Cannot Copy a Text File through fwrite and fread in C [closed] - c

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Here is the Code:
#include <stdio.h>
#include <stdlib.h>
int main() {
FILE *f, *fp;
char buff[512];
int buff_size;
int bytes;
fp = fopen("File.txt", "rb");
if (fp == NULL) {
printf("Cannot Open Source File!\n");
exit(1);
}
f = fopen("append.txt", "ab+");
if (f == NULL) {
printf("Cannot Open Target File!\n");
fclose(fp);
exit(1);
}
buff_size = sizeof(buff);
while (bytes = fread(&buff, buff_size, 1, fp) > 0) {
if (bytes > 0)
fwrite(&buff, buff_size, 1, f);
else
break;
printf("Appending...\n\n");
}
rewind(f);
while (bytes = fread(&buff, buff_size, 1, f) > 0)
if (bytes > 0)
printf("%s", buff);
fclose(fp);
fclose(f);
}
So, it happens to be that this doesn't output anything and when I check the file "append.txt" it also does not contain anything.
Note that the Source File "File.txt" is not empty.
Can anyone tell me what is wrong with it?
EDIT:
I fixed the problem by replacing buff_size with strlen(buff) as this:
bytes = fread(&buff, strlen(buff), 1, f) > 0 and the same in fwrite() and second fread().
Can someone explain why this worked?

Answer 1

char buff[512];
int buff_size;
// [...]
bytes = fread(&buff, buff_size, 1, fp)
This will attempt to read one block of 512 bytes. The return value is the number of blocks read, so it won't be bytes. But leaving this aside, if your file is shorter than 512 bytes, this won't read anything.
What you want is read 512 times 1 byte, then you will get the byte count back, so swap places for buff_size and 1.
Side notes:
if you do your checks in the loop condition correctly like:
while ((bytes = fread(buff, 1, buff_size, fp)) > 0 )
the extra check for if (bytes > 0) is redundant.
when writing, you only want to write the amount of bytes you actually read:
fwrite(buff, 1, bytes, f);
For sizes, always use size_t -- int could very well be wrong:
size_t buff_size;
size_t bytes;
printing your buff with printf("%s", ) is undefined behavior because you don't add a '\0' byte after the data read by fread(). A C string must end with '\0'. When the data read by fread() doesn't contain a '\0' by accident, printf() will read and use uninitialized data and possibly even read beyond the bounds of your buff.

Answer 2

Felix Palmen listed a number of problems in your code, your fix is completely wrong as buff does not even have a null terminator.
Here is a better version:
#include <stdio.h>
#include <stdlib.h>
int main(void) {
FILE *f, *fp;
char buff[512];
size_t bytes;
fp = fopen("File.txt", "rb");
if (fp == NULL) {
fprintf(stderr, "Cannot open source file!\n");
exit(1);
}
f = fopen("append.txt", "ab+");
if (f == NULL) {
fprintf(stderr, "Cannot open target file!\n");
fclose(fp);
exit(1);
}
while ((bytes = fread(buff, 1, sizeof buff, fp)) != 0) {
if (fwrite(buff, 1, bytes, 1, f) != bytes) {
fprintf(stderr, "Error writing to the target file\n");
break;
}
printf("Appending...\n\n");
}
rewind(f);
while ((bytes = fread(buff, 1, sizeof buff, f)) != 0) {
printf("%.*s", (int)bytes, buff);
}
fclose(fp);
fclose(f);
return 0;
}

Answer 3

bytes = fread(&buff, buff_size, 1, fp) > 0
Take a look at the C Precedence Chart. The operator > is above =, so the return value of fread will be compared to zero, and then the result of that comparison will be stored in bytes. You intended to write this
(bytes = fread(&buff, buff_size, 1, fp)) > 0

Answer 4

Here's my solution to your problem, given fixed file names. Were it my own code, it would take the file names as arguments.
#include <stdio.h>
int main(void)
{
const char src_file[] = "File.txt";
FILE *fp = fopen(src_file, "rb");
if (fp == NULL)
{
fprintf(stderr, "Cannot Open Source File '%s'!\n", src_file);
return(1);
}
const char tgt_file[] = "append.txt";
FILE *f = fopen(tgt_file, "ab+");
if (f == NULL)
{
fprintf(stderr, "Cannot Open Target File '%s'!\n", tgt_file);
fclose(fp);
return(1);
}
char buff[512];
int bytes;
while ((bytes = fread(buff, sizeof(char), sizeof(buff), fp)) > 0)
{
fwrite(buff, sizeof(char), bytes, f);
printf("Appending...\n\n");
}
rewind(f);
while ((bytes = fread(buff, sizeof(char), sizeof(buff), f)) > 0)
printf("%.*s", bytes, buff);
fclose(fp);
fclose(f);
return 0;
}
There are a variety of fixes, most of them articulated in comments somewhere along the line.
I made the file names into arrays so that the name could be used in both fopen() and in the error messages, which are printed to stderr, not stdout. This is helpful to other users in generalized code. If the file names come from command line arguments, it is trivial.
The calls to fread() were fixed so that the number of bytes is reported, rather than the number of 512-byte blocks (which will be 0 or 1). This involved reversing the order of the size/count arguments to fread(). The buffer was passed rather than the address of the buffer, too.
The number of bytes read was captured correctly.
The number of bytes read was used to control the size of the fwrite().
The number of bytes read was used to control the number of bytes printed by printf().
I don't like the special rule in C99 and later that allows main() — but only main() — to return 0 by default. AFAIAC, it's a function defined as returning an int; it should return an int. However, there are others who disagree.