Array Arithmetic Issue in C

Question

I have a C code in long file that is compiled using cc. But when I tried to compile on gcc it gives error. I took that particular code in small program and try to compile on cc but it failed over there.

Here is source:

#include <stdio.h>
int main (int argc, char **argv)
{
     char unsigned   FileName[100];
     char            test[100];
     FileName[strstr(FileName,test) - FileName] = 0;   
     return 0;
}

This line is causing the problem: FileName[strstr(FileName,test) - FileName] = 0;

error on CC is :

"foo.c", line 10: operands have incompatible types:
         int "-" pointer to unsigned char

and on gcc is :

foo.c:10: error: invalid operands to binary - Both are same.

But when I compile original file on CC it compiled and just give a warning. Like this:

"dbtprc.c", line 643: warning: argument #1 is incompatible with prototype:
        prototype: pointer to const char : "/usr/include/iso/string_iso.h", line 133
        argument : pointer to unsigned char
"dbtprc.c", line 643: warning: improper pointer subtraction

Can you please help why here it is giving warning "improper pointer subtraction" and sample program it is showing error?

First off, as Acme mentioned, neither bar or foo point to anything. Second, you're trying to subtract a char* from a char*? — Ricardo Ferreira
– Ricardo Ferreira, Commented Mar 29, 2011 at 16:40

Laurent G · Accepted Answer · 2011-03-29 17:03:26Z

1

Don't you missed to include <string.h> ??

If yes, the prototype of strsrt is guessed and by default it returns an int, hence the invalid pointer operation.

Otherwise, it appears that the signedness mismatch is the cause of the warning/error. Use a (char*)cast before the two occurence of your table and it will go.

edited Mar 29, 2011 at 17:03

answered Mar 29, 2011 at 16:45

Laurent G

3,2141 gold badge20 silver badges10 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

QMG Over a year ago

Thank you for reply.Yes, you are partially right. Adding string.h on CC, it works and gives warning. But on GCC it still complaining for the same

QMG Over a year ago

Please tell ,Alternate to do the same?

Laurent G Over a year ago

Alternate to do what ? Are you trying to avoid casts ?

Sadique · Accepted Answer · 2011-03-29 16:54:03Z

1

Is there anything called Array Arithmetic in C? Read this:

Arrays are not Pointers.

And see how to use strstr().

answered Mar 29, 2011 at 16:54

Sadique

22.8k7 gold badges68 silver badges92 bronze badges

1 Comment

QMG Over a year ago

Thank you for reply. Besides this Arrays are not pointer but the issue is CC is compiling a piece of code and GCC is not compiling same source. So why is it happening ?

karlphillip · Accepted Answer · 2011-03-29 16:58:03Z

1

void * bar;
void * foo;
...
foo = bar + 1;

That's undefined behaviour, right there! You are referring to a memory location that wasn't even allocated.

EDIT:

Now you have another problem: even though you sucessfully declared both arrays, you failed to clean/initialize them. Only god knows what strstr() will return to you.

The problem you have compiling this code is that strstr() takes 2 const char* and you defined Filename as unsigned: char unsigned FileName[100];

char *strstr(const char *haystack, const char *needle);

edited Mar 29, 2011 at 16:58

answered Mar 29, 2011 at 16:38

karlphillip

93.7k38 gold badges254 silver badges448 bronze badges

5 Comments

karlphillip Over a year ago

Updated answer. Even if you fix the type of Filename, you still need to #include <string.h> on your file. Even though all of this will get your code compiled, I doubt it will do anything good.

QMG Over a year ago

Thank you karl for your help. Adding <string.h> solve the problem on CC but on GCC it still there.

QMG Over a year ago

Dear I just have to compile it :)

karlphillip Over a year ago

The 2nd tip was to define Filename appropriately: char FileName[100]; since strstr() does not take unsigned. Have you done that?

karlphillip Over a year ago

It's important to all of us that you accept the answer that solved your problem. Good luck. And if you have another problem, please ask another question on a different thread.

Bo Persson · Accepted Answer · 2011-03-29 16:51:44Z

0

An error or a warning is not much different, just showing how serious the compiler believes the issue is.

But why do you use unsigned char for the filename? That's in conflict with strstr which only handles char*, both for its parameter and return type.

That's what the compilers try to tell you, in different ways.

answered Mar 29, 2011 at 16:51

Bo Persson

92.9k31 gold badges153 silver badges211 bronze badges

Collectives™ on Stack Overflow

Array Arithmetic Issue in C

4 Answers 4

3 Comments

1 Comment

5 Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

1 Comment

5 Comments

Comments

Related