Pointer difference and size_t - c

Question

I want to allocate memory for holding a field extracted from a given string. The size of the field is determined by the difference of two pointers, see the following minimal example:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(int argc, char *argv[])
{
const char line[] = "foo,bar,baz";
char *field_start = line;
char *field_end;
char *field;
field_end = strchr(line, ',');
field = malloc(field_end - field_start + 1);
memcpy(field, field_start, field_end - field_start);
*(field + (field_end - field_start)) = '\0';
printf("field=\"%s\"\n", field);
/* ... */
return (0);
}
Compiling this code with clang -Weverything -o ex ex.c results in the following warnings:
ex.c:14:41: warning: implicit conversion changes signedness: 'long' to 'unsigned long'
[-Wsign-conversion]
field = malloc(field_end - field_start + 1);
~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~^~~
ex.c:15:39: warning: implicit conversion changes signedness: 'long' to 'unsigned long'
[-Wsign-conversion]
memcpy(field, field_start, field_end - field_start);
~~~~~~ ~~~~~~~~~~^~~~~~~~~~~~~
As I understand it, the result of the pointer difference is of ptrdiff_t type while the malloc/memcpy expect an argument of type size_t.
So my question is how to address this and to eliminate the warning? As
field_end >= field_start the difference cannot become negative, so could the
above be safely casted to size_t
field = malloc(size_t(field_end - field_start + 1));
memcpy(field, size_t(field_start, field_end - field_start));
or are the any problems I'm overlooking?
Note:
There are no checks for return values in the above just for simplicity. field_start and _end should be const of course.

Answer 1

field_end >= field_start only holds in case strchr does not return NULL, i.e. nothing in the type system tells the compiler that this indeed always holds. Hence the warning is warranted. However, if you make sure that this is not the case, then (size_t)(field_end - field_start) should be fine. In order to not duplicate this all over, I'd add
size_t field_len;
/* memchr & null-check go here */
field_len = (size_t)(field_end - field_start);
...and then use field_len all over.
That being said, you may want to replace your malloc/memcpy combination with a call to strndup.