strcpy dilemma - it works half way

Vaclav_

I am going crazy over this simple test code.

char *Result ={"Result init "};
char *Source = {"123"};
char *Destination = {"456"};

I can verify that all strings are as initialized. Now I run this strcpy

Result = strcpy(Source,Destination);

Now I can verify, using same approach as before , that Result = 123 - AKA orignal Source, it was Result init before strcpy was run. and Source is STILL 123. Result suppose to be Source ( source destination swap) , but I expect the Source to be COPY of the original Destination AKA 456 What am I doing wrong? strcpy executes giving correct return, but unmodified Destination. Am I doing the initialization correctly? Am I nuts? Standard stdio using GCC compiler /tools on Arduino IDE. Thank for your help. Vaclav

PIEBALDconsult

I think you have the parameter names backward so it's confusing to me. After the strcpy, Result and Source should contain the same address, the value there should be 465, and the value at Destination should also be 456. Is that what you see? Is that not what you want? Be aware that you are not implementing a "swap" at all. Unless there are other concerns, if you want to "swap" then swap the pointers, not the values:

Result = Destination;
Destination = Source;
Source = Result;

Lost User

It should not work at all. String constants declared as above are read-only, so you should not be able to use strcpy to overwrite one of them by another. When I try it I get an access violation on the write, as I would expect. I am not sure why your system does not give the fault, maybe a peculiarity of Arduino. But since strcpy is supposed to return the pointer to the destination field, Result will now point to the data that Source points to, i.e. "123", but the original data should be untouched.

CPallini

You cannot do that, it is a mistake. Your variables point to statically allocated (i.e. constant) strings you must NOT try to change. The following program

#include <stdio.h>
#include <string.h>

int main()
{
char Result[] ="Result init ";
char Source[] = "123";
char Destination[] = "456";

strcpy(Destination, Source);

printf("%s %s\n", Destination, Source);

return 0;
}

on the other hand, outputs

123 123

Please note, parameter order in strcpy is important (like in any other C function, after all).

Vaclav_

Thanks, one of these days I'll hope to learn. I did change the initialization and it did the trick with the exception of Result. Can you explain to me why Result needs to be declared this way? I appreciate all you guys time and I am sorry about the poor choice of variable names I did. Thanks. I am enclosing my full test code, just FYI , so do not pay much attention to it. char *Result = {"0007890"}; char Destination[] = {"456"}; char Temp[] = {" "}; char Source[] = {"123"}; #ifdef DEBUG lcd_i2c.clear(); //lcd_i2c.print("BuildAttribute..."); lcd_i2c.setCursor(0, 0); lcd_i2c.print("Result "); lcd_i2c.print(Result ); //for(;;); lcd_i2c.setCursor(0, 1); lcd_i2c.print("Source " ); lcd_i2c.print(Source ); //strcpy(PassedAttribute, "attribute"); lcd_i2c.setCursor(0, 2); lcd_i2c.print("Dest " ); lcd_i2c.print(Destination ); lcd_i2c.setCursor(0, 3); //lcd_i2c.print(Attribute); //delay(2000); //for(;;); //return PassedAttribute; #endif //Result = strncpy(Source, "TESTDestination",2 ); printf("%s %s\n", Destination, Source); Result = strcpy(Destination, Source); printf("%s %s\n", Destination, Source); //for (;;); #ifdef DEBUG lcd_i2c.clear(); //lcd_i2c.print("BuildAttribute..."); lcd_i2c.setCursor(0, 0); lcd_i2c.print("Result "); lcd_i2c.print(Result ); //for(;;); lcd_i2c.setCursor(0, 1); lcd_i2c.print("Source " ); lcd_i2c.print(Source ); //strcpy(PassedAttribute, "attribute"); lcd_i2c.setCursor(0, 2); lcd_i2c.print("Dest " ); lcd_i2c.print(Destination ); lcd_i2c.setCursor(0, 3); lcd_i2c.print("Temp "); lcd_i2c.print(Temp); //delay(2000); for (;;); //return PassedAttribute; #endif for(;;);

CPallini

Result must be declared as pointer to character (char *) because (as already explained by Richard) you are assigning it with strcpy return value. Please note such an assignment is useless since strcpy result is already in Destination array.

Vaclav_

Thanks. I know the results are already known, but I was just using it to learn about the strcpy.

Arthur V Ratz

This should exactly work:

#include #include int main(void)
{
char *Result = "Result init ";
char Source[256] = "123";
char Destination[256] = "456";

Result = strcpy(Source, Destination);

printf("Result = %s\\nSource = %s\\nDestination = %s\\n", Result, Source, Destination);

return 0;

}