[Documentation]read(buffer, len)

[sterretje]

https://www.arduino.cc/en/Reference/FileRead states that the read method returns a character or -1. That however only applies to ::read(). read(buffer, len) returns the number of bytes read or -1 on error.

From SDFile.cpp

//------------------------------------------------------------------------------
/**
 * Read data from a file starting at the current position.
 *
 * \param[out] buf Pointer to the location that will receive the data.
 *
 * \param[in] nbyte Maximum number of bytes to read.
 *
 * \return For success read() returns the number of bytes read.
 * A value less than \a nbyte, including zero, will be returned
 * if end of file is reached.
 * If an error occurs, read() returns -1.  Possible errors include
 * read() called before a file has been opened, corrupt file system
 * or an I/O error occurred.
 */
int16_t SdFile::read(void* buf, uint16_t nbyte) {
  uint8_t* dst = reinterpret_cast<uint8_t*>(buf);