[SOLVED] sizeof() issue

Caltrop · 09-18-2017, 12:20 PM

I have not been programming in 'C' since University but it's coming back like a Bad Dream... fun figuring out these BUGS!

I am trying to determine the size of my character array inside a subroutine. Compile generates an error: GPX_split_file.c:52:17: error: invalid application of ‘sizeof’ to incomplete type ‘char[]’

exerpts from my project:

GPX.h
extern char tmp_str[];
extern int tmp_int;
void gpx_split_file();

GPX.c
#include "GPX.h"
char tmp_str[512]; // sizeof issue with this array
int tmp_int;
main()
{
GPX_split_file()
}

GPX_split_file.c
#include "GPX.h"
void gpx_split_file()
{
tmp_int = sizeof(tmp_str) / sizeof (tmp_str[0]); // ERROR!
}

rtmistler · 09-18-2017, 12:26 PM

An array size does not work this way. What you should do is instead of using a literal value of 512, you should #define a constant as part of an include H file and use that definition:

Code:

#define TMP_ARRAY_LEN 512

// In source
char tmp_str[TMP_ARRAY_LEN];

Meanwhile, are you looking for the size of that array, regardless of it's contents? Meaning the maximum possible size? Or are you looking to determine if there is any information stored in that array? If it is a string, you can use a function call to get the string length, however if the data in that array can be any value from 0-255, then string length is not the best choice. Noticing that it is not an unsigned array, so this is why I'm asking about if you wish a string length.

NevemTeve · 09-18-2017, 12:40 PM

@OP: And what is your question?

astrogeek · 09-18-2017, 01:46 PM

Please place your code snippets inside [CODE]...[/CODE] tags for better readability. You may type those yourself or click the "#" button in the edit controls.

As rtmistler has pointed out, you cannot use sizeof to get the size of an array, and the size may not be what you actually need anyway.

It would be helpful if you could provide a better description of what you are trying to test (size vs existence, value vs length?). Putting it into simplest words often highlights the objective and the solution.

sundialsvcs · 09-19-2017, 07:43 AM

It is my understanding – and good ol' WikiPedia supports me on this, that the sizeof() an array should be the amount of storage required to store the entire array ... and that, in some optimization situations, that might be larger than you expect. (The compiler might choose to use a cache-aligned storage layout.)

The size of an array element is as for any other scalar.

pan64 · 09-19-2017, 07:45 AM

From the other hand I think the error message is correct.
You wrote in GPX.c:

Code:

...
char tmp_str[512]; // sizeof issue with this array
...

and you wrote in GPX_split_file.c:

Code:

#include "GPX.h"
tmp_int = sizeof(tmp_str) / sizeof (tmp_str[0]); // ERROR!

amd in GPX.h:

Code:

extern char tmp_str[];

so the size of tmp_str array is unknown in GPX_split_file.c (in compile time), because this type is currently inclomplete.
You could/may calculate the size runtime, but that is another function.
http://en.cppreference.com/w/c/language/sizeof

Caltrop · 09-25-2017, 11:45 AM

I rewrote the entire project
must have been a punctuation problem & other programmer errors
used some suggestions, thanks