string handling bug or feature?

[Dirk Petry <petry at milkyway dot gsfc dot nasa dot gov>]

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;

}