How send a binary file in segments over network in C?

Question

I am a bit puzzled over how to send a file from an http server to a client(web-browser).

First I send the header and my next task is to send the file content. However I want to send it in segments of say 512 bytes instead of the whole file at once as I ran into some problems.

I am a bit lost on how to achieve that. Here is what I want

read file1;
while (seg=get_next_segment(file1)){
  do
    send(seg)
  until (seg_is_sent)
}

However I can't seem to find the appropriate functions to achieve that. fread and fseek crossed through my mind but the first one reads the whole file at once and with fseek I don't see a way to just grab a portion from a file(instead of reading from the file pointer until the end of the file).

If fread reads the whole file at once, why does it have parameters telling it how much to read? And fseek can't be used for reading at all. — Some programmer dude
– Some programmer dude, Commented Nov 17, 2011 at 15:48
can I ask why do you want to send it in segments? Also, sendfile is a lot more efficient. — Karoly Horvath
– Karoly Horvath, Commented Nov 17, 2011 at 15:58
@yi_H the reason is that I sent it before as a whole and it didn't arrive as a whole. I was getting a message in the browser like "Could not read file" — Pithikos
– Pithikos, Commented Nov 17, 2011 at 16:42
@Joachim I think only people that already know how the functions work are able to read and fully understand some manpages. — Pithikos
– Pithikos, Commented Nov 17, 2011 at 16:47

Gian · Accepted Answer · 2011-11-17 15:46:36Z

3

freaddoes not read a whole file to the end. It reads exactly how much you tell it to read.

size_t
 fread(void *restrict ptr, size_t size, size_t nitems,
     FILE *restrict stream);

 size_t
 fwrite(const void *restrict ptr, size_t size, size_t nitems,
     FILE *restrict stream);

Both functions take a size and nitems. You could set the size to 512, and the number of items to 1 (or vice versa) and read just that portion of the file.

answered Nov 17, 2011 at 15:46

Gian

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2 Comments

Pithikos Over a year ago

Now I see that. However I am curious why the function is implemented as that instead of just giving right away the number of bytes to read.

Gian Over a year ago

A very common usage pattern is fread(foo, sizeof(some_complex_datastructure_t), x, fp);. I'm not sure there's a sound technical reason, except that you're then talking about discrete elements rather than bytes.

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

From the man page:

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

The function fread() reads nmemb elements of data, each size bytes long, from the stream pointed to by stream, storing them at the location given by ptr.

As you can see, you don't have to read the whole file with fread, you can read it in nmemb size chunks.

If you are writing your own web-server in C, you may want to have a look at libmicrohttpd, which is a web-server in C for embedding into other applications, it can handle streaming a file for you.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Nov 17, 2011 at 15:49

Silas Parker

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Comments

Some programmer dude · Accepted Answer · 2011-11-17 15:55:21Z

1

I'm feeling kind... How about something like this:

for (;;)
{
    char buffer[512];
    size_t nread = fread(buffer, sizeof(char), 512, fp);
    if (nread > 0)
        send_to_socket(buffer, nread);

    if (nread < 512)
    {
        if (feof(fp))
            printf("End of file\n");
        if (ferror(fp))
            printf("Error reading\n");
        break;
    }
}

answered Nov 17, 2011 at 15:55

Some programmer dude

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