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);
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