Error: Control may reach end of non-void function - cat function - c

Question

char *wcat(char *str, size_t n, FILE *fp){
if (fp == NULL) {
printf("wcat cannot open file\n");
fclose(fp);
perror("File cannot be opened");
return NULL;
exit(1);
}else{
if ((str = fgets(str,n,fp)) != NULL){
printf("%s",str);
return str;
exit(0);
}
}
}
Terminal:
gcc -o wcat wcat.c
Error: wcat.c:36:1: warning: control may reach end of non-void function [-Wreturn-type]
The fp already equals fopen(...).
I am not sure why this is happening. I wanted to create this wcat file to work like:
./wcat file1.c file2.c

Answer 1

Your else clause also needs an else, or at least a default return. Your ifs don't cover every possible case. The warning says exactly what the problem is.
char *wcat(char *str, size_t n, FILE *fp){
if (fp == NULL) {
printf("wcat cannot open file\n");
fclose(fp);
perror("File cannot be opened");
return NULL;
//exit(1);
}
else if (fgets(str,n,fp))
{
printf("%s",str);
return str;
// exit(0);
}
return NULL; /// or whatever it is that you expect to happen here.
}
Neither of the calls to exit makes sense. They'll never be executed. It looks like you're trying to use those to return some sort of success/failure flag, but:
they never execute because they follow a return
exit terminates the program.
The parameter is passed back to the calling process. In my experience, this is basically never used unless you're writing a console utility.
Do you really understand what exit does? And return?
There's a lot wrong with this one. I suggest stepping through in your debugger.

Answer 2

the following changes, with comments, is the correct way to handle this function:
char *wcat(char *str, size_t n, FILE *fp){
// note: this check should have been handled
// where/when 'fopen()' was called
if (fp == NULL) {
// this changes the value in 'errno'
// so 'perror() will not display the right message
//printf("wcat cannot open file\n");
//fclose(fp); // <-- never open'd so cannot close
perror("File cannot be opened");
return NULL;
//exit(1); // <-- will never be executed
}else{
if (fgets(str,n,fp)){
printf("%s",str);
return str;
//exit(0); // <-- will never be executed
}
return NULL; // <-- need to handle when 'fgets()' fails
}
}
after applying the corrections AND moving the check for a failure of 'fopen()', the code would look like the following:
char *wcat(char *str, size_t n, FILE *fp)
{
if ( fgets( str, n, fp ) )
{
printf("%s",str);
return str;
}
return NULL;
}