include the mbed library with this snippet

#include "mbed.h"

LocalFileSystem local("local");               // Create the local filesystem under the name "local"

DigitalOut led1(LED1);
DigitalOut led2(LED2);

Serial pc(USBTX, USBRX);    // Serial console

#define BUFFER_SIZE 5000    // Buffer to store the data to be written on file

int main() 
{
    // Set the serial baud rate
    pc.baud(921600); 
    
    // Initialize the source buffer
    uint8_t buff[BUFFER_SIZE];
    for(int i = 0; i < BUFFER_SIZE; i++)
        buff[i] = 'a' + i%10;   // abcdefghij
       
    // Create the file
    FILE *pFile = NULL;
    pFile = fopen("/local/a.txt","w");
    
    // Initialize the timer related variables
    Timer timer;
    int nTime_us = 0;
    int nDT_us[BUFFER_SIZE];
    memset((void* )nDT_us, 0, BUFFER_SIZE*sizeof(int));
    
    // Initialize the writing parameters
    int nBlockSize = 1;         // Size of one data block of data to write
    int nBytesToWrite = 513;    // Total number of bytes to write to the file

    int nBytesWritten = 0;      // Number of bytes written in a single write operation
    int nTotalBytesWritten = 0; // Total number of bytes written
    int nNbBlocksToWrite = 1;   // Number of data blocks to write in a single write operation
    int nBlocksWritten = 0;     // Number of data blocks written in a single write operation
    int nNbWriteOp = 0;         // Number of write operations
    
    pc.printf(
            "\r\n\nNB bytes to write: %d (Block size: %d)\r\n", 
            nBytesToWrite,
            nBlockSize
            );

    // Start the data writing
    timer.start();
    while(nBytesToWrite > 0)
        {
        nTime_us = timer.read_us();
        // Write the data block
        nBlocksWritten = fwrite(
                                buff,               // const void *ptr
                                nBlockSize,         // size_t size
                                nNbBlocksToWrite,   // size_t nmemb
                                pFile               // FILE *stream
                                );
        // Store the time required for the write operation
        if(nNbWriteOp < BUFFER_SIZE)
            nDT_us[nNbWriteOp++] = timer.read_us() - nTime_us;
        
        // Update the remaining number of bytes to write
        if(nBlocksWritten > 0)  // Write success
            {
            nBytesWritten = nBlocksWritten * nBlockSize;
            nBytesToWrite -= nBytesWritten;
            nTotalBytesWritten += nBytesWritten;
            led1 = !led1;
            }
        else
            {
            led2 = !led2;       // Write error    
            }
        }
    
    // Display the time required for each write operation
    pc.printf(
                "\r\n%d bytes written\r\n", 
                nTotalBytesWritten
                );
    pc.printf("\r\nWrite time:\r\n");
    for(int i = 0; i < nNbWriteOp; i++)
        {
        if(nDT_us[i] > 10)
            pc.printf("#%d:", i); // Mark the slower write operations
        pc.printf(
                    "%d ", 
                    nDT_us[i]
                    );
        }
    pc.printf("\r\n");
    
    // Close the file
    fclose(pFile);
        
}