Question

I’m trying to open a file using an absolute path. I’m currently doing this in Windows but will also need this to work in a Unix environement.

The path is composed using an environment variable as shown below.

char *dataPath = getenv ("DATA");
strcat(dataPath, "/index");
char indexPath[255] = {0};
strcat(indexPath, dataPath);
strcat(indexPath, "/index.tbl");
printf("Path: %s\n", indexPath);
ip = fopen(indexPath, "r");

This code prints out C:\Data/index/index.tbl but the application fails to open the file.

What am I doing wrong?

Answer 1

This is incorrect:

char *dataPath = getenv ("DATA");
strcat(dataPath, "/index"); 

and may be overwriting a part of the process’ environment block. From man getenv:

As typically implemented, getenv() returns a pointer to a string within the environment list. The caller must take care not to modify this string, since that would change the environment of the process.

You need to allocate a buffer large enough to contain the full path and copy in getenv("DATA") and then strcat() or sprintf():

const char* dataPath = getenv("DATA");
char* fullPath = 0;
if (dataPath)
{
    /* 6 for "/index" and 1 for terminating null character. */
    fullPath = malloc(strlen(dataPath) + 6 + 1);
    if (fullPath)
    {
        sprintf(fullPath, "%s/index", dataPath);

        free(fullPath);
    }
}