hi
i want to make a function that will take for input a number n and a character b
the function will return a string that contains n times the character b
i tried this but it doesnt work
after some thinking i tried dynamic memory allocationCode:char *s(int n, char b) { int i; char r[n]; for (i=0;i<n;i++){ r[i] = b; } return r; }
and it workedCode:char *s(int n, char b) { int i; char *r; r = (char *)malloc(n*sizeof(char)); for (i=0;i<n;i++){ r[i] = b; } return r; }
can anyone tell me why?
ty
Because in the first example, you're returning a pointer to a local variable.
When the function returns, variable goes out of scope, and you're left with a pointer to junk.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
Because
is a local stack variable. That means it is part of function s()'s stack, and only accessible locally, that is, inside s(), because each function has it's own stack and cannot access any others.Code:char r[n];
Malloc() allocates heap memory, which there is only one heap for the entire program and it is globally accessible.
When you return a pointer to a local variable, it is useless because the memory addresses used by a function call (a "stack frame") are recycled when the function returns (or else they do not match the addressing used outside of the frame, I'm not sure).
C programming resources:
GNU C Function and Macro Index -- glibc reference manual
The C Book -- nice online learner guide
Current ISO draft standard
CCAN -- new CPAN like open source library repository
3 (different) GNU debugger tutorials: #1 -- #2 -- #3
cpwiki -- our wiki on sourceforge
thanks for both of you, I now understand it
