Why Is Escaping Data Considered So Magical?
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From: Rami Chowdhury on 1 Jul 2010 23:17 On Thursday 01 July 2010 16:50:59 Lawrence D'Oliveiro wrote: > Nevertheless, it it at least self-consistent. To return to my original > macro: > > #define Descr(v) &v, sizeof v > > As written, this works whatever the type of v: array, struct, whatever. > Doesn't seem to, sorry. Using Michael Torrie's code example, slightly modified... [rami(a)tigris ~]$ cat example.c #include <stdio.h> #define Descr(v) &v, sizeof v int main(int argc, char ** argv) { char *buf = malloc(512 * sizeof(char)); const int a = 2, b = 3; snprintf(Descr(buf), "%d + %d = %d\n", a, b, a + b); fprintf(stdout, buf); free(buf); return 0; } /*main*/ [rami(a)tigris ~]$ clang example.c example.c:11:18: warning: incompatible pointer types passing 'char **', expected 'char *' [-pedantic] snprintf(Descr(buf), "%d + %d = %d\n", a, b, a + b); ^~~~~~~~~~ example.c:4:18: note: instantiated from: #define Descr(v) &v, sizeof v ^~~~~~~~~~~~ <<snip>> [rami(a)tigris ~]$ ./a.out Segmentation fault ---- Rami Chowdhury "Passion is inversely proportional to the amount of real information available." -- Benford's Law of Controversy +1-408-597-7068 / +44-7875-841-046 / +88-01819-245544
From: Lawrence D'Oliveiro on 2 Jul 2010 22:20 In message <mailman.136.1278040489.1673.python-list(a)python.org>, Rami Chowdhury wrote: > On Thursday 01 July 2010 16:50:59 Lawrence D'Oliveiro wrote: > >> Nevertheless, it it at least self-consistent. To return to my original >> macro: >> >> #define Descr(v) &v, sizeof v >> >> As written, this works whatever the type of v: array, struct, whatever. > > Doesn't seem to, sorry. Using Michael Torrie's code example, slightly > modified... > > char *buf = malloc(512 * sizeof(char)); Again, you misunderstand the difference between a C array and a pointer. Study the following example, which does work, and you might grasp the point: ldo(a)theon:hack> cat test.c #include <stdio.h> int main(int argc, char ** argv) { char buf[512]; const int a = 2, b = 3; snprintf(&buf, sizeof buf, "%d + %d = %d\n", a, b, a + b); fprintf(stdout, buf); return 0; } /*main*/ ldo(a)theon:hack> ./test 2 + 3 = 5
From: Rami Chowdhury on 2 Jul 2010 23:07 On Friday 02 July 2010 19:20:26 Lawrence D'Oliveiro wrote: > In message <mailman.136.1278040489.1673.python-list(a)python.org>, Rami > Chowdhury wrote: > > On Thursday 01 July 2010 16:50:59 Lawrence D'Oliveiro wrote: > >> Nevertheless, it it at least self-consistent. To return to my original > >> > >> macro: > >> #define Descr(v) &v, sizeof v > >> > >> As written, this works whatever the type of v: array, struct, whatever. > > > > Doesn't seem to, sorry. Using Michael Torrie's code example, slightly > > modified... > > > > char *buf = malloc(512 * sizeof(char)); > > Again, you misunderstand the difference between a C array and a pointer. > Study the following example, which does work, and you might grasp the > point: > > ldo(a)theon:hack> cat test.c > #include <stdio.h> > > int main(int argc, char ** argv) > { > char buf[512]; > const int a = 2, b = 3; > snprintf(&buf, sizeof buf, "%d + %d = %d\n", a, b, a + b); > fprintf(stdout, buf); > return > 0; > } /*main*/ > ldo(a)theon:hack> ./test > 2 + 3 = 5 I'm sorry, perhaps you've misunderstood what I was refuting. You posted: > >> macro: > >> #define Descr(v) &v, sizeof v > >> > >> As written, this works whatever the type of v: array, struct, whatever. With my code example I found that, as others have pointed out, unfortunately it doesn't work if v is a pointer to a heap-allocated area. ---- Rami Chowdhury "A man with a watch knows what time it is. A man with two watches is never sure". -- Segal's Law +1-408-597-7068 / +44-7875-841-046 / +88-01819-245544
From: David Cournapeau on 2 Jul 2010 23:09 On Mon, Jun 28, 2010 at 6:44 PM, Gregory Ewing <greg.ewing(a)canterbury.ac.nz> wrote: > Carl Banks wrote: > >> Indeed, strncpy does not copy that final NUL if it's at or beyond the >> nth element. Â Probably the most mind-bogglingly stupid thing about the >> standard C library, which has lots of mind-boggling stupidity. > > I don't think it was as stupid as that back when C was > designed Actually, strncpy had a very specific use case when it was introduced (dealing with limited-size entries in very old unix filesystem). It should never be used for C string handling, and I don't think it is fair to say it is stupid: it does exactly what it was designed for. It just happens that most people don't know what it was designed for. David
From: Lawrence D'Oliveiro on 3 Jul 2010 21:23
In message <mailman.182.1278126257.1673.python-list(a)python.org>, Rami Chowdhury wrote: > I'm sorry, perhaps you've misunderstood what I was refuting. You posted: >> >> macro: >> >> #define Descr(v) &v, sizeof v >> >> >> >> As written, this works whatever the type of v: array, struct, >> >> whatever. > > With my code example I found that, as others have pointed out, > unfortunately it doesn't work if v is a pointer to a heap-allocated area. It still correctly passes the address and size of that pointer variable. It that's not what you intended, you shouldn't use it. |