Get partially overwritten by the copying. You don't want the two blocks to overlap because one of them could =27492= Source and destination overlap in memcpy(0xbffff294, 0xbffff280, 21) The POSIX standards have wording along the lines 'If copying takes placeīetween objects that overlap, the behavior is undefined.' Therefore, Memory block to another (or something similar): The following C library functions copy some data from one

Note that the firstĤ.2.6. Overlapping source and destination blocks because the program has (a) written uninitialised junkįrom the heap block to the standard output, and (b) passed an Syscall param exit(error_code) contains uninitialised byte(s)Īt 0x25A21B44: _GI_exit (in /lib/tls/libc-2.3.3.so) Syscall param write(buf) points to uninitialised byte(s)Īt 0x25A48723: _write_nocancel (in /lib/tls/libc-2.3.3.so)īy 0x259AFAD3: _libc_start_main (in /lib/tls/libc-2.3.3.so)īy 0x8048348: (within /auto/homes/njn25/grind/head4/a.out)Īddress 0x25AB8028 is 0 bytes inside a block of size 10 alloc'dĪt 0x259852B0: malloc (vg_replace_malloc.c:130) Here's an example of two system calls with invalid parameters: Precisely reflect any changes in memory state caused by the system Your program, Memcheck checks that the entire buffer is addressableĪlso, if the system call needs to write to a user-suppliedīuffer, Memcheck checks that the buffer is addressable.Īfter the system call, Memcheck updates its tracked information to It checks all the direct parameters themselves, whether they areĪlso, if a system call needs to read from a buffer provided by Sources of uninitialised data tend to be: X so it can turn it into the corresponding ASCII string,Īnd it is at this point that Memcheck complains. The value being passed to _IO_printf and thence to Issued only when your program attempts to make use of uninitialisedĭata in a way that might affect your program's externally-visible behaviour. Memcheck observes thisĪnd keeps track of the data, but does not complain. Junk (uninitialised) data as much as it likes. It is important to understand that your program can copy around Reported when running the following small program: Here, the undefined value is used somewhere inside the Uses a value which hasn't been initialised - in other words, is In this particular example, reading junk on the stack isĬonditional jump or move depends on uninitialised value(s)Īn uninitialised-value use error is reported when your program Result in a segmentation fault, you program will still suffer the sameįate - but you will get a message from Memcheck immediately prior to So, if your program makes an access which normally would Note that Memcheck only tells you that your program is about toĪccess memory at an illegal address. In this particular case it's probably caused by GCC generating invalidĬode, a known bug in some ancient versions of GCC. Stack address - it is below the stack pointer and that isn't allowed. The address is on the stack, but, for some reason, this is not a valid In this example, Memcheck can't identify the address. If you use the -read-var-info optionīut may give a more detailed description of any illegal address.

The end of a heap block, a common result of off-by-one-errors inĪrray subscripting, you'll be informed of this fact, and also where theīlock was allocated. Likewise, if it should turn out to be just off Which has already been freed, you'll be informed of this, and also where Memcheck tries to establish what the illegal address might relate Same library, called from line 326 of qpngio.cpp, Library libpng.so.2.1.0.9, which was called from somewhere else in the In this example, the program did aĤ-byte read at address 0xBFFFF0E0, somewhere within the system-supplied This happens when your program reads or writes memory at a place

You don't have to, though, since MemcheckĤ.2.1. Illegal read / Illegal write errorsĪt 0x40F6BBCC: (within /usr/lib/libpng.so.2.1.0.9)īy 0x40F6B804: (within /usr/lib/libpng.so.2.1.0.9)īy 0x40B07FF4: read_png_image(QImageIO *) (kernel/qpngio.cpp:326)īy 0x40AC751B: QImageIO::read() (kernel/qimage.cpp:3621)Īddress 0xBFFFF0E0 is not stack'd, malloc'd or free'd To use this tool, you may specify -tool=memcheck

Building and installing the wrappers 4.9.2. Debugging MPI Parallel Programs with Valgrind 4.9.1. Memory Pools: describing and working with custom allocators 4.9. Details of Memcheck's checking machinery 4.5.1. Overlapping source and destination blocks 4.2.7. When a heap block is freed with an inappropriate deallocationįunction 4.2.6. Use of uninitialised or unaddressable values in systemĬalls 4.2.4. Illegal read / Illegal write errors 4.2.2. Explanation of error messages from Memcheck 4.2.1.

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