File I/O

The FILE * Data type

In C, the FILE * type is commonly used to represent a stream.
Streams are largely categorized 2 ways: text and binary.
The file pointer is automatically updated by the standard library to point to the next character in the file after each read operation.

Reading text files

Text files are logically a sequence of lines separated by newlines.

Read char by char

#include <stdio.h>

int main(void){
    FILE *fp;
    int c; //EOF is the reason we need to use integer, but deep down it's a char

    fp = fopen("../hello.txt", "r");

    while ((c = fgetc(fp)) != EOF ){
        printf("%c",c);
    };

    fclose(fp);
}

Read line by line

#include <stdio.h>

int main(void){
    FILE *fp;
    char s[1024]; // Big enough for any line this program will encounter

    int linecount = 0;

    fp = fopen("../hello.txt", "r");
    while (fgets(s, sizeof s, fp) != NULL){
        printf("%d: %s", ++linecount, s);
    }
    fclose(fp);
}

Read and store values on variables

#include <stdio.h>

int main(void){
    FILE *fp;
    char name[1024];
    float length;
    int mass;

    fp = fopen("whales.txt","r");
    while (fscanf(fp, "%s %f %d", name, &length, &mass) != EOF){
        printf("%s whale, %d tonnes, %.1f meters\n", name, mass, length);
    }

    fclose(fp);
}

Writing Text files

#include <stdio.h>

int main(void){
    FILE *fp;
    int x = 32;

    fp = fopen("output.txt", "w");

    fputc('B', fp);
    fputc('\n', fp);
    fprintf(fp, "x = %d\n", x);
    fputs("Hello World!\n", fp);
}

Writing bytes to files:

With binary files, we only get a raw stream of bytes, that’s all.
Because it’s a stream of bytes, it can contain NUL characters.
Use fread and fwrite instead.

#include <stdio.h>

int main(void){
    FILE *fp;
    unsigned char bytes[6] = {5,37,0,88,255,12};

    fp = fopen("output.bin", "wb"); // wb mode for "write binary"!

    // Data to write, size of each "piece" data, counter of each "piece" of data, FILE *
    fwrite(bytes, sizeof(char), 6, fp);
}

Reading bytes to files

#include <stdio.h>

int main(void){
    FILE *fp;
    unsigned char c;

    fp = fopen("output.bin", "rb"); //rb for "read binary"!

    while (fread(&c, sizeof(char), 1, fp) > 0){
        printf("%d\n", c);
    }
}

Struct and Number Caveats

=> Solution: serialization (like Protocol Buffer)