question on how to correctly communicate with external library functions which return malloc()'ed strings - Mailing list pgsql-hackers
From | Vladimir Volovich |
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Subject | question on how to correctly communicate with external library functions which return malloc()'ed strings |
Date | |
Msg-id | 871w5d86r6.fsf@external.prs-office.com Whole thread Raw |
Responses |
Re: question on how to correctly communicate with external library functions which return malloc()'ed strings
(Martijn van Oosterhout <kleptog@svana.org>)
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List | pgsql-hackers |
Hi! i'm using a module for postgresql (8.2 and 8.3) which links with a 3rd-party library and calls a function from there which returns a malloc()'ed string. [it's a libunac, and unac_string() but the question is general] there was a pg_unac-8.2.tar.gz tarball distributed on the net, which has the following implementation: ============================================= Datum unac(PG_FUNCTION_ARGS) { text *str = PG_GETARG_TEXT_P(0); text *result; int tlen, nlen, rsize; char *tstr, *nstr, *rptr; tlen = VARSIZE(str) -VARHDRSZ; tstr = (char *) palloc(tlen + 1); memcpy(tstr, VARDATA(str), tlen); tstr[tlen] = '\0'; nstr = NULL; nlen = 0; unac_string("UTF-8", tstr, strlen(tstr), &nstr, &nlen); /* It may happen that unac_string returns NULL, because iconv * can't translate the input string. In this case we output * the string as it is. */ if (nstr == NULL) nstr = tstr; rsize = strlen(nstr) + VARHDRSZ; result = (text *) palloc(rsize);rptr = VARDATA(result); memcpy(rptr, nstr, rsize - VARHDRSZ); SET_VARSIZE(result, rsize); PG_RETURN_TEXT_P(result); } ============================================= clearly there's a memory leak here: nstr never gets free()'d: its value is copied to a palloc()-ated string but nstr itself is never free()'d. this call to palloc() appears to be necessary because one cannot just pass malloc()-created nstr as a result of a function - there will be a segfault, as far as i remember; therefore one needs to copy nstr to a palloc()-created string and then to free() nstr (which is missing in the above code). i had to modify that function to remove the memory leak and to add some more functionality i needed. in a simplified form, it looks like: ============================================= /* tstr is a palloc()'ed string */ size_t nlen = 0; char *nstr = NULL; unac_string("UTF-8",tstr, strlen(tstr), &nstr, &nlen); if (nstr == NULL) { nstr = strdup(tstr); } pfree(tstr); result =(text *) palloc(strlen(nstr) + VARHDRSZ); memcpy(VARDATA(result), nstr, strlen(nstr)); #if PG_VERSION_NUM >= 80300 SET_VARSIZE(result, strlen(nstr) + VARHDRSZ); #else VARATT_SIZEP(result) = strlen(nstr) + VARHDRSZ; #endif free(nstr); PG_RETURN_TEXT_P(result); ============================================= it worked fine with postgresql 8.2; with 8.3 it started segfaulting, and it appeared that the reason is because in postgresql 8.3, the "free" is a macro defined in snowball/header.h: #define free(a) pfree(a) i solved it by adding these lines: #ifdef free #warning undefining free #undef free #endif #include <stdlib.h> but i want to ask you, postgresql hackers, on how we are supposed to work with external library functions which return malloc()'ed strings? should we create palloc()'ed copy and free() the original string? then, is it correct to have that "#define free(a) pfree(a)" in snowball/header.h? Best, v.
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