How to read const char * from files - c

Question

I need to read a filename from a text file.
Then I have to use it as an argument for fopen .
Consequently , I need to read it as a const char* because this is the type fopen accepts as a first argument. How do I do this?
I tried something like:
FILE *a;
a=fopen("a.txt","r");
const char *filename
fgets(filename,100,a);
image=fopen(filename,"rb");
Something is be wrong as I receive a segmentation fault when I do this. I think that the variable filename is not well-received by the fopen function.

Answer 1

You may have seen that fopen() takes an argument of type (const char *), but you need to be able to modify your string before you pass it to the function. Also, you need to allocate space for your string, and you might consider allocating space for more than 100 chars. The stdio.h header file contains the macro FILENAME_MAX, which expands to an integer constant of the correct size for an array that will hold the longest possible file name string on a system. For example, you can use char filename[FILENAME_MAX];. When the identifier filename is encountered in the call to fopen(), it decays to a pointer to char, which is converted to a pointer to const char, as per the function prototype.
Furthermore, fgets() keeps the newline when if fetches a line of text, so you will need to remove that. You should check the result of fgets(), as it returns a NULL pointer in the event of an error or if it is called at end-of-file; otherwise it returns a pointer to the first char in filename[].
FILE *a;
a=fopen("a.txt","r");
char filename[FILENAME_MAX];
char *ch;
ch = fgets(filename,FILENAME_MAX,a);
/* Remove trailing newline */
if (ch) {
while (*ch != '\0' && *ch != '\n') {
++ch;
}
if (*ch == '\n') { // replace '\n' with '\0'
*ch = '\0';
}
image=fopen(filename,"rb");
}

Answer 2

First, your seg fault likely comes from trying to use memory that you do not own. i.e. by creating the variable:
const char *filename;
And not giving it any memory ([c][m]alloc) before trying to use it.
Regarding your statement:
Consequently , i need to read it as a const char*.
Not necessarily.
The first argument of the fopen prototype: "FILE *fopen(const char *filename, const char *mode)" simply guarantees that the argument passed in that position will be treated as a const char * within the fopen function.
The argument can be passed using a simple char *, eg. either of these forms:
char *filename = {"somefilename.txt"};
or
char filename[80];
strcpy(filename, "somefilename.txt");
And, as mentioned in comments, and other answers, remove the newline character, \n before passing as an argument.