Question 12.27

comp.lang.c FAQ list · Question 12.27

Q: I wrote this routine which is supposed to open a file:

myfopen(char *filename, FILE *fp)
 {
  fp = fopen(filename, "r");
 }

But when I call it like this:

FILE *infp;
  myfopen("filename.dat", infp);

the infp variable in the caller doesn't get set properly.

---

A: Functions in C always receive copies of their arguments, so a function can never ``return'' a value to the caller by assigning to an argument. See question 4.8.
For this example, one fix is to change myfopen to return a FILE *:

FILE *myfopen(char *filename)
 {
  FILE *fp = fopen(filename, "r");
  return fp;
 }

and call it like this:

FILE *infp;
  infp = myfopen("filename.dat");

Alternatively, have myfopen accept a pointer to a FILE * (a pointer-to-pointer-to-FILE):

myfopen(char *filename, FILE **fpp)
 {
  FILE *fp = fopen(filename, "r");
  *fpp = fp;
 }

and call it like this:

FILE *infp;
  myfopen("filename.dat", &infp);