how to pass a char variable to FS_write() function -- EmFile? - c

Question

I`m trying to use EmFile library for embedded programing based on C.
this is the structure of FS_write in API documentation :
U32 FS_Write (FS_FILE * pFile,
const void * pData,
U32 NumBytes);
I want to write my own function for my later use like:
void SD_write_to_file(char buff)
{
if(pFile)
{
if(0 != FS_Write(pFile, buff, strlen(buff)))
{
}
else
{
FontWrite_Position(100,148);//Text written to the position
PutStringLCD("Failed to write data to file");
}
}
}
the problem is that when I call this function and pass my char value, it can`t use it for FS_write.
for example:
sprintf(tempwordshow, "%f", realSamples[realsamplescounter]);
SD_write_to_file(tempwordshow);
realSample[ ] is float and tempwordshow is char.
Note that if use the function like :
if(0 != FS_Write(pFile, "0123456789", 10u))
it is working.
I think the problem is from the way of passing my data to this function .
any ideas?
Thanks

Answer 1

In general, make sure functions which take a pointer as argument do indeed receiver pointers. And conversely if you intend to use a pointer, you should make sure the function's signature takes a pointer as argument.
In this particular case your intention seems to be to write a buffer of characters (which can be passed to a function by passing a pointer to the memory location corresponding to the buffer), which would in all likelyhoods be more than a single char. Correspondingly, the signature of the SD_write_to_file should be changed to:
void SD_write_to_file(const char* buff)
You may have a similar problem with sprintf's first argument is expected to be of type char* whereas you mention that tempwordshow is of type char (note also that there should be memory allocated wherever tempwordshow points to). If it wasn't a trivial typo in your question, then you would have to convert tempwordshow's declaration to something such as:
char tempwordshow[40]; // a 40-char buffer should be large enough for %f
As a final note, strlen is designed to handle null-terminated character sequences (or strings). So, if you intend to use this function strictly for null-terminated strings, then this is fine. Otherwise, you may find it more appropriate to pass along the length of the buffer that you wish to write (something like SD_write_to_file(const char* buff, int bufferLength)).

Answer 2

SleuthEye was right. but after those modification I still couln`t write the correct float number in the file. some crap were written in the text file.
So I wrote my own function for converting float to string and the problem solved.
you need to do all the modification that SleuthEye mentioned above. additionally, use this function for changing type:
/** Number on countu**/
int n_tu(int number, int count)
{
int result=1;
while(count-- > 0)
result *=number;
return result;
}
/Convert float to string/
void float_to_string(float f, char r[])
{
long long int length, length2, i, number, position, sign;
float number2;
sign=-1; // -1 == positive number
if (f <0)
{
sign='-';
f *= -1;
}
number2=f;
number=f;
length=0; // size of decimal part
length2=0; // size of tenth
/* calculate length2 tenth part*/
while( (number2 - (float)number) != 0.0 && !((number2 - (float)number) < 0.0) )
{
number2= f * (n_tu(10.0,length2+1));
number=number2;
length2++;
}
/* calculate length decimal part*/
for(length=(f> 1) ? 0 : 1; f > 1; length++)
f /= 10;
position=length;
length=length+1+length2;
number=number2;
if(sign=='-')
{
length++;
position++;
}
for(i=length; i >= 0 ; i--)
{
if(i== (length))
r[i]='\0';
else if(i==(position))
r[i]='.';
else if(sign=='-' && i==0)
r[i]='-';
else
{
r[i]= (number % 10)+'0';
number /=10;
}
}
}
Thanks again . hope that it helps the others in future. :)