lvalue required as increment operand compile error

canadatom · 06-13-2009, 10:56 AM

Code:

int

read_itr_seq(void *buf, int itype, FILE *f)

{

  int l, ch;

  void *p = buf ;

  if ( itype < 7 || (itype > 13 && itype != 20) ) return -2 ;

  l = (ch = getc(f)) ;

  if ( l == 0 ) {

    while ( (ch = getc(f)) == 0 ) ;

    ch = getc(f) ;		/* Second byte of chunk word */

    l = (ch = getc(f)) ;

  }

  else if ( ch == 0xff ) return 0 ;

  *((u_char *)p)++ = l ; 

  if ( itype < 11 || itype == 20 ) fread(p, (size_t)(l + 22), (size_t)1, f) ;

  else fread(p, (size_t)(l + 23), (size_t)1, f) ;

  if ( ferror(f) ) return -1 ;

  return 1 ;

}

Code:

  *((u_char *)p)++ = l ;

has the compile error lvalue required as increment operand, I am using GCC4.3.3 and this code works under GCC 2.96, would someone give me a hint on this? Thanks

MK27 · 06-13-2009, 11:03 AM

I'm gonna guess that is better written as

Code:

*(p++) = l ;

You'll have to test it yourself, but this is my reasoning:

the cast is unnecessary [edit] okay it is
the increment should be inside the parentheses, or else you are kind of saying add one to the dereferenced value, but then setting it (=)

**laserlight** · 06-13-2009, 11:05 AM

hmm... you cast p to a u_char*, assign l to the u_char that this temporary pointer points to, then increment this temporary pointer. The increment is not useful, so it should suffice to write:

Code:

*(u_char *)p = l ;

EDIT:

Quote:

Originally Posted by MK27

the cast is unnecessary

Since p is a pointer to void, the cast is necessary in order to dereference the pointer and/or perform pointer arithmetic.

EDIT #2:

Quote:

Originally Posted by MK27

the increment should be inside the parentheses, or else you are kind of saying add one to the dereferenced value, but then setting it (=)

Actually, *(p++) is equivalent to *p++.

canadatom · 06-13-2009, 11:23 AM

thanks for helping guys,

*(u_char *)p = l; works for me,

and

Code:

int

extract_text_nosource(char *ip, int il, FIELD_TYPE field_type,

		      Extraction_regime reg, int (*stemfn)(char *, int, char *),

		      Gsl *gs, void *outptr, int outflag, int rec, int diag)

{

  int l = il ;

  char *p = ip ;

  char *sp ;

  int pl, sl ;

  int nk ;

  int numterms = 0 ;



  int stopped ;

  int outlen ;

  void *opp = outptr ;

  while ( l > 0 ) {

    pl = find_para(p, l, INDENT|GAP) ; 

    stopped = 0 ;

    l -= pl ; sp = p ; p += pl ;

    while ( pl > 0 ) {

      sl = find_sentence(sp, pl, field_type, TEXTPOS_MAX_T, 0) ;

      nk = extract(sp, sl, reg, gs, stemfn, opp, outflag,

		   rec, 0, 0, 0, (int *)NULL, &stopped, &outlen) ;

      if ( nk == -1 ) {

	numterms = nk ;

	break ;

      }

      pl -= sl ;

      sp += sl ;

      numterms += nk ;

      (char *)opp += outlen ;

    }

  }

  return numterms ;

}

Code:

(char *)opp += outlen ;

I got same errors here. I don't see anything wrong with this line.

Thanks again for helping here

**laserlight** · 06-13-2009, 11:30 AM

It's the same problem: from what I infer, the C standard now states that the result of a cast expression may not be used as an lvalue, i.e., it may not be modified. This makes sense, since the result of a cast expression is a temporary that would typically end its lifetime after the given statement.

So, this statement:

Code:

(char *)opp += outlen ;

causes a temporary pointer of type char* to be advanced by outlen. But this temporary is destroyed after this statement, so the statement has no net effect, were it legal in the first place.

In this case, it looks like opp should just be a pointer to char to begin with.

MK27 · 06-13-2009, 11:41 AM

So make opp a char* and cast it to void where necessary.

ps. they don't call you "Stompin'", do they?

canadatom · 06-13-2009, 11:42 AM

any hint for this line to be able to compile? Thanks

**laserlight** · 06-13-2009, 11:43 AM

Quote:

Originally Posted by canadatom

any hint for this line to be able to compile?

Both MK27 and I have already given you such a suggestion.

canadatom · 06-13-2009, 11:49 AM

thanks a lot for helping : )

06-13-2009, 11:03 AM	#2
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	I'm gonna guess that is better written as Code: (p++) = l ; You'll have to test it yourself, but this is my reasoning: the cast is unnecessary [edit] okay it is the increment should be inside the parentheses, or else you are kind of saying add one to the dereferenced value, but then setting it (=) __________________ Accuracy and integrity mean nothing if you don't make it past the censors...*PYTHAGORAS Last edited by MK27; 06-13-2009 at 11:11 AM.
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06-13-2009, 11:30 AM	#5
laserlight C++ Witch Join Date: Oct 2003 Location: Singapore Posts: 10,352	It's the same problem: from what I infer, the C standard now states that the result of a cast expression may not be used as an lvalue, i.e., it may not be modified. This makes sense, since the result of a cast expression is a temporary that would typically end its lifetime after the given statement. So, this statement: Code: (char )opp += outlen ; causes a temporary pointer of type char to be advanced by outlen. But this temporary is destroyed after this statement, so the statement has no net effect, were it legal in the first place. In this case, it looks like opp should just be a pointer to char to begin with. __________________ C + C++ Compiler: MinGW port of GCC Build + Version Control System: SCons + Bazaar Look up a C/C++ Reference and learn How To Ask Questions The Smart Way
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06-13-2009, 11:41 AM	#6
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	So make opp a char* and cast it to void where necessary. ps. they don't call you "Stompin'", do they? __________________ Accuracy and integrity mean nothing if you don't make it past the censors...PYTHAGORAS
MK27 is offline

06-13-2009, 11:42 AM	#7
canadatom Registered User Join Date: Jun 2009 Posts: 4	any hint for this line to be able to compile? Thanks
canadatom is offline

06-13-2009, 11:49 AM	#9
canadatom Registered User Join Date: Jun 2009 Posts: 4	thanks a lot for helping : )
canadatom is offline

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