* malloc question @ 2011-04-26 10:33 Randi Botse 2011-04-26 10:40 ` Daniel Baluta 2011-04-26 15:05 ` Glynn Clements 0 siblings, 2 replies; 6+ messages in thread From: Randi Botse @ 2011-04-26 10:33 UTC (permalink / raw) To: linux-c-programming Hi All, I want to ask malloc() behaviour, consider these codes; ... char *ptr = malloc(1); strcpy(ptr, "what"); puts(ptr); .... Confusingly, the strcpy() copied all bytes to ptr, but I just manage to allocate ptr only for 1 byte, I guess I will have segfault here, why this happen? why the string successfully copied into ptr? , is those code legal? Randi, ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: malloc question 2011-04-26 10:33 malloc question Randi Botse @ 2011-04-26 10:40 ` Daniel Baluta 2011-04-26 10:54 ` ratheesh kannoth 2011-04-26 15:05 ` Glynn Clements 1 sibling, 1 reply; 6+ messages in thread From: Daniel Baluta @ 2011-04-26 10:40 UTC (permalink / raw) To: Randi Botse; +Cc: linux-c-programming On Tue, Apr 26, 2011 at 1:33 PM, Randi Botse <nightdecoder@gmail.com> wrote: > Hi All, > > I want to ask malloc() behaviour, consider these codes; > > ... > char *ptr = malloc(1); > strcpy(ptr, "what"); > puts(ptr); > .... > > Confusingly, the strcpy() copied all bytes to ptr, but I just manage > to allocate ptr only for 1 byte, I guess I will have segfault here, > why this happen? why the string successfully copied into ptr? , is > those code legal? You didn't get segfault because you were lucky. Memory is allocated in multiples of page size (usually 4K). The memory after your allocated byte is valid in your case. thanks, Daniel. ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: malloc question 2011-04-26 10:40 ` Daniel Baluta @ 2011-04-26 10:54 ` ratheesh kannoth 2011-04-26 10:57 ` Daniel Baluta 0 siblings, 1 reply; 6+ messages in thread From: ratheesh kannoth @ 2011-04-26 10:54 UTC (permalink / raw) To: Daniel Baluta; +Cc: Randi Botse, linux-c-programming On Tue, Apr 26, 2011 at 4:10 PM, Daniel Baluta <daniel.baluta@gmail.com> wrote: > On Tue, Apr 26, 2011 at 1:33 PM, Randi Botse <nightdecoder@gmail.com> wrote: >> Hi All, >> >> I want to ask malloc() behaviour, consider these codes; >> >> ... >> char *ptr = malloc(1); >> strcpy(ptr, "what"); >> puts(ptr); >> .... >> >> Confusingly, the strcpy() copied all bytes to ptr, but I just manage >> to allocate ptr only for 1 byte, I guess I will have segfault here, >> why this happen? why the string successfully copied into ptr? , is >> those code legal? > > You didn't get segfault because you were lucky. > > Memory is allocated in multiples of page size (usually 4K). > The memory after your allocated byte is valid in your case. > > thanks, > Daniel. > -- U could read a little more about vm_page_struct. ( virtual address space to physical page ). Try below code also for more clarification..... char *ptr = malloc(1); strcpy(ptr, "what"); free(ptr); strcpy(ptr, "hell"); > To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: malloc question 2011-04-26 10:54 ` ratheesh kannoth @ 2011-04-26 10:57 ` Daniel Baluta 2011-04-26 11:50 ` ratheesh kannoth 0 siblings, 1 reply; 6+ messages in thread From: Daniel Baluta @ 2011-04-26 10:57 UTC (permalink / raw) To: ratheesh kannoth; +Cc: Randi Botse, linux-c-programming On Tue, Apr 26, 2011 at 1:54 PM, ratheesh kannoth <ratheesh.ksz@gmail.com> wrote: > On Tue, Apr 26, 2011 at 4:10 PM, Daniel Baluta <daniel.baluta@gmail.com> wrote: >> On Tue, Apr 26, 2011 at 1:33 PM, Randi Botse <nightdecoder@gmail.com> wrote: >>> Hi All, >>> >>> I want to ask malloc() behaviour, consider these codes; >>> >>> ... >>> char *ptr = malloc(1); >>> strcpy(ptr, "what"); >>> puts(ptr); >>> .... >>> >>> Confusingly, the strcpy() copied all bytes to ptr, but I just manage >>> to allocate ptr only for 1 byte, I guess I will have segfault here, >>> why this happen? why the string successfully copied into ptr? , is >>> those code legal? >> >> You didn't get segfault because you were lucky. >> >> Memory is allocated in multiples of page size (usually 4K). >> The memory after your allocated byte is valid in your case. >> >> thanks, >> Daniel. >> -- > > U could read a little more about vm_page_struct. ( virtual address > space to physical page ). Can you elaborate on this? Daniel. -- To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: malloc question 2011-04-26 10:57 ` Daniel Baluta @ 2011-04-26 11:50 ` ratheesh kannoth 0 siblings, 0 replies; 6+ messages in thread From: ratheesh kannoth @ 2011-04-26 11:50 UTC (permalink / raw) To: Daniel Baluta; +Cc: Randi Botse, linux-c-programming On Tue, Apr 26, 2011 at 4:27 PM, Daniel Baluta <daniel.baluta@gmail.com> wrote: > On Tue, Apr 26, 2011 at 1:54 PM, ratheesh kannoth > <ratheesh.ksz@gmail.com> wrote: >> On Tue, Apr 26, 2011 at 4:10 PM, Daniel Baluta <daniel.baluta@gmail.com> wrote: >>> On Tue, Apr 26, 2011 at 1:33 PM, Randi Botse <nightdecoder@gmail.com> wrote: >>>> Hi All, >>>> >>>> I want to ask malloc() behaviour, consider these codes; >>>> >>>> ... >>>> char *ptr = malloc(1); >>>> strcpy(ptr, "what"); >>>> puts(ptr); >>>> .... >>>> >>>> Confusingly, the strcpy() copied all bytes to ptr, but I just manage >>>> to allocate ptr only for 1 byte, I guess I will have segfault here, >>>> why this happen? why the string successfully copied into ptr? , is >>>> those code legal? >>> >>> You didn't get segfault because you were lucky. >>> >>> Memory is allocated in multiples of page size (usually 4K). >>> The memory after your allocated byte is valid in your case. >>> >>> thanks, >>> Daniel. >>> -- >> >> U could read a little more about vm_page_struct. ( virtual address >> space to physical page ). > > Can you elaborate on this? > > Daniel. > Daniel, Note: Pls read Linux Kernel internals 2.6. malloc() and free() works on virtual address space. malloc(1) - this allotes a virtual address space of 4k. strcpy(ptr, "what" ) - the 4k virtual address is mapped to a 4k page frame thru page fault exception. Now you have a valid virtual address of 4k. free(ptr) - tells OS that this virtual address space ( 4k ) can be reallotted if there is a need . But each program has a virtual address space of 3GB ( 32 bit ,4GB minus 1GB (kernel) ). SO ptr is a valid pointer unless 1) vitual address space is realloted. 2) page frame is realloted for some other page ( LRU algorithm ) so if ptr is valid , you could do following *(ptr ++ ), *ptr ... etc . -ratheesh -- To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: malloc question 2011-04-26 10:33 malloc question Randi Botse 2011-04-26 10:40 ` Daniel Baluta @ 2011-04-26 15:05 ` Glynn Clements 1 sibling, 0 replies; 6+ messages in thread From: Glynn Clements @ 2011-04-26 15:05 UTC (permalink / raw) To: Randi Botse; +Cc: linux-c-programming Randi Botse wrote: > I want to ask malloc() behaviour, consider these codes; > > ... > char *ptr = malloc(1); > strcpy(ptr, "what"); > puts(ptr); > .... > > Confusingly, the strcpy() copied all bytes to ptr, but I just manage > to allocate ptr only for 1 byte, I guess I will have segfault here, > why this happen? why the string successfully copied into ptr? , libc typically requests memory from the kernel in large chunks, then uses portions of this memory to satisfy malloc() requests. The memory following the allocated block is likely to be valid (i.e. accessing it won't cause a segfault), but it may have been allocated to something else, or it may be allocated to something else in the future. > is those code legal? No. Any memory following the one byte block which you requested will be deemed available for use by other parts of the code. If you modify memory immediately beyond the end of a malloc()d block, the most common result is corruption of the heap's internal data, resulting in a subsequent malloc(), realloc(), free() etc call crashing. For such a small string, you'll typically get away with it, as any practical malloc() implementation will align blocks to at least a word boundary and probably more (e.g. GNU libc uses 16-byte boundaries to ensure that a "long double" won't straddle a page boundary), so there will be some padding between the end of the allocated block and any following block. -- Glynn Clements <glynn@gclements.plus.com> ^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2011-04-26 15:05 UTC | newest] Thread overview: 6+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2011-04-26 10:33 malloc question Randi Botse 2011-04-26 10:40 ` Daniel Baluta 2011-04-26 10:54 ` ratheesh kannoth 2011-04-26 10:57 ` Daniel Baluta 2011-04-26 11:50 ` ratheesh kannoth 2011-04-26 15:05 ` Glynn Clements
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