使用 fread 快速阅读而不使用 fwrite

Question

I have a graph in the following format:

3 15 12
15 15
12 35
12 54

where the first line is some info about the graph and the first letter on the first line is the number of edges. The following m lines are the edges. I can easily read this file with scanf. But now I am looking to understand fread to read all the edges at once and not each line one at a time.

int main () {
int32_t row1[3];

printf("row1[0]=%d, row1[1]=%d, row1[2]=%d\n", row1[0], row1[1], row1[2]);
FILE* in;
in = fopen("in", "r");
fread(&row1, sizeof(int32_t), 3, in);

fclose(in);
printf("row1[0]=%d, row1[1]=%d, row1[2]=%d\n", row1[0], row1[1], row1[2]);
}

But I do not get the output 3, 15, 12 as I expect from the last printf.

Do you need to create the in file with fwrite or can you create it in any way?

Answer 1

It's possible to read the whole file at once. It can be done like this. I omitted error checking for malloc and fread.

fseek(in, 0L, SEEK_END);
size_t size = ftell(in);
rewind(in);
char buffer = malloc(size);
fread(buffer, size, 1, in);

But then you need to parse the buffer. There's not much to gain from this. If you want to read a text file, then fgets is usually the way to go. Reading line by line with fread makes little sense. The function is intended for binary files. You would have to manually search for line breaks.