File I/O

File access in C goes through a FILE *, a pointer to a structure the standard library uses to track an open file's position and buffering. The functions that operate on it mirror printf and scanf closely, since stdin and stdout are themselves just pre-opened FILE * streams.

Opening and closing a file

1#include <stdio.h>
2 
3FILE *file = fopen("data.txt", "r");
4 
5if (file == NULL) {
6    printf("Could not open file.\n");
7    return 1;
8}
9 
10// ... work with the file ...
11 
12fclose(file);

The second argument to fopen is the mode: "r" for reading, "w" for writing (creates the file, or erases it if it already exists), "a" for appending. fopen returns NULL on failure — a missing file, no permission, and so on — and that always has to be checked before use.

Writing to a file

1FILE *file = fopen("output.txt", "w");
2if (file == NULL) return 1;
3 
4fprintf(file, "Score: %d\n", 95);
5fprintf(file, "Name: %s\n", "Nour");
6 
7fclose(file);

fprintf works exactly like printf, but takes the target stream as its first argument.

Reading a file line by line

1FILE *file = fopen("data.txt", "r");
2if (file == NULL) return 1;
3 
4char line[256];
5while (fgets(line, sizeof(line), file) != NULL) {
6    printf("%s", line);
7}
8 
9fclose(file);

fgets returns NULL once it reaches the end of the file, which is what naturally ends the loop above — no separate "end of file" check needed.

Reading structured data

fscanf reads formatted data from a file the same way scanf reads from the keyboard:

1int id;
2float score;
3fscanf(file, "%d %f", &id, &score);