Thread: How can I download an Image from the web

If I remember correctly, this question has been answered before on the forum with a complete working example. I suggest doing a forum search to see what you can find.

Originally Posted by rags_to_riches

Binary data == array of bytes. You should be able to issue a HTTP HEAD command for the image URL to get the size of the image, then allocate memory for the array, then use GET to retrieve the data into the array.

The problem is that the server may be using Chunked encoding. If so, it won't return the size of the returned content. This is why it's usually better to use a library like libcurl than to write your own HTTP client.

Yes, as bithub mentioned, libcurl. Or where you're on Windows there's winhttp.

So I started out just by trying to copy a file that is on my own system. The code works, it can copy an image but my problem when I want to use some block reading function like fread or fwrite. Why wont the image copy? When I use fread/write the file isn't the same size as the original. My method of writing on a character by character basis may be slow, but it works. So can someone tell me why the commented code wont work for an image? It does for text files but not images.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


void error(char *err){
	printf("%s\n",err);
	exit(-1);
}

char *get_copy(char* filename){

	int i = 0;
	char *file = malloc(strlen(filename) + strlen("_copy.ext"));
	while(*filename != '.') file[i++] = *filename++;
	strcat(file,"_copy");
	strcat(file,filename);
	
	return file;
}

int main(int argc, char *argv[]){

	if(argc != 2){
		char err_msg[32];
		snprintf(err_msg,sizeof(err_msg),"Usage: %s <file_to_copy>",argv[0]);
		error(err_msg);
	}
		
	
	FILE *ifile, *ofile;
	char *out_file, c;
	int file_size = 0, nread = 0;
	
	if(!(ifile = fopen(argv[1], "r")))
		error("Error opening read file");
	
	out_file = get_copy(argv[1]);
	
	fseek(ifile,0,SEEK_END);

	file_size = ftell(ifile);
	
	fseek(ifile,0,SEEK_SET);
	 
	if(!(ofile = fopen(out_file,"w")))
		error("Error opening write file");

	printf("Writing %d bytes to %s\n", file_size ,out_file);
	
	while(nread < file_size){
		c = fgetc(ifile);
		fputc(c,ofile);
		nread++;
	}

	printf("%d bytes written to %s\n",nread, out_file);

	fclose(ifile);
	fclose(ofile);
	
	return 0;
}

So can someone tell me why the commented code wont work for an image?

What commented code?

You should be opening the files in binary mode ("rb"/"wb", nor "r"/"w")

This code:

Code:

char *get_copy(char* filename){
        int i = 0;
	char *file = malloc(strlen(filename) + strlen("_copy.ext"));
	while(*filename != '.') file[i++] = *filename++;
	strcat(file,"_copy");
	strcat(file,filename);
	
	return file;
}

can be greatly improved by the following:

Code:

char *get_copy(char* filename){

    // You forgot space for the null terminator
    char *file = malloc(strlen(filename) + strlen("_copy.ext") + 1);
    if (file) {
        sprintf(file, "%s_copy.ext", filename);
    }
    return file;
}

Yay for boobs in the screenshot...cheers!

Hahahaha, I would have used other images but those were the only ones on the computer I used to write the code. And the router is downstairs and this particular machine has no wireless nic nor do I have a really long ethernet cable. I finally had decided to share my laptops internet connection so thats why it shows that I have a connection in that screenshot. Also rags, the reason my get_copy() is the way it is is because I append the file extension to the _copy filename. The extension shouldn't be ".ext". I just wanted to allocate enough space for the "up to three letter" extension. You know, .c, .mp4 or .jpg. Otherwise, the file would be misinterpreted, wouldn't it? I mean, what uses the extension ".ext"?


Example: if I supplied socket.c as argv[1] using my code would produce the copy file as "socket_copy.c". Yours would produce "socket_copy.ext". I intentionally wrote it this way. Do you know of a better way to get the extension or is what I have done fine?

EDIT: disregard that statement about the commented code. I took it out. I think i may be getting undesired results because I forgot to make the buffer unsigned. Could that be part of the problem?