Question

I am trying to read Data from a Text file & storing it inside a structure having one char pointer & an int variable.
During fetching data from file I know that there will be one string to fetch & one integer value.
I also know the position form where I have to start fetching.
What I don’t know is size of the string.
So, how can I allocate memory for that String.
Sample code is here :

struct filevalue
{
 char *string;
 int integer;
} value;    
fseek(ptr,18,SEEK_SET);//seeking from start of file to position from where I get String
fscanf(ptr,"%s",value.string);//ptr is file pointer

fseek(ptr,21,SEEK_CUR);//Now seeking from current position
fscanf(ptr,"%d",value.integer);

Thanks in advance for your help.

Answer 1

Either

1. malloc the maximum possible length
2. read that much into the malloc’d block
3. figure out where the real end of the string is
4. write a \0 into your malloc’d block there so it behaves correctly as a nul-terminated string (and/or save the length too in case you need it)
5. optionally realloc your block to the correct size

Or

1. malloc a reasonable guesstimate N for the length
2. read that much
3. if you can’t find the end of the string in that buffer:
   1. grow the buffer with realloc to 2N (for example) and read the next N bytes into the end
   2. goto 3
4. write a \0 etc. as above

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You said in a comment that the max. string length is bounded, so the first approach is probably fine. You haven’t said how you figure out where the string ends, but I’m assuming there is some delimiter, or it’s right-filled with spaces, or something.