How to write a file to a buffer memory in C - c

Question

So i got my function here that works to write back any file
int write_file(FILE *f_write) {
// Temp variables
FILE *img = fopen("test.pdf", "wb");
unsigned char buffer[255];
while ( (bytes_read = fread(buffer, 1, sizeof(buffer), f_write) ) > 0) {
fwrite(buffer, 1, bytes_read, img);
}
fclose(img);
return 1;
}
So this works perfecly ive tried with pnj / pdf / jpg etc..
But now i want to stock what ive writen in the memory so i can use it later and not write right away
like an array of uint8_t (maybe) that will contain all the bytes ive writen and that i can send later with sockets to my server and store the file
no idea how to do it
Or maybe i'm making it too complicated and i can just
send(client_socket, FILE, sizeof(FILE), 0); ?

Answer 1

One way to do it would be to create a buffer that exactly fits the size of the file.
In order to do so, you can write a function to get the size of an openned file like so:
size_t get_file_size(FILE *f)
{
size_t pos = ftell(f); // store the cursor position
size_t size;
// go to the end of the file and get the cursor position
fseek(f, 0L, SEEK_END);
size = ftell(f);
// go back to the old position
fseek(f, pos, SEEK_SET);
return size;
}
Then create and fill your buffer:
FILE *f = fopen("your_file", "r");
size_t size = get_file_size(f);
char *buffer = malloc(size);
if (fread(buffer, 1, size, f) != size) { // bytes read != computed file size
// error handling
}
// use your buffer...
// don't forget to free and fclose
free(buffer);
fclose(f);
It is worth mentioning that you should check if the file was opened correctly, and to check if you have enough memory to store the buffer (the one created with malloc).
Edit:
As Andrew Henle said, fseek()/ftell() to get the size of a file is non-portable. Instead, to get the size of your file, you should use one of these techniques depending on your OS (assuming you are trying to open a 'normal' file):
On Linux / MacOS:
#include <sys/stat.h>
struct stat st;
size_t size;
if (stat("your_file", &st) != 0) {
// error handling...
}
size = st.st_size;
On Windows (as answered here) :
__int64 FileSize(const wchar_t* name)
{
HANDLE hFile = CreateFile(name, GENERIC_READ,
FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL, NULL);
if (hFile == INVALID_HANDLE_VALUE)
return -1; // error condition, could call GetLastError to find out more
LARGE_INTEGER size;
if (!GetFileSizeEx(hFile, &size)) {
CloseHandle(hFile);
return -1; // error condition, could call GetLastError to find out more
}
CloseHandle(hFile);
return size.QuadPart;
}