FileSystem - fwrite() with large buffer input (over 1024) fails

[Alonof]

when fwrite() function received a buffer size larger than 1024 then the write is failing

• Type: Bug
• Priority: Major

---

Bug

Target
K64F
Toolchain:
ARM

Toolchain version:
5.06 (did not test with ARM v6.0)

mbed-cli version:
1.2.0

mbed-os sha:
4c256f0 (HEAD, tag: mbed_lib_rev148, tag: mbed-os-5.5.4, tag: latest) Merge pull request #4824 from ARMmbed/release-candidate

Steps to reproduce

FILE* fd = fopen("/fs/big.txt", "w+");
size_t write_count = 0;
char buff_big[1025] = {0};
write_count = fwrite(buff_big, 1, 1025, fd);
printf("number of written bytes %d\n\r",write_count);   <----- write_count is 0 insted of 1025
fclose(fd);

example code

1. deploy https://github.com/ARMmbed/mbed-os-example-fat-filesystem
2. follow the instruction for use with an SD card
3. replace main.c with the following code and compile to ARMCC

#include "mbed.h"
#include "FATFileSystem.h"
#include "SDBlockDevice.h"
#include <stdio.h>
#include <errno.h>

#define BIG_BUFF_SIZE 1025
#define SMALL_BUFF_SIZE 1024

SDBlockDevice bd(MBED_CONF_SD_SPI_MOSI, MBED_CONF_SD_SPI_MISO, MBED_CONF_SD_SPI_CLK, MBED_CONF_SD_SPI_CS);
FATFileSystem fs("fs");

void return_error(int ret_val){
	if (ret_val)
		printf("Failure. %d\r\n", ret_val);
	else
		printf("done.\r\n");
}

void errno_error(void* ret_val){
	if (ret_val == NULL)
		printf(" Failure. %d \r\n", errno);
	else
		printf(" done.\r\n");
}

int main()
{
	int error = 0;
	printf("Welcome to the filesystem example.\r\n"
			"Formatting a FAT, RAM-backed filesystem. ");
	error = FATFileSystem::format(&bd);
	return_error(error);

	printf("Mounting the filesystem on \"/fs\". ");
	error = fs.mount(&bd);
	return_error(error);

	printf("Big Buffer Test.\r\n");

	FILE* fd = fopen("/fs/big.txt", "w+");
	size_t write_count = 0;
	char buff_big[BIG_BUFF_SIZE] = {0};\
	char buff_small[SMALL_BUFF_SIZE] = {0};
	write_count = fwrite(buff_big, 1, BIG_BUFF_SIZE, fd);
	printf("number of written bytes %d\n\r",write_count);
	fclose(fd);

	printf("Small Buffer Test.\r\n");
	fd = fopen("/fs/small.txt", "w+");
	write_count = 0;
	write_count = fwrite(buff_small, 1, SMALL_BUFF_SIZE, fd);
	printf("number of written bytes %d\n\r",write_count);
	fclose(fd);
	fflush(stdout);

	printf("End Big Buffer Test.\r\n");
	while (true) {}
}

[0xc0170]

cc @geky