I want to pass a char pointer to a function and have it set the value and pass it back. Here is my attempt but the printf prints garbage, where am I going wrong?
int a() {
char *p;
b(p);
printf("%s", p);
return 0;
}
int b(char * ptr) {
ptr = "test string";
return 0;
}
Pass a reference to the pointer.
int b(char **ptr)
{
*ptr = "Print statement";
//your code
}
//call
b(&p);
char ** to const char** because it is a constant string.
"Print statement" is a constant string and will most likely be placed inside the read-only section of the executable. Modifying it will result in a crash. const char * ''prevents'' the modification of the string.If you want to set the value of the pointer and pass it back, then b() needs to return a pointer:
int a(void) {
char *p = NULL;
p = b(p);
printf("%s", p);
return 0;
}
char * b(char * ptr) {
ptr = "test string";
return ptr;
}
Here p is initialized to NULL to avoid undefined behavior from passing an uninitialized value to a function. You could also initialize p to another string literal. Inside b, ptr is a copy of the pointer p that was passed to b. When you reassign the value stored in ptr to the address of the string literal "test string", the original pointer p is unchanged. By passing ptr back to the calling function, and reassigning p to the return value of b, the calling function can use the updated value.
As @M.M points out in the comments, this is somewhat redundant. The function b() could instead declare a pointer of its own, initialized to a string literal:
int a(void) {
char *p = b();
printf("%s", p);
return 0;
}
char * b(void) {
char *ptr = "test string";
return ptr;
}
b a return type of char *. But in this code the function parameter is redundant and should be removed. (In fact b(p) causes undefined behaviour by using an uninitialized pointer value).