CODEWITHSUNDEEP
cstring
Billy
Billy

Reputation: 1293

fwrite, but to string instead of file in C

I'm writing a C program to get a JPEG frame from a camera and do various stuff with it. I have the following function:

static void process_frame(const void *p, int size) {
        fwrite(p, size, 1, stdout);
}

It works great for writing the frame to stdout, but what I really need is something like this ("swrite" being like fwrite, but to a char instead of a file):

static char* process_frame(const void *p, int size) {
        char* jpegframe;
        swrite(p, size, 1, jpegframe);
        return jpegframe;
}

Unfortunately, there is no swrite function (at least not that I could find). So...

Are there any functions similiar to an "swrite" function?

or

How can I use the process_frame function with printf (and therefore sprintf) instead of fwrite?

Upvotes: 4

Views: 1574

Answers (2)

Chris McGrath
Chris McGrath

Reputation: 1936

Under the hood, it seems your pointer p is just a big array of bytes. You can cast it to a char array and use it:

char *jpegframe = (char *) p;

If you'd like to modify the frame, you might want to allocate some more memory and copy it in:

static char* process_frame(const void *p, int size) {
    char* jpegframe = malloc(size);
    memcpy(jpegframe, p, size);

    /* Do the processing... */

    return jpegframe;
}

Upvotes: 6

Marcus Müller
Marcus Müller

Reputation: 36462

a char is just a single character. What you mean is a char buffer; and lo! your p is already one. Just use (char*)p (cast your void pointer to a char pointer), or memcpy the things that p points to to the char buffer of your choice.

Deamentiaemundi raises a good point: you probably pass p as const void* for a reason; if you plan to modify the data, you should memcpy the contents, definitely.

Upvotes: 8

Related Questions