Function that gets a chars array separated by commas - c

Question

I have a char array that i get from serial port (Arduino) .
the data is separated by commas so its like this :
header:data
The chars array that holds this structure is pre defined like this : char content[50];
I am trying to write a function , that will get a parameter content
and return only the header , and then another function to return only the data. I know i have to start with this - but dont really know how to continue:
void getHeader( char* localString)
{
char delimiters[] = ":";
char *valPosition=NULL;
char newString=NULL ;
valPosition = strtok(localString, delimiters); //what is this doing ?
.... //whats now ?
//also how do you return the header to the argument
I have tried that according to an answer here ,but i get nothing to print:
char delimiters[] = ":";
char *valPosition=NULL;
char newString=NULL ;
char * header = NULL;
valPosition = strtok(content, delimiters);
malloc(strlen(valPosition) + 1);
strcpy(header, valPosition);
Serial.println(header);

Answer 1

Let's have a look at the man page for strtok(). It says
char *strtok(char *str, const char *delim);
The strtok() function parses a string into a sequence of tokens..... The delim argument specifies a set of bytes that delimit the tokens in the parsed string. ...... Each call to strtok() returns a pointer to a null-terminated string containing the next token.
That means, when you call
valPosition = strtok(localString, delimiters); /
strtok() will search localString for the delimiter specified in delimiters
and if it finds any, it will return the token as a null-terminated string.
Beware, strtok()
modify their first argument.
and
cannot be used on constant strings.
so, the localString should be modifiable, i.e., cannot be a string literal.
Next, as per your format, strtok() will return the header, without the :.
so, you need to copy the returned string into another and return that. You can use dynamic memory allocation, as per following algorithm
Define a char * header = NULL;
Check the returned value of strtok(), if not NULL, allocate memory to header, like header = malloc(strlen(valPosition) + 1);
copy the data using strcpy(header, valPosition);
return the header from the function.
I hope, you understand that you need to change the function prototype also return a pointer, like
char * getHeader( char* localString){....
Also, once you're done using the returned value, you need to free() it.

Answer 2

If header:data is the only use case you have you want to look for strchr().
Example:
#include <string.h> /* for strchr() and strlen() */
#include <errno.h> /* for EINVAL */
int get_header_and_data(const char * input, char ** pheader, char ** pdata)
{
int result = 0;
if (NULL == input
|| NULL == pheader || NULL == *pheader
|| NULL == pdata || NULL == *pdata
)
{
/* Trivial case of "no" input and/or missing references to store the result. */
errno = EINVAL;
result = -1;
}
else
{
char * pcolon = strchr(input, ':');
if (NULL == pcolon)
{
/* No delimiter found. */
errno = EINVAL;
result = -1;
}
else
{
/* Delimiter found. */
if (pcolon == input)
{
/* No header found. */
errno = EINVAL;
result = -1;
}
else
{
if (1 == strlen(pcolon))
{
/* No data found. */
errno = EINVAL;
result = -1;
}
else
{
/* Success. */
*pcolon = '\0';
++pcolon;
(*data) = pcolon;
(*pheader) = input;
}
}
}
}
return result;
}
and use it like this:
#include <stdio.h>
int get_header_and_data(const char *, char **, char **);
...
char content[50] = "";
/* Load content here. */
char * header = NULL;
char * data = NULL;
if (-1 == get_header_and_data(content, &header, &data)
{
perror("get_header_and_data() failed.");
abort(); /* Or what ever to handle the error. */
}
else
{
/* Dereference and/or use header and data here .*/
}
Please not that on success header and data (still) refer to the memory of content, as well as that on success the latter is modifed.
Just for fun, the code above can be shrunk down to:
int get_header_and_data(const char * input, char ** pheader, char ** pdata)
{
int result = 0;
if (NULL == input
|| NULL == pheader || NULL == *pheader
|| NULL == pdata || NULL == *pdata
)
{
/* Trivial case of "no" input and/or missing references to store the result. */
errno = EINVAL;
result = -1;
}
else
{
char * pcolon = strchr(input, ':');
if (NULL == pcolon /* No delimiter found. */
|| pcolon == input /* No header found. */
|| 1 == strlen(pcolon) /* No data found. */
)
{
errno = EINVAL;
result = -1;
}
else
{
/* Success. */
*pcolon = '\0';
++pcolon;
(*data) = pcolon;
(*pheader) = input;
}
}
return result;
}