Hi Avinash, On Wed, Aug 31, 2011 at 06:12:41PM +0530, Avinash Sonawane wrote: > Respected Sir, > I am using gcc version 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4) > > I have one doubt regarding malloc () in gcc. GCC is right whatever happens, your code is illegal C (so you get "undefined behaviour" :-)). > When I tried to execute the program given below, it got executed. > > But since 256 = 1 0000 0000 (9 bits) , How malloc can store it at 'p' > since I have allocate only 1 byte of memory (8 bits of memory) ? > Please guide me. You are misunderstanding C. For general C-questions there are more adequate mailing lists. According to the C language, "256" is understood as an integer number of type "int" with the value 256. The memory size (=number of bits needed & used) for an "int" is ABSOLUTELY INDEPENDENT of its value -- "1" needs as many bits as "256" (simply starting with some "0"-bits). The precise size of an "int" depends on your hardware, the only limitation is that it has to have at least 2 bytes (maybe that limit got increased with newer C-standards?). So: 256 needs 16 bits or more. When you call now "malloc(1)", this library function returns you a memory address at which you have at least 1 byte "free" which is reserverd for you and won't be used by anybody else (if all code is written correctly). You write now 2 or more bytes at this address. This means: - the first byte is fine. - the second and all following bytes are written in a memory range, which MIGHT BE used for something else. So many different things can happen: - nobody uses this region. Then the code runs fine (but nobody guarantees it will run correctly when you restart it). - the implementation of malloc on your machine always reserves MORE memory (e.g. 4 bytes) -- then your code works always ON YOUR MACHINE, you don't know what will happen on other machines - another variable of your program is stored there: then that variable is overwritten / might overwrite p - your operating system does not allow your program to access this memory address (e.g. because it was distributed to another user). Then your code get's killed by the operating system - ...... Axel
