C Files

programming C MOC

---

Reading files is almost identical to reading the command line, except the file handler is opened with fopen and closed with fclose.

#include <stdio.h>
#include <string.h>
 
int main() {
    char data[20];
    FILE *file = fopen("test.txt", "r");
 
    fgets(data, sizeof data, file);
    if (strlen(data) > 0){
        if (data[strlen(data) - 1] != '\n') {
            data[strlen(data) - 1] = '\n';
        }
    }
    printf("The file contains: \n%s", data);
        fclose(file);
 
    return(0);
}

File Permissions

Files need to be opened with explicit permissions.

Character	Permission
r	Read
w	Write
a	Append
b	Binary (with r, w, or a)

fscanf

fscanf() reads bytes until a whitespace character and formats each chunk, unlike fgets(), which reads to a \n.

/*
Reads hexadecimal data from file.
*/
#include <stdio.h>
 
int main() {
	FILE *fp = fopen("message.txt", "r");
	char buff[2];
	while (fscanf(fp, "%hhx", buff) != EOF){
		printf("%s", buff);
	}
	fclose(fp);
}

fseek

The fseek(<file>, <offset>, <origin>) reads bytes at specific positions. The origin is the position to begin the offset from.

Origin	Location
SEEK_SET	Start of file
SEEK_CUR	Current position of file pointer
SEEK_END	End of file

#include <stdio.h>
 
int main() {
    FILE *fp = fopen("message.txt", "r");
    char buff[2];
 
    printf("File pointer at byte %ld\n", ftell(fp));
    fseek(fp, 50, SEEK_SET);
    printf("File pointer at byte %ld\n", ftell(fp));
 
    for (int i = 0; i < 14; i++) {
        fscanf(fp, "%hhx", buff);
        printf("%d: %s ", i, buff);
    }
 
    printf("\nFile pointer at byte %ld\n", ftell(fp));
    rewind(fp); // Point file pointer at beginning again
    printf("File pointer at byte %ld\n", ftell(fp));
    fclose(fp);
 
    return 0;
}