How to get int and string from file and save it in structure?

Question

Let's assume that we have file which contains:

 1 John 
 2 Alex 
 3 Michael

We can get one line using fscanf() function, but how to save it to below structure:

 typedef struct entry { 
 int n; 
 char *name; 
 } entry_t;

I want to create the array of structures and save values from file to it, and do it dynamically. I've tried doing this in that way

entry_t *prt = malloc ( size * sizof(entry_t) ); 
//opening file 
prt[0].name = malloc ( sizeof("John") ); 
fscanf (fp,"%d %s", prt[0].n, prt[0].name);

Ok, it works, but how to allocate memory for every name before get it from text file? I decided to use array of structures, because I'll use it to implement hash table.

You can't allocate _before_, as you don't know the size. `strlen` each of your names from the file and then `malloc` — mihai, Dec 16 '16 at 19:05
You have at least two options. (1) Most portable: `char buffer[1024]; if (fscanf(fp, "%d %1023s", &prt[0].n, buffer) == 2) { prt[0].name = strdup(buffer); …use values after checking strdup worked… } else { …handle error… }` which uses a large buffer to read the value and then allocates the memory afterwards; or (2) Using POSIX features of [`fscanf()`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fscanf.html): `if (fscanf(fp, "%d %ms", &prt.n, &prt[0].name) == 2) { …use values read… } else { …handle error… }`. Note the size limit on the first call. — Jonathan Leffler, Dec 16 '16 at 19:10
The call to `fscanf()` shouldn't compile — you omitted the file pointer argument. — Jonathan Leffler, Dec 16 '16 at 19:10

score 2 · Accepted Answer · edited May 23 '17 at 11:53

sizeof("John") works fine for a string literal, but the names in the file are not known prior, so size must be determined dynamically.

Use fgets() to read a line.
Use sscanf(), strtol(), strtok() to parse that line.

Example:

int read_entry(FILE *istream, struct entry *record) {
  char buf[200];
  if (fgets(buf, sizeof buf, istream) == NULL) return -1;  // EOF
  buf[strcspn(buf, "\n")] = 0;  // lop off potential trailing \n

  int start;
  int end = 0;
  sscanf(buf, "%d %n%*s%n", &record->n, &start, &end); 

  if (end == 0) {
    return 0;  // failed to parse
  }
  record->name = strdup(&buf[start]);
  return 1; // Success
}

Usage

struct entry record;
while (read_entry(stdin, &record) == 1) {
  printf("%d '%s'\n", record.n, record.name);
  ...
  // when done with the record,
  free(record.name);
}

strdup() is a common way to "duplicate" a string, yet it is not part of the standard C library. Easy enough to code: Example implementation

Additional error checking could be had with `if (record->name == NULL)` after the allocation and trimming any excess white-space at the end of the name. — chux - Reinstate Monica, Dec 16 '16 at 19:40

score 0 · Answer 2 · answered Dec 16 '16 at 19:34

^{Converting comment into answer.}

You have at least two options.

Most portable:
```
 char buffer[1024];
 if (fscanf(fp, "%d %1023s", &prt[0].n, buffer) != 2)
     …handle I/O (format?) error…
 else if ((prt[0].name = strdup(buffer)) == 0)
     …handle out-of-memory error…
 else
 {
     …use values, or continue loop…
 }
```
This uses a large buffer to read the value and then allocates the appropriate memory for use in the structure afterwards. Note the overflow protection in the argument (and the difference by one is necessary). Note that strdup() is part of POSIX and not part of standard C. It is easy to write, though:
```
char *strdup(const char *str)
{
    size_t len = strlen(str) + 1;
    char *copy = malloc(len);
    if (copy != 0)
        memmove(copy, str, len);
    return copy;
}
```
There'll be the usual debate of memmove() vs memcpy(); both work in this context, but memmove() works everywhere and memcpy() doesn't.
Using POSIX features of fscanf():
```
if (fscanf(fp, "%d %ms", &prt.n, &prt[0].name) != 2)
    …handle I/O (format?) error…
else
{
    …use values, or continue loop…
}
```
Note that in this context, you do pass the address of the pointer prt[0].name as fscanf() allocates the necessary memory for you.

You need to free the allocated memory for each name later, of course, whichever solution you're using.

How to get int and string from file and save it in structure?

2 Answers2