Re: string handling bug or feature?

[Andrew Pinski <pinskia at physics dot uc dot edu>]

Feature, use `-Wwrite-strings' to warn about it.
Use `-fwritable-strings' to get them as writable.

Thanks,
Andrew Pinski

On Saturday, May 17, 2003, at 17:52 US/Eastern, Dirk Petry wrote:

> Hi!
>
> The following problem is present in C programs compiled with
> gcc 2.95.2 and gcc 3.2.3 :
>
> It is apparently not possible to create an array of strings
> with a statement like e.g.
> char *c[] = { "abcd", "cdef" };
> and then modify the strings with statements like e.g.
> c[0][0] = 'x';
>
> However, when you create the array of strings using
> char c[2][5] = { "abcd", "cdef" };
> everything works fine.
>
> Apparently, the two versions of the array "c" have
> internally a different data type and behave differently
> although they should both behave like "char**" for the
> programmer.
>
> Is this a bug or a feature?
>
>
> Below are included three little programs. The first two (test.c and
> test2.c) give segmentation faults. The lines in which the faults
> arrise are marked.
>
> The third program (test3.c) uses the statement
> char c[2][5] = { "abcd", "cdef" };
> to create the array of strings and runs fine.
>
> Regards,
>
> Dirk Petry
>
>
> --
> Dirk Petry - NASA/GSFC, Code 661, Greenbelt, MD 20771
> mailto:petry@milkyway.gsfc.nasa.gov   +1-301-286-0810
>
> -----------
> test.c:
>
> #include <string.h>
> #include <stdio.h>
>
> typedef struct {
>   char xxx[5];
>   int xy;
> } row;
>
> void writerow( row * );
>
> int main(){
>
>   row a;
>
>   sprintf(a.xxx, "1234");
>   a.xy = 1;
>
>   writerow( &a );
>
>   return(0);
> }
>
> void writerow( row *b ){
>
>   char *c[] = { "abcde", "cdefe" };
>
>   fprintf( stdout, "%s, %s, %d\n", c[0], b->xxx, b->xy );
>
>   fprintf( stdout, "%d, %d\n", sizeof(c[0]), sizeof(b->xxx) );
>
>   sprintf( c[0], "%s",  b->xxx ); /* causes segmentation fault */
>
>   fprintf( stdout, "%s, %d\n", c[0], b->xy );
>
>   return;
>
> }
>
> --------
> test2.c:
>
> #include <string.h>
> #include <stdio.h>
>
> typedef struct {
>   char xxx[5];
>   int xy;
> } row;
>
> void writerow( row * );
>
> int main(){
>
>   row a;
>
>   sprintf(a.xxx, "1234");
>   a.xy = 1;
>
>   writerow( &a );
>
>   return(0);
> }
>
> void writerow( row *b ){
>
>   int i;
>
>   char *c[] = { "abcde", "cdefe" };
>
>   fprintf( stdout, "%s, %s, %d\n", c[0], b->xxx, b->xy );
>
>   fprintf( stdout, "%d, %d\n", sizeof(c[0][0]), sizeof((b->xxx)[0]) );
>
>   for(i=0; i<5; i++){
>     fprintf(stdout, "%c %c\n", c[0][i], (b->xxx)[i]);
>   }
>
>   (b->xxx)[0] = 'x';
>   c[0][0] = 'x'; /* causes segmentation fault */
>
>   fprintf( stdout, "%s, %s, %d\n", c[0], b->xxx, b->xy );
>
>   return;
>
> }
>
> ------------
> test3.c:
>
> #include <string.h>
> #include <stdio.h>
>
> typedef struct {
>   char xxx[5];
>   int xy;
> } row;
>
> void writerow( row * );
>
> int main(){
>
>   row a;
>
>   sprintf(a.xxx, "1234");
>   a.xy = 1;
>
>   writerow( &a );
>
>   return(0);
> }
>
> void writerow( row *b ){
>
>   char c[2][5] = { "abcd", "cdef" };
>
>   fprintf( stdout, "%s, %s, %d\n", c[0], b->xxx, b->xy );
>
>   fprintf( stdout, "%d, %d\n", sizeof(c[0]), sizeof(b->xxx) );
>
>   sprintf( c[0], "%s",  b->xxx ); /* OK */
>   c[0][0] = 'x'; /* OK */
>
>   fprintf( stdout, "%s, %s, %d\n", c[0], b->xxx, b->xy );
>
>   return;
>
> }