Question

Just got query regarding c malloc() function. I am read()ing x number of bytes from a file to get lenght of filename, like ‘ read(file, &namelen, sizeof(unsigned char)); ‘ .

The variable namelen is a type unsigned char and was written into file as that type (1 byte). Now namelen has the lenght of filename ie namelen=8 if file name was ‘data.txt’, plus extra /0 at end, that working fine.

Now I have a structure recording file info, ie filename, filelenght, content size etc.

   struct fileinfo
        {
              char *name;
              ...... other variable like size etc

        };
struct fileinfo *files;

Question: I want to make that files.name variable the size of namelen ie 8 so I can successfully write the filename into it, like ‘ files[i].name = malloc(namelen) ‘

However, I dont want it to be malloc(sizeof(namelen)) as that would make it file.name[1] as the size of its type unsigned char. I want it to be the value thats stored inside variable &namelen ie 8 so file.name[8] so data.txt can be read() from file as 8 bytes and written straight into file.name[8?

Is there a way to do this my current code is this and returns 4 not 8

files[i].name = malloc(namelen);
//strlen(files[i].name) - returns 4

//perhaps something like
malloc(sizeof(&namelen)) but does not work

Thanks for any suggestions

Have tried suggested suggestions guys, but I now get a segmentation fault error using:

printf("\nsizeofnamelen=%x\n",namelen); //gives 8 for data.txt
files[i].name = malloc(namelen + 1);
read(file, &files[i].name, namelen);
int len=strlen(files[i].name); printf("\nnamelen=%d",len);
printf("\nname=%s\n",files[i].name);  

When I try to open() file with that files[i].name variable it wont open so the data does not appear to be getting written inside the read() &files[i].name and strlen() causes segemntation error as well as trying to print the filename

Answer 1

1. a string is strlen(name) + 1 bytes long (because of the terminating \0).
2. if namelen = strlen("data.txt") (i.e. 8) then you must allocate files[i].name = malloc(namelen + 1)
3. any memory you allocate using malloc() is undefined. You cannot perform strlen() on memory you just allocated – you must set the memory you’ve allocated first
4. only after you allocate the memory can you use the memory, i.e. strncpy( files[i].name, "data.txt", namelen + 1 ) and only then can you strlen( files[i].name )