在gdb中使用print命令是否安全？

Question

I know that gdb allows to print expressions via print <exp> command. As I understand result of such exression is saved to gdb variables (which can be accesed with $N - N - number of variable . Is it safe to print result of execution of functions? Suppose I have functions:

const char* foo(){ return "foo";}

char* foo2(){static char b[10]; /*change b*/; return b}

char* foo3(){char* b; /*Create string on heap*/ return b;}

How does gdb treat results of print foo(), print foo2(), print foo3() ? Will it release memory of variables and can it lead to errors?

Answer 1

Gdb does nothing different than calling the function and catch the result. So if the function have side effects like allocating memory, the memory is still allocated after the function call from gdb.

So for you foo3() function the memory will be allocated on the heap until you will free it ( manually ). But this is not an "unsafe" behavior, it simply produces memory leaks. If that is acceptable in your debug environment, there is no problem. If you allocate tons of memory but never free it, you will run in "out of memory" problems. On an small embedded system this is potentially a problem.

gdb itself will never release any memory. Maybe you want to call two functions where the first allocates memory and the second needs to use it. So gdb is not allowed to decide how to handle allocated storage. As a result gdb will not release memory!

If you open files, sockets or use semaphores, you have the same effect. The file will not be closed or a semaphore will not released. You as the user have to know what you call and what side effects your call will produce in the given environment, not only with memory but with all other resources and states in your prog.

Answer 2

gdb cannot know what is in your functions so cannot decide to free a returned pointer. If you're returning a literal or a static variable, there's no need to free the memory, since no allocation is performed (and it would be a fault to free it).

The third example, on the other hand, allocates some memory, so it's a kind of memory leak, yes, unless you're able to call another function from gdb like free on the gdb variable.