Reading and writing pdf or binary data in C - c

Question

I'm implementing ftp and I want to upload and download files, when I download or upload pdf files they are corrupted. How can handle reading any file, using read() and write() or mmap? below is my simplified code of what I have tried.
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
int is_regular_file(const char *path)
{
struct stat path_stat;
stat(path, &path_stat);
return (S_ISREG(path_stat.st_mode));
}
int ft_get_file_size(const char *filename)
{
struct stat file;
int fd;
if (!is_regular_file(filename))
return (-1);
fd = open(filename, O_RDONLY);
memset(&file, 0, sizeof(struct stat));
fstat(fd, &file);
close(fd);
return (file.st_size);
}
char *read_file(const char *filename)
{
char *content;
int file_size;
int fd;
ssize_t retval;
if ((file_size = ft_get_file_size(filename)) <= 0)
return (NULL);
content = (char *)malloc(sizeof(char) * file_size + 1);
fd = open(filename, O_RDONLY);
retval = read(fd, content, file_size);
content[retval + 1] = '\0';
close(fd);
return (content);
}
void write_file(char *file, char *content)
{
int fd;
fd = open(file, O_CREAT | O_TRUNC | O_WRONLY, S_IRUSR | S_IWUSR);
if (fd)
write(fd, content, strlen(content));
close(fd);
}
int main() {
char *test = read_file("ftp.en.pdf");
write_file("copy.pdf", test);
return EXIT_SUCCESS;
}
The process of downloading and uploading the file, is reading all the data from the file and then send that data to the socket. I have tried using mmap and I still get corrupted file.
Document is damaged error message

Answer 1

As binary data can have \0 characters, you cannot treat your content as a string, so strlen(content) is wrong. You must return the size of the content from your read_file function.
For example, define your function as char *read_file(const char *filename, int *size) and return the size in *size. Likewise define your write function as void write_file(char *file, char *content, int size)
(and forget the +1 in malloc)