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arduino_universal_serial_ad.../Libraries/SdFat/extra/examplesV1/SdFatRewrite/SdFatRewrite.ino

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/*
* Rewrite Example
*
* This sketch shows how to rewrite part of a line in the middle
* of the file created by the SdFatAppend.pde example.
*
* Check around line 30 of pass 50 of APPEND.TXT after running this sketch.
*/
#include <SdFat.h>
#include <SdFatUtil.h> // use functions to print strings from flash memory
const uint8_t SD_CHIP_SELECT = SS;
SdFat sd;
SdFile file;
// store error strings in flash to save RAM
#define error(s) sd.errorHalt_P(PSTR(s))
void setup(void) {
Serial.begin(9600);
while (!Serial) {} // wait for Leonardo
PgmPrintln("Type any character to start");
while (Serial.read() <= 0) {}
delay(200); // Catch Due reset problem
// initialize the SD card at SPI_HALF_SPEED to avoid bus errors with
// breadboards. use SPI_FULL_SPEED for better performance.
if (!sd.begin(SD_CHIP_SELECT, SPI_HALF_SPEED)) sd.initErrorHalt();
char name[] = "APPEND.TXT";
// open for read and write
if (!file.open(name, O_RDWR)) {
PgmPrint("Can't open ");
Serial.println(name);
PgmPrintln("Run the append example to create the file.");
error("file.open failed");
}
// seek to middle of file
if (!file.seekSet(file.fileSize()/2)) error("file.seekSet failed");
// find end of line
int16_t c;
while ((c = file.read()) > 0 && c != '\n');
if (c < 0) error("file.read failed");
// clear write error flag
file.writeError = false;
// rewrite the begining of the line at the current position
file.write("**rewrite**");
if (file.writeError) error("file.write failed");
if (!file.close()) error("file.close failed");
Serial.print(name);
PgmPrintln(" rewrite done.");
}
void loop(void) {}