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segfault caused depending on how a function is called

I have this function to trim whitespace which it segfaults
if i call it this way:

Code:

  /*bad*/

  btrimws("  a\n");

or this way:

Code:

  /*bad*/

  char *str1="  b\n";

  btrimws(str1);

but not this way:

Code:

  /*works ok*/

  char *str2= (char *)malloc(10);

  strcpy(str2,"  c\n");

  btrimws(str2);

or this way:

Code:

  /*works ok*/

  char str3[]= "  d\n";

  btrimws(str3);

what should i do? I think i know its segfaulting because I am trying to write over something that hasn't been allocated, but I am not 100% sure. Also I don't understand why or not quite sure what I can check for in my trim function to make it work for all cases.

thanks a lot in advance.

Code:

#define BLOCK_COPY(D,S,L)        { if ((L) > 0) memmove ((D),(S),(L)); }

#define WSPACE(c)                (isspace ((unsigned char) c))



int btrimws (char* b) {

  int i, j;

  if (b == NULL) return -1;

  int slen = strlen(b);

  if(slen <= 0) return -1;



  for (i = slen - 1; i >= 0; i--) {

      if (!WSPACE (b[i])) {

        if (b[i+1] != (unsigned char) '\0')

            b[i+1] = (unsigned char) '\0';      /* segfaults here*/

        for (j = 0; WSPACE(b[j]); j++) {}

        return bdelete (b, 0, j);

      }

  }

  b[0] = (unsigned char) '\0';

  slen = 0;

  return 0;

}

*note: this code is modified from the The Better String Library

here is the other function that gets called from the trim function.

Code:

/*  int bdelete (char* b, int pos, int len)

 *

 *  Removes characters from pos to pos+len-1 inclusive and shifts the tail of 

 *  the char* starting from pos+len to pos.  len must be positive for this 

 *  call to have any effect.  The section of the string described by (pos, 

 *  len) is clamped to boundaries of the char* b.

 */

int bdelete (char* b, int pos, int len) {

  /* Clamp to left side of bstring */

  if (pos < 0) {

      len += pos;

      pos = 0;

  }

  int slen = strlen(b);

  if (len < 0 || b == NULL || slen < 0 ) 

      return -1;

  if (len > 0 && pos < slen) {

      if (pos + len >= slen) {

        slen = pos;

      } else {

        BLOCK_COPY ((char *) (b + pos),

                    (char *) (b + pos + len), 

                    slen - (pos+len));

        slen -= len;

      }

      b[slen] = (unsigned char) '\0';

  }

  return 0;

}

you cant modify a string literal.

Quote:

Originally Posted by xhi

you cant modify a string literal.

So if you want it to work with literals then you need to change the signature of the function.Maybe a function that accepts a character array and returns the modified character array is what you want.

Code:

char * trim (const char *)

Quote:

Originally Posted by graemef

Code:

char * trim (const char *)

i agree.. and another option would be a function that requires the return memory to be passed to the function.. allowing the return to notify of success..

ie

Code:

int trim(const char* input, char* output);

avoiding calls to malloc, this could be used as such

Code:

char buffer[STR_MAX];

if(!trim("    First char is F", buffer)

  printf("error trimming");

or something like that anyhow

Hya,

In the old days, --fwritable-stings or something used to take care of this issue.

Happy Penguins!

Quote:

Originally Posted by kaz2100

Hya,

In the old days, --fwritable-stings or something used to take care of this issue.

Happy Penguins!

that sounds like pure evil