Naive Local Pointer Update in C

A programmer wanted to implement a C method with an output parameter, that is, a parameter used to pass a value back from the method without using (or used in addition to) the return value of the method.

The programmer mistakenly thought that C supports passing arguments by reference, and implemented createSomethingFromCode with an (supposedly) output parameter ‘error_if_fail’ that passes back an explanation error when it fails to achieve its task:

1 #include <stdio.h>

2 #include “something.h”

3 #include “error.h”
4
5 Something* createSomethingFromCode (int code, Error *error_if_fail)
6 {
7 /* Try to create Something here... */

8 if (/* cannot create Something */) {
9 error_if_fail = createError(“Failed to create Something from code %d”, code);

10 return NULL;

11 }

12 return something;
13 }
14
15 int main (int argc, char *argv[])
16 {
17 int INVALID_CODE = 0xdeadbeef;
18 Error *error = NULL;
19 Something* something = createSomethingFromCode(INVALID_CODE, error);
20 if (something == NULL) {
21 printf("Error: %s", error->message);

22 destroyError(error);

23 return 1;
24 }

25 /* else, use something... */
26 return 0;
27 }

This program gets a null pointer exception whenever executes line 21, because error_if_fail above is not an output parameter neither it reflects the value assigned to it by createSomethingFromCode() after the method returns. This is because C does not support passing arguments by reference and because method parameters are local variables when accessed from the method body.

C only supports pass-by-value
C simulates pass-by-reference by passing by value pointer values

On createSomethingFromCode invocation the argument given is copied into variable error_if_fail. Updates to variable error_if_fail inside createSomethingFromCode body will only update the copy, not the original variable from where the copy obtained its initial value. Assigning error_if_fail on line 9 assigns a value to createSomethingFromCode’s error_if_fail local variable, not to main’s error variable.

A proper implementation of the intended output parameter would be:

1 #include <stdio.h>

2 #include “something.h”

3 #include “error.h”
4
5 Something* createSomethingFromCode (int code, Error **error_if_fail)
6 {
7 /* Try to create Something here... */

8 if (/* cannot create Something */ && error != NULL) {
9 *error_if_fail = createError(“Failed to create Something from code %d”, code);

10 return NULL;

11 }

12 return something;
13 }
14
15 int main (int argc, char *argv[])
16 {
17 int INVALID_CODE = 0xdeadbeef;
18 Error *error = NULL;
19 Something* something = createSomethingFromCode(INVALID_CODE, &error);
20 if (something == NULL) {
21 printf("Error: %s", error->message);

22 destroyError(error);

23 return 1;
24 }

25 /* else, use something... */
26 return 0;
27 }