Question

My goal is to gather input and open files based on that input.

FILE* 
open_input_file (char* fileName) //opens source file to be read
{ 
 return fopen(fileName, "r");
}

In an earlier function, I collect input from the user and save it to fileName. When I debug the program, it tells me fopen is returning NULL. That’s not what I want, and I’m not sure where the problem is.

int main(void)
{    FILE* inFile = NULL;
     char infileName[32] = {'\0'};
     gather_input(infileName); // infileName is an output parameter for this
     inFile = open_input_file(infileName);
}

I don’t know what the problem is. Any thoughts?

Answer 1

If fopen returns NULL, the open failed. errno will hold the failure code and strerror(errno) will return a short description of why the open failed.

#include <errno.h>
#include <string.h>

...

int main(void)
{    FILE* inFile = NULL;
     char infileName[32] = {'\0'};
     gather_input(infileName); // infileName is an output parameter for this
     if (!(inFile = open_input_file(infileName))) {
         fprintf(stderr, "Error opening '%s': %s\n", 
                 infileName, strerror(errno));
     } else {
         // open successful
         ...
     }
}

Off-topic

gather_input better make sure infileName is null-terminated to prevent buffer overflows. The simplest way to do this is to define the size of the file name buffer as a macro and set the last character to 0.

#define FILENAMELEN 32
void gather_input(char infileName[]) {
    ...
    infileName[FILENAMELEN-1]=0;
}

 int main(void)
{    FILE* inFile = NULL;
     char infileName[FILENAMELEN] = {'\0'};

This isn’t very flexible. You could instead pass the size of the file name buffer into gather_input.

#define LENGTH(a) (sizeof(a) / sizeof(a[0]))
void gather_input(char infileName[], size_t len) {
    ...
    infileName[len-1]=0;
}

 int main(void)
{    FILE* inFile = NULL;
     char infileName[32] = {'\0'};
     gather_input(infileName, LENGTH(infileName)); // infileName is an output parameter for this

An alternative to setting the last character, if using standard string manipulation functions, is to use the strl* functions (strlcpy and strlcat) rather than their unbounded cousins. If you aren’t using strl*, you should be using strncpy and strncat.