Am Dienstag, 13. September 2005 13:23 schrieb Fco. J:
> Block A: and B: are the same, but B: segfault.
>
> > #include <stdio.h>
> >
> > #define SIZE_MARCA (256 + 62)
> >
> > typedef struct Marca
> > {
> > unsigned char str[SIZE_MARCA];
> > unsigned int returned;
> > } marc;
> >
> > static struct Marca
> > marca (const unsigned char *page, const unsigned int id)
> > {
> > struct Marca marca;
> >
> > if (sprintf (marca.str, "%sTralara%u",page, (unsigned int) id) <= 0)
> > {
> > printf ("Error de salida de sprintf\n");
> > marca.returned = 0;
> > };
> > marca.returned = 1;
> > return marca;
> > }
> >
> > int
> > main ()
> > {
> > struct Marca tmp;
> > /*struct Marca (*puntero) (const unsigned char *page, const unsigned int
> > id);*/ /* A: */
> > tmp.str = marca("server_form.html", 1).str;
> > printf("TMP=%s\n",tmp.str);
> >
> > /* B: */
> > printf ("1=%s \n", marca("server_form.html", 1).str);
> > exit (0);
> > }
>
> Output:
> > ./tes2
> > TMP=server_form.htmlTralara1
> > Fallo de segmento
>
The statement tmp.str = marca(...).str copies the CONTENTS of the string
value, returned by the function which is ok, since when returning a variable
of type struct Marca is shipped through the stack.
The second form in block B this variable will also ships through the stack.
This time not the CONTENTS of the string is copied but the pointer to str
field of the variable that has been created on the stack frame is given as an
argument to printf. The problem is, that while printf executes the stack
frame contains the printf arguments. The stack frame of the previously
executed marca function has been deleted at this time. But you still point
to this stack frame by trading the str field pointer.
You should be carefull exchanging such objects through the stack. It is much
better to trade object references:
int marca( struct Marca* x, const char* y, int v) -> operate on x->str
and use the integer return value as an error indicator. You are on the save
side.
int main (...) {
int ret=0;
struct Marca marca;
if ( 0 == (ret=marca(&marca,"bla.html",1)) ) printf( "OK=%s\n", marca.str);
return ret;
}
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