w mode, and that worked beautifully. So I'm pretty sure I'm just doing something wrong when it comes to writing binary files using wb mode. Can anyone help? Thanks.
The main problem:
you didn't seek to the beginning of the file before reading, so your call to fread to read the entire file will fail
Change:
bigBuffer = malloc(i * sizeof(char));
fread(bigBuffer, sizeof(char), i, inFile);
to:
bigBuffer = malloc(i); // allocate buffer
rewind(inFile); // reset file pointer to start of file
fread(bigBuffer, 1, i, inFile); // read entire file
Additional notes:
sizeof(char) is 1 by definition, and therefore redundant
you should not cast the result of malloc in C
you should add error checking to any call that might fail, especially I/O calls
malloc-ing a single char is inefficient - just use a local variable
reading a file one char at a time to determine its length is very inefficient
The file with the opening in wb truncated to 0 length. Use for simultaneous read / write mode or addition mode rb +, a.
I'm trying to make a program that uses fgets to take the text from a preexisting file, invert it and then write it to another file. This is the code I've written so far:
#include <stdio.h>
#include <string.h>
int main()
{
int c, d;
FILE *file1, *file2;
char string [100], *begin, *end, temp;
file1 = fopen("StartingFile.txt", "rt");
if (file1 == NULL)
{
printf ("Error - Couldn't open file\n");
return (-1);
}
fgets(string, 100, file1);
fclose (file1);
begin = string;
end = begin + strlen(string) - 1;
while (end > begin)
{
temp = *begin;
*begin = *end;
*end = temp;
++begin;
--end;
}
file2 = fopen("FinalFile.txt", "wt");
fprintf (file2, "%s", string);
fclose (file2);
printf ("%s\n", string);
return 0;
}
It works fine if the text in the preexisting file is all in one line, but if it has more than one line, only the first one is inverted and written to the new file. I think that fgets can only read one line, so I think I'll have to use a loop, but I'm having trouble implementing it. Can someone give me a hand? Thanks in advance!
To read each line separately from file use fgets in while loop as below,
while(fgets(string, sizeof(string), file1) != NULL)
{
...
}
fclose(file1);
Inside the loop operate on each line to reverse it.
Your code has quite a few logical errors in it. I would recommend using other f* methods instead.
If you want an easy solution, open the file, determine its length, create two buffers of the size of the file, fill the first buffer with the file's contents and then do a loop to copy the reverse to the other buffer, then write that buffer back. Roughly that would look like this:
#include <stdio.h>
#include <string.h>
int main()
{
FILE *file;
file = fopen("StartingFile.txt", "rt");
if (file == NULL)
{
printf ("Error - Couldn't open file\n");
return (-1);
}
fseek(file, 0, SEEK_END); // move file pointer to end of file
long size = ftell(file); // file pointer position == character count in file
fseek(file, 0, SEEK_SET); // move back to beginning of file
char* buffer = malloc(size * sizeof(char));
fread(buffer, sizeof(char), size, file) // read file contents to buffer
for(long i = 0; i < size/2; ++i)
{
buffer[i] = buffer[size-i-1];
}
fseek(file, 0, SEEK_SET); // The fread set the file pointer to the end so we need to put it to the front again.
fwrite(buffer, sizeof(char), size, file); // Write reverted content
delete buffer;
fclose (file);
}
I haven't tested it and it may contain a few errors since I haven't programmed in C for some time. The only reason to still be programming in C anyways is efficiency and if you want your program to be efficient, the two buffer solution isn't the best either. At least not in terms of memory usage.
I highly recommend getting familiar with all the functions available in C (stdio and so on) cplusplus.com is a great reference for that.
Regards, Xaser
I'm using the get line function in C to read through the lines of a file. I want to loop over the function so that I can read through the file n number of times. For some reason though, it only reads through it once (I think there's a pointer somewhere still pointing to the last line) at the beginning of the subsequent loops. How do I get it to reset?
To make things clearer, if there were 100 lines in the file, below, the largest val would get would be 100 even though it should get up to 300.
Thanks!
FILE* fp = myfopen (inf, "r");
char* line = NULL;
size_t len = 0;
int num=3
int val=0
for (i=0;i<num;i++)
{
while (getline (&line, &len, fp) != -1)
{
val++;
}
}
Once you read past the end of the file, you have to call
rewind(fp);
to start at the beginning of the stream again.
As mentioned you can use the rewind() function to start over again, the other option is of course to move the open/close of the file into your loop:
FILE* fp; = myfopen (inf, "r");
for (i=0;i<num;i++)
{
fp = myfopen(inf, "r");
while (getline (&line, &len, fp) != -1)
{
val++;
}
fclose(fp);
}
I am try to write a buffer so I can remove a lot of null "00" characters in a file. The characters are useless and are completely random. They are wreaking havoc on the searcher in the program. The code below compiles but just seems to hang when a file is passed to it. Any suggestions will be helpful.
void ReadFile(char *name)
{
FILE *dbg;
char *buffer;
unsigned long fileLen;
//Open file
dbg = fopen(dbg, "w+");
if (!dbg)
{
fprintf(stderr, "Unable to open file %s", name);
return;
}
//Get file length
fseek(dbg, 0, SEEK_END);
fileLen = ftell(dbg);
fseek(dbg, 0, SEEK_SET);
//Allocate memory
buffer = (char *)malloc(fileLen+1);
if (!buffer)
{
fprintf(stderr, "Memory error!");
fclose(dbg);
return;
}
//Read file contents into buffer
fread(buffer, fileLen, 1, dbg);
for(i = fileLen-1; i >= 0 && buffer[i] == 0; i--);
i++;
if (i > 0)
{
fwrite(buffer, 1, i, dbg);
}
fclose(dbg);
//Do what ever with buffer
free(buffer);
}
Change
dbg = fopen(dbg, "w+");
to
dbg = fopen(name, "w+");
Also, if you want to read the file, change it then write it, you shouldn't open it with "w+". You should first open the file with "r", read from it, do whatever change you want, then fclose it, then again open it but this time with "w" so that you write over it. After you have opened it in "w"rite mode, you can write the modified buffer back into the file.
You opened a file for writing and then you try to read from it.
Check the return value of fread and all the other calls.