Pass by address, scanf, de-referencing - c

Question

I have written a program employing the following concept:
I create an integer x pass by address to a function, along with a filename, said function opens file if available, scans the first line and sets the value that pX points to equal to the scanned line.
Only it's not working, and I don't see what I'm doing wrong. As far as I can tell the code below is generally how one would accomplish it, but I'm not sure if I'm not using scanf() correctly with the pointer or what.
void foo() {
char input[20] = "test.txt";
int x = 1;
bar(input, &x);
}
void bar(char *fileName, int *pX) {
FILE *fp = fopen(fileName, "r");
char *buffer = malloc(15 * sizeof(int));
fgets(buffer, 15, fp);
scanf(buffer, "%d", *pX);
free(buffer);
fclose(fp);
}

Answer 1

Change line :
scanf(buffer, "%d", *pX);
to :
sscanf(buffer, "%d", pX);
You need function sscanf for what you are trying to do.
Both scanf and sscanf take a pointer as an argument. pX is of type int *, therefore a pointer to an int and should work for you. What you pass, *pX, is the contents of this pointer, in other words an int.
Also, change line :
char *buffer = malloc(15 * sizeof(int));
to :
char *buffer = malloc(15 * sizeof(char));
or simply :
char *buffer = malloc(15);
and always check the result of malloc :
if (buffer == NULL){
...
}

Answer 2

First of all, there's no pass-by-reference in C, function parameters are all passed by value. By passing a pointer-to-data, we make the simulation of the achieving the same effect as pass-by-reference, but that does not mean C has any pass-by-reference concept.
That said, the problem seems to be
scanf(buffer, "%d", *pX);
^^^^
where
the current syntax is invalid and invokes undefined behavior. Probably you need sscanf().
px is already a pointer to int. Passing px will be correct and suffice.
Moral of the story: enable compiler warnings and pay heed to them. They are there for a reason. With proper warnings enabled, you should see something like
warning: format %d expects argument of type int *, but argument 3 has type int [-Wformat=]
Finally,
Always check the return value of fopen() for success before using the file pointer.
Check for the return value of scanf() to ensure successful scanning.
Check for the return value of fgets() to ensure success
...basically, check return value of all library calls to make sure they worked as expected.

Answer 3

You use scanf() incorrectly: either use scanf directly to parse standard input or use sscanf() to parse the string read by fgets(). Furthermore, pX is already a pointer to int, which is what sscanf() expects to store the int value it converts, pass it directly: sscanf(buffer, "%d", pX);
Here is a modified version:
int bar(const char *fileName, int *pX) {
char buffer[15];
FILE *fp = fopen(fileName, "r");
int success = 0;
if (fp != NULL) {
fgets(buffer, sizeof buffer, fp);
if (sscanf(buffer, "%d", pX) == 1)
success = 1;
fclose(fp);
}
return success;
}
void foo(void) {
int x = 1;
bar("test.txt", &x);
/* do something with x */
}
Notes:
allocating buf is unnecessary, just make it a local array with automatic storage.
char *buffer = malloc(15 * sizeof(int)); is incorrect: you allocate space for 15 int instead of 15 characters, which have a size of 1 by definition. Use the size of the destination type to avoid any inconsistencies:
char *buffer = malloc(15 * sizeof(*buffer));
always check the return value of malloc() to avoid potential undefined behavior.
reading from fp without checking if fopen succeeded has potential undefined behavior.
the contents of the array pointed to by filename is not modified, make it a const char *.
it may be useful for bar to return a success indicator.
enable more warnings at compile time: gcc -Wall -Wextra -Werror or clang -Weverything -Werror might have caught the mistakes in scanf.