C Programming Language :: Why this de-referencing does not work?


	Why this de-referencing does not work? Hi, My program crashes on the following code, void lower(char* src, char* ret){ while ( src ) { char c = tolower(src); ret = c; <----------------------------- crash here ++ret; ++src; } return; } the calling of this function is like this, char* s1 = "aBcDeFg"; char* s2 = "1234567"; lower(s1, s2); Do you see what is wrong? <linq@hotmail.com> wrote in message news:1180656028.806833.56960@o11g2000prd.googlegroups.com... > Hi, > My program crashes on the following code, > void lower(char* src, char* ret){ > while ( src ) { > char c = tolower(src); > ret = c; <----------------------------- crash here > ++ret; > ++src; > } > return; > } You continue your loop until the pointer assumes a null value. You want to continue until the character pointed to by the pointer is a null value. karl m linq@hotmail.com said: > Hi, > My program crashes on the following code, > void lower(char* src, char* ret){ > while ( src ) { This should be: while(src) { That's the first fix. > char c = tolower(src); > ret = c; <----------------------------- crash here > ++ret; > ++src; > } > return; > } > the calling of this function is like this, > char s1 = "aBcDeFg"; > char* s2 = "1234567"; > lower(s1, s2); > Do you see what is wrong? Yes, you're trying to write to memory you don't own. Here's the second fix: char s1 = "aBcDeFg"; char s2[] = "1234567"; lower(s1, s2); -- Richard Heathfield "Usenet is a strange place" - dmr 29/7/1999 http://www.cpax.org.uk email: rjh at the above domain, - www. In article <1180656028.806833.56@o11g2000prd.googlegroups.com>, - Hide quoted text - <linq@hotmail.com> wrote: > My program crashes on the following code, >void lower(char src, char* ret){ > while ( src ) { > char c = tolower(src); > ret = c; <----------------------------- crash here > ++ret; > ++src; > } > return; >} >the calling of this function is like this, > char* s1 = "aBcDeFg"; > char* s2 = "1234567"; > lower(s1, s2); >Do you see what is wrong? char* s2 declares s2 to be a pointer to a character. char* s2 = "1234567" goes further and initializes that pointer to the address of a character array that has '1' '2' '3' '4' '5' '6' '7' and a terminating 0 (not '0'). char* s2 = "1234567" is not defined as "allocate an area of memory big enough to hold the literal string, copy the literal string into that memory, and store the pointer to that memory in s2". Instead, as far as the C language is concerned, when you use the form char* s2 = "1234567", the area pointed to is -required- to have static storage duration, and is -permitted- to be in read-only memory. It appears that in your implementation, literal strings are indeed stored in read-only memory, so when you attempt to write to that string in lower(), you get a fault. In other systems, there might not be a fault -- but there might be the side effect that other places that happened to also use "1234567" as literal strings might suddenly start containing the string assigned in lower(). So Don't Do That! What you can do instead is use char s2[] = "1234567"; I know this -looks- like it should have exactly the same effect as char* s2 = "1234567"; but when you use the char s2[] = literalstring form, C is defined to allocate memory and copy the literal string in, leaving you with an array of characters that you can modify. When you use the char* s2 = literalstring form, you cannot modify the string. -- All is vanity. -- Ecclesiastes - Hide quoted text - linq@hotmail.com writes: > My program crashes on the following code, > void lower(char* src, char* ret){ > while ( src ) { > char c = tolower(src); > ret = c; <----------------------------- crash here > ++ret; > ++src; > } > return; > } > the calling of this function is like this, > char* s1 = "aBcDeFg"; > char* s2 = "1234567"; > lower(s1, s2); > Do you see what is wrong? Yes. The comp.lang.c FAQ is at <http://www.c-faq.com/>. You've asked question 1.32. -- Keith Thompson (The_Other_Keith) k@mib.org <http://www.ghoti.net/~kst> San Diego Supercomputer Center <> <http://users.sdsc.edu/~kst> "We must do something. This is something. Therefore, we must do this." -- Antony Jay and Jonathan Lynn, "Yes Minister" -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 - Hide quoted text - linq@hotmail.com wrote: > Hi, > My program crashes on the following code, > void lower(char src, char* ret){ > while ( src ) { > char c = tolower(src); > ret = c; <----------------------------- crash here > ++ret; > ++src; > } > return; > } > the calling of this function is like this, > char* s1 = "aBcDeFg"; > char* s2 = "1234567"; > lower(s1, s2); > Do you see what is wrong? Yes 1) Your loop will terminate when src equals (char )0 (the NULL pointer). You don't want this; you want the loop to terminate when src equals \0 2) If indeed you are calling your lower() function with the s1 and s2 you presented, then you are doing it wrong. As s2 is a pointer to a fixed string (/what/ does the standard call that?), alteration of the characters in that string invokes (IIRC) undefined behaviour. Your operating system /may/ cause the program to abend with the equivalent of a "trying to alter unalterable memory" error. Instead, you want to call lower() with ret pointing to alterable memory, something like... { void lower(char , char ); char s1 = "This is a string", / unalterable / s2[60]; / alterable / lower(s1,s2); } 3) In lower(), you don't length check the space pointed to by the ret argument. What would happen if you called lower() with a ret that points to a smaller area than the src points to? As in... { void lower(char , char ); char s1[] = "This is a long string", s2[] = "Short"; lower(s1,s2); } HTH - -- Lew Pitcher Master Codewright & JOAT-in-training \| Registered Linux User #112576 http://pitcher.digitalfreehold.ca/ \| GPG public key available by request - ---------- Slackware - Because I know what I'm doing. ------ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (GNU/Linux) Comment: Armoured with GnuPG iD8DBQFGX2WTagVFX4UWr64RAl4BAKCAwV+6kG6gOzkrt15dg6H4FVGbFACeJ/LL /LUp15tkEqcp41XPHAof9+U= =3L12 -----END PGP SIGNATURE----- - Hide quoted text - linq@hotmail.com wrote: > My program crashes on the following code, > void lower(char src, char* ret){ > while ( src ) { > char c = tolower(src); > ret = c; <----------------------------- crash here > ++ret; > ++src; > } > return; > } > the calling of this function is like this, > char* s1 = "aBcDeFg"; > char* s2 = "1234567"; > lower(s1, s2); > Do you see what is wrong? Yes. Change to: char s2[] = "1234567"; and read up on non-modifiable char strings. The lower function will receive the same parameters, but ret will be modifiable. -- <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt> <http://www.securityfocus.com/columnists/423> <http://www.aaxnet.com/editor/edit043.html> <http://kadaitcha.cx/vista/dogsbreakfast/index.html> cbfalconer at maineline dot net -- Posted via a free Usenet account from http://www.teranews.com "Lew Pitcher" <lpitc@teksavvy.com> ha scritto nel messaggio news:bdae7$465f6584$cef8bfac$24302@TEKSAVVY.COM-Free... > 3) In lower(), you don't length check the space pointed to by the ret > argument. What would happen if you called lower() with a ret that points > to a > smaller area than the src points to? As in... > { > void lower(char , char ); > char s1[] = "This is a long string", > s2[] = "Short"; > lower(s1,s2); > } AFAIK there is no way to check that from within lower(). You can only prevent lower() from being called with such arguments. -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 - Hide quoted text - Army1987 wrote: > "Lew Pitcher" <lpitc@teksavvy.com> ha scritto nel messaggio > news:bdae7$465f6584$cef8bfac$24302@TEKSAVVY.COM-Free... >> 3) In lower(), you don't length check the space pointed to by the ret >> argument. What would happen if you called lower() with a ret that points >> to a >> smaller area than the src points to? As in... >> { >> void lower(char , char ); >> char s1[] = "This is a long string", >> s2[] = "Short"; >> lower(s1,s2); >> } > AFAIK there is no way to check that from within lower(). You can only > prevent lower() from being called with such arguments. Yup. You are correct. Sort of. With the function prototype that the OP uses, there is no way that lower() can perform any sort of check. If the function prototype for lower() were changed a bit, the function could perform a rudimentary check, as in lower(char source, char target, size_t targetlength) { if (targetlength <= 0 strlen(source)) return; or similar, then some small level of assurance could be established. It becomes a little bit harder to shoot yourself in the foot with the OP's lower() function. Ideally, the function would take absolute pains to ensure that the target string allocation was of a suitable size for the converted data. This /could/ be done by either a) having the lower() function allocate the target string itself (returning a char target, and possibly obliging the caller to perform the free() ), or b) having the lower() function replace in place the source string with the target string, In any case, my point is valid, and a solution is achievable. - -- Lew Pitcher Master Codewright & JOAT-in-training \| Registered Linux User #112576 http://pitcher.digitalfreehold.ca/ \| GPG public key available by request - ---------- Slackware - Because I know what I'm doing. ------ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (GNU/Linux) Comment: Armoured with GnuPG iD8DBQFGYuXqagVFX4UWr64RAmfHAKDB0wmIoSF6CTAXeQPnDBq5MVGhFACgqEBg 2J92IlBywrY5nhv1C8Fa46g= =DkMM -----END PGP SIGNATURE----- Lew Pitcher wrote: ... snip ... > If the function prototype for lower() were changed a bit, the function could > perform a rudimentary check, as in > lower(char source, char target, size_t targetlength) > { > if (targetlength <= 0 strlen(source)) return; size_t is unsigned. Such a value cannot be < 0. -- <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt> <http://www.securityfocus.com/columnists/423> <http://www.aaxnet.com/editor/edit043.html> <http://kadaitcha.cx/vista/dogsbreakfast/index.html> cbfalconer at maineline dot net -- Posted via a free Usenet account from http://www.teranews.com -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 CBFalconer wrote: > Lew Pitcher wrote: > ... snip ... >> If the function prototype for lower() were changed a bit, the function could >> perform a rudimentary check, as in >> lower(char source, char target, size_t targetlength) >> { >> if (targetlength <= 0 strlen(source)) return; > size_t is unsigned. Such a value cannot be < 0. Oops... sorry for the typo I meant if (targetlength <= strlen(source)) return; I don't know how that stray zero got in there :-S - -- Lew Pitcher Master Codewright & JOAT-in-training \| Registered Linux User #112576 http://pitcher.digitalfreehold.ca/ \| GPG public key available by request - ---------- Slackware - Because I know what I'm doing. ------ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (GNU/Linux) Comment: Armoured with GnuPG iD8DBQFGY3xeagVFX4UWr64RAihnAJ4hJeyzpmK5dcFLfUbVc5nFIAzNhgCg57rF JX2Ed9Furqtex79WBgkBxsk= =VfMy -----END PGP SIGNATURE----- CBFalconer <cbfalco@yahoo.com> writes: > Lew Pitcher wrote: > ... snip ... >> If the function prototype for lower() were changed a bit, the function could >> perform a rudimentary check, as in >> lower(char source, char target, size_t targetlength) >> { >> if (targetlength <= 0 strlen(source)) return; > size_t is unsigned. Such a value cannot be < 0. I'm guessing the 0 is a typo. As it stands, it's a syntax error. -- Keith Thompson (The_Other_Keith) k@mib.org <http://www.ghoti.net/~kst> San Diego Supercomputer Center <> <http://users.sdsc.edu/~kst> "We must do something. This is something. Therefore, we must do this." -- Antony Jay and Jonathan Lynn, "Yes Minister"