Fwrite wrong answer

Question

in my project I've write a.bin file which has sensor data in the form of unsigned short(16 bit LE values). but I've get wrong value. Can you guys suggest what is going wrong here?

#include<stdio.h>
int main()
{
    FILE *fp = fopen("a.bin","ab");
    unsigned short us;
    us=123;
    fwrite(&us,2,1,fp);
    printf("%04X",us); // 0x:007B
    us=1234;
    fwrite(&us,2,1,fp);
    printf("%04X",us); // 0x04D2
    us=-3145;
    fwrite(&us,2,1,fp);
    printf("%04X",us); // 0xF3B7
    fclose(fp);
}

After I do the fwrite()

a.bin
7b 00 d0 bc 04 d0 b8 d0 b7

But I want

a.bin
7b 00 04 d2 f3 b7

Answer 1

After fixing your errors, the code works for me just fine.

Note that opening the file in mode "a" means "append". I changed it to "w", which will overwrite the existing file:

fwrite.c:

#include<stdio.h>
int main(void)
{
    FILE *fp = fopen("a.bin","wb");
    unsigned short us;

    us = 123;
    fwrite(&us, sizeof(us), 1, fp);
    printf("%04hX\n",us); // 007B

    us = 1234;
    fwrite(&us, sizeof(us), 1, fp);
    printf("%04hX\n",us); // 04D2

    us = -3145;
    fwrite(&us, sizeof(us), 1, fp);
    printf("%04hX\n",us); // F3B7

    fclose(fp);        

    return 0;
}

Result:

$ gcc -Wall -Werror fwrite.c 
$ ./a.out 
007B
04D2
F3B7
$ hexdump -Cv a.bin 
00000000  7b 00 d2 04 b7 f3                                 |{.....|
00000006