Double Ended Queue, Call by Reference (Create Header) - c

Question

i have a simple problem with my c code .... I want to call the variable with call by reference.
I have this function:
void createHeader(HEADER *yourHeader)
{
yourHeader = malloc(sizeof(HEADER));
if (yourHeader == NULL)
printError();
yourHeader->first = NULL;
yourHeader->last = NULL;
yourHeader->length = 0;
}
My main-call:
createHeader(&header);
if (header == NULL)
exit(-1);
Now I have the following problem: "The program '[6044] My-C-Project.exe' has exited with code -1 (0xffffffff)", so that means my function createHeader doens't work ... Can someone please explain what's the mistake is?
Regards
Alex

Answer 1

Change the function to:
HEADER *createHeader(void)
{
HEADER *yourHeader = malloc(sizeof(HEADER));
if (yourHeader == NULL)
printErrorAndExit();
yourHeader->first = NULL;
yourHeader->last = NULL;
yourHeader->length = 0;
return yourHeader;
}
Use:
Header *header = createHeader();
if (header == NULL)
exit(-1);
Alternatively, use a pointer to a pointer as the argument:
void createHeader(HEADER **yourHeader)
{
*yourHeader = malloc(sizeof(HEADER));
if (*yourHeader == NULL)
printError();
(*yourHeader)->first = NULL;
(*yourHeader)->last = NULL;
(*yourHeader)->length = 0;
}
Use:
Header *header = NULL;
createHeader(&header);
if (header == NULL)
exit(EXIT_FAILURE);
These two techniques can be applied to most 'allocate pointer' operations — creating linked lists is a particularly common variation. Note that fopen() and fclose() match the first model; the allocator (fopen()) takes some arguments to tell it what to do and the function returns a pointer to a structure that can be used and eventually passed to fclose() for release.
Note that printErrorAndExit() must not return. Or, if it does, the functions have to be revised:
HEADER *createHeader(void)
{
HEADER *yourHeader = malloc(sizeof(HEADER));
if (yourHeader == NULL)
printError();
else
{
yourHeader->first = NULL;
yourHeader->last = NULL;
yourHeader->length = 0;
}
return yourHeader;
}
It might be better for the function not to print any error message — that limits its reusability, in general.
HEADER *createHeader(void)
{
HEADER *yourHeader = malloc(sizeof(HEADER));
if (yourHeader != NULL)
{
yourHeader->first = NULL;
yourHeader->last = NULL;
yourHeader->length = 0;
}
return yourHeader;
}
The alternative design should probably return a status instead of nothing, so you can test the function result directly:
int createHeader(HEADER **yourHeader)
{
int rc = 0;
*yourHeader = malloc(sizeof(HEADER));
if (*yourHeader == NULL)
rc = -1; /* Or perhaps errno */
else
{
(*yourHeader)->first = NULL;
(*yourHeader)->last = NULL;
(*yourHeader)->length = 0;
}
return rc;
}
Use:
if (createHeader(&header) != 0)
…report error and bail out…
Using 0 for success and non-zero for failure is very common (many Unix system calls fall into this category). Very often, a negative return value indicates failure, especially if a positive value can be used for success — think open(). But the POSIX pthread functions frequently return a positive error number on failure and zero on success.