Question

I have a structure

typedef struct store
{
   char name[11];
    int age;
} store;

and a main function(below is part of it):

int main()
{
    int i=0;
    int inputs;
    char line[100];
    char name[11];
    char command[11];
    store read[3000];

    while(i < 3000 && gets(line) != NULL)  
    {
        int tempage;
        inputs = sscanf(line, "%10s %10s %d", command, name, &tempage);
        if (inputs == 3)
        {
            if (strcmp(command, "register") == 0)
             {
             strncpy(read[i].name, name,10);
             read[i].age = tempage;
             i++;
             ....

I need to modify it so that it can read a line of arbitrary length, and store the name from the line which is also a string of arbitrary length using malloc and realloc.

How should I approach this?

Answer 1

What you need to do is read the line in smaller increments, and resize your buffer as you go.

As an example (not tested and not meaning to be particularly elegant, just an example):

char *readline(FILE *f)
{
   char *buf = NULL;
   size_t bufsz = 0, len = 0;
   int keep_going = 1;

   while (keep_going)
   {
      int c = fgetc(f);
      if (c == EOF || c == '\n')
      {
         c = 0;             // we'll add zero terminator
         keep_going = 0;    // and terminate the loop afterwards
      }

      if (bufsz == len)
      {
         // time to resize the buffer.
         //
         void *newbuf = NULL;
         if (!buf)
         {
            bufsz = 512; // some arbitrary starting size.
            newbuf = malloc(bufsz);
         }
         else
         {
            bufsz *= 2; // issue - ideally you'd check for overflow here.
            newbuf = realloc(buf, bufsz);
         }

         if (!newbuf)
         {
            // Allocation failure.  Free old buffer (if any) and bail.
            //
            free(buf);
            buf = NULL;
            break;
         }

         buf = newbuf;
      }

      buf[len++] = c;
   }

   return buf;
}