Writing image into file- Wrong output

Question

I am trying to put an image into a file that has been retrieved from a server. I am using the fwrite function, but it doesn't really work the way I want it to. It looks like the biggest problem is that it can't write the \ character. Or maybe not. I don't know what to do with it. Does anyone know what am I doing wrong? Thanks in advance.

Here is my fwrite code:

FILE * pFile;
if((pFile = fopen ("test", "wb")) == NULL) error(1);
fwrite (buffer.c_str() , 1, buffer.size(), pFile);

where buffer contains data retrieved from server. When it contains pure html, it works just fine.

Here is my output:

GIF89a¥ÈÔ

Here is what it was supposed to write:

GIF89a\A5\C8\00

Answer 1

fwrite() won't automatically do the conversion you want. You should implement some code to convert what you want to convert to "\ character".

example:

#include <cstdio>
#include <string>

void error(int no) {
    printf("error: %d\n", no);
    exit(1);
}

int main(void) {
    char data[] = "GIF89a\xA5\xC8"; // '\0' is automatially added after string literal
    std::string buffer(data, sizeof(data) / sizeof(*data));

    FILE * pFile;
    // use text mode because it seems you want to print text
    if((pFile = fopen ("test", "w")) == NULL) error(1);

    for (size_t i = 0; i < buffer.size(); i++) {
        if (0x20 <= buffer[i] && buffer[i] <= 0x7E && buffer[i] != '\\') {
            // the data is printable ASCII character except for \
            putc(buffer[i], pFile);
        } else {
            // print "\ character"
            fprintf(pFile, "\\%02X", (unsigned char)buffer[i]);
        }
    }

    fclose(pFile);
    return 0;
}