Return an array of files - c

Question

I am trying to create a function that returns an array of files but I couldn't find a way to access that array outside of the function.
Here's what I tried:
FILE* *arquivoescrita(int k) //gerar nome de arquivo
{
char filename[sizeof("file1000000000000.txt")];
static FILE* arquivos[2];
for (int i = k; i < k+2; ++i)
{
sprintf(filename, "file%d.txt", i);
arquivos[i] = fopen(filename,"r");
}
return arquivos;
}
And I'm calling the function inside main like this:
FILE* psaidas[2];
psaidas = arquivoescrita(0);
And the error I get is
error: array type 'FILE *[2]' is not assignable
psaidas = arquivoescrita(0);
How can I access this kind of array?

Answer 1

You are returning a pointer to the static array. That's fine. But C will not magically copy the array for you into another location. You can change your declaration:
FILE**psaidas;
psaidas = arquivoescrita(0);
And this will work. But at this point, psaidas will be pointing in to your static FILE * array. So if you call arquivoescrita again, it will be overwritten.
If that works - and it probably will work - then proceed. If you have to worry about calling arquivoescrita multiple times before you finish with the files, then you must either use malloc or calloc to dynamically allocate your result within arquivoescrita or you may choose to copy the array after you receive it:
FILE * psaidas[2];
memcpy(psaidas, arquivoescrita(0), sizeof(psaidas));

Answer 2

There is one issue in returning a FILE ** array of open file stream, you need to know how many are actually open. You have a couple of choices, (1) pass a pointer to the function and update the pointer to reflect the number of files successfully opened, or (2) what probably makes more sense, is to pass a FILE * array as a parameter to the function and return the number of files successfully opened to be use as an index back in the calling function. (you could also simply check the stream before attempting to read from it)
For example, if you declared file pointer array FILE *fparray[MAXF] = {NULL}; in main, you could simply pass that as a parameter to be filled in your function. e.g.
nfiles = openmulti (fparray, 0, MAXF);
Your function would then take the array, just as any other type of array, and fill the elements with open and validated FILE * elements:
int openmulti (FILE **fp, size_t k, size_t n)
{
char fname[MAXFN] = "";
size_t i, idx = 0;
for (i = k; i < n; i++) {
sprintf (fname, "file%zu.txt", i);
fp[idx] = fopen (fname, "r");
if (!fp[idx])
continue;
idx++;
}
return (int)idx;
}
(the beginning number file#.txt is passed as k and then number of files to open is passed as n)
A short example opening file0.txt and file1.txt in the function for reading back in the calling function could be:
#include <stdio.h>
enum { MAXF = 2, MAXFN = 256 };
int openmulti (FILE **fp, size_t k, size_t n);
int main (void) {
FILE *fparray[MAXF] = {NULL};
size_t i, nfiles;
nfiles = openmulti (fparray, 0, MAXF); /* populate array */
if (nfiles < MAXF) /* validate number of open files */
fprintf (stderr, "warning: '%zu' of %d files open.\n",
nfiles, MAXF);
for (i = 0; i < nfiles; i++) { /* read/output file contents */
printf ("\ncontents of 'farray[%zu]'\n\n", i);
char buf[MAXFN] = "";
while (fgets (buf, MAXFN, fparray[i]))
printf ("%s", buf);
fclose (fparray[i]); /* close file after reading */
}
return 0;
}
int openmulti (FILE **fp, size_t k, size_t n)
{
char fname[MAXFN] = "";
size_t i, idx = 0;
for (i = k; i < n; i++) {
sprintf (fname, "file%zu.txt", i);
fp[idx] = fopen (fname, "r");
if (!fp[idx])
continue;
idx++;
}
return (int)idx;
}
Example Input Files
$ cat file0.txt
1.
2. hello -- file0.txt
3.
$ cat file1.txt
1.
2. hello -- file1.txt
3.
Example Use/Output
$ ./bin/filemultiopen
contents of 'farray[0]'
1.
2. hello -- file0.txt
3.
contents of 'farray[1]'
1.
2. hello -- file1.txt
3.
Unless there is some critical need to return FILE **, it seems to make things a bit more straight forward to pass an array to the function and return the number of files successfully opened. Both ways are possible, whatever fits your needs.
Let me know if you have any additional questions.