My application runs on an ESP32-CAM. It takes pictures and stores to SPIFFS. Periodically SPIFFS is checked and if images are found: 1- The image is saved to a FTP server 2- The image is summited to a Web Service that returns some informations about the image 3- Values are saved to mySQL 4- The image is deleted from SPIFFS

To read the image from SPIFFS I use the following fragment of code:

String imageName   = file.name();  
imageBuffer = "";
while (file.available()){
   imageBuffer += char(file.read());

The FTP transfer works perfectly with the following command:


The service the image is submited for analizes works OK if I use the command:


But as it is not usable in the current implementation of my project I am trying to work with:

webClient.write((uint8_t*) imageBuffer.c_str(), imageBuffer.length());

I get no errors in return but it just does not work and the web service at the destination returns nothing.

What Am I doing wrong?

  • 1
    What you're doing is treating binary data as text. As soon as you get a 0 byte the string will be truncated.
    – Majenko
    Apr 7, 2021 at 12:48
  • Your webClient.write() looks fine, but how are you framing this raw data? See How does HTTP file upload work?. Apr 7, 2021 at 12:50
  • Short knowledge on how to handle this sort of data. "framing raw data"? What does it mean? Apr 7, 2021 at 13:03
  • @Majenko: A String object does not truncate on the first NUL: it keeps all the bytes you push with +=, and the length() method returns the total number of bytes, including embedded NULs. Apr 7, 2021 at 13:04
  • 1
    @EdgarBonet I'd rather not, thanks. I stay well away from String and anything to do with it. Never use String for anything. Not only is it generally a bad idea, you just can't trust what it's doing. And regardless: storing raw binary data in a string is asking for trouble.
    – Majenko
    Apr 7, 2021 at 13:16

2 Answers 2


You should not use String to store binary data.

Instead allocate yourself an array of uint8_t of the right size.

uint8_t *imageData;
size_t len = file.size();

imageData = alloca(len);
file.readBytes(imageData, len);

webClient.write(imageData, len);

With that said, without knowing what the rest of your program is doing I can only surmise that you're just blasting raw data at the web server and hoping it sticks. That's not how web data transfers work. You have to craft the upload page properly as multi-part mime form data. Read this for more information

  • Hi, thanks, on one side I am submitting it to the FTP library and assume it will handle the flow. The submission to the web service is HTTP so I do not know, but when I use webClient.write(fb->buf,fb->len); it works so I am assuming it is handling the 24K or so bytes of the image. What do you suggest me to do? Thanks Apr 7, 2021 at 16:49

OK, I got it to work as follows:

Not passing the binary anymore, passing the file name so the FTPPost function will do its own reading of the file on SPIFFS and will post in 256 bytes chunks. The same strategy works with HTTP post to the WebService.

  File picFile = SPIFFS.open(fileName,FILE_READ);
  const size_t bufferSize = 256;
  uint8_t buffer[bufferSize];
  while (picFile.available()) {
    size_t n = picFile.readBytes((char*) buffer, bufferSize);
    if (n != 0) {
      ftp.WriteData(buffer, n);
    } else {
      Serial.println(F("Buffer was empty"));

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