Dynamic storage of a binary file in C - c

Question

Participant *readRankingList(const char *filename, int *length) {
length = 0;
int i = 0;
FILE *fp;
fp = fopen(filename, "rb");
Participant *list;
if (fp == NULL) {
fprintf(stderr, "*** open of %s failed ***\n", filename);
return 0;
}
length = fread((void *)list, sizeof(Participant), LIST_LEN, fp);
while (fread(list, sizeof(length), 1, fp) > 0) {
list[i] = list;
i++;
}
free(list);
fclose(fp);
*length = i;
return list;
}
I have a binary file, which I open in this part. Now I want to store it with a dynamic storage.
At the moment I get a
Warning : assignment makes pointer from interfer without a cast
and an
Error: incompatible types when assigning to type Participant from type struct Participant

Answer 1

You must allocate space for the structures with malloc() or calloc(). Seeking to the end of the file will give the number of entries. Reading the entries can be done in a single call to fread.
#include <errno.h>
#include <stdio.h>
#include <string.h>
Participant *readRankingList(const char *filename, int *length) {
Participant *list;
size_t count;
// open the file
FILE *fp = fopen(filename, "rb");
if (fp == NULL) {
fprintf(stderr, "*** open of %s failed: %s ***\n",
filename, strerror(errno));
*length = 0;
return NULL;
}
// get the number of structures
fseek(fp, 0L, SEEK_END);
count = ftell(fp) / sizeof(Participant);
if (count == 0) {
fclose(fp);
*length = 0;
return NULL;
}
// allocate memory for the array of Participants
list = calloc(count, sizeof(*list));
if (list == NULL) {
fclose(fp);
*length = 0;
return NULL;
}
// read the structures, update the length, return the pointer.
rewind(fp);
*length = fread(list, sizeof(Participant), count, fp);
fclose(fp);
return list;
}