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C File Handling Cheat Sheet (DRAFT) by

C File Handling Cheatsheet

This is a draft cheat sheet. It is a work in progress and is not finished yet.

File Functions

int fscanf­(FILE stream, const char format, ...)
reads formatted input from a stream.
int fprint­f(FILE stream, const char format, ...)
sends formatted output to a stream.
FILE fopen(­const char filename, const char *mode)
opens the filename pointed to, by filename using the given mode.
"­r" Opens a file for reading. The file must exist.
"­w" Creates an empty file for writing. If a file with the same name already exists, its content is erased and the file is considered as a new empty file.
"­a" Appends to a file. Writing operat­ions, append data at the end of the file. The file is created if it does not exist.
int fseek(FILE *f, long int offset, int origin)
go forward offset times without reading the files
origins:
SEEK_SET - from start of file
SEEK_END - end of file
SEEK_CUR - move from current location
int ftell(FILE *f)
returns the distance from cursor to start of file
void rewind­(FILE *f)
go back to start of file
int ferror­(FILE *F)
returns 0 if no errors occured
fclose­(FILE* F)
closes the file
size_t fread(void ptr, size_t size, size_t nmemb, FILE stream)
reads data from the given stream into the array pointed to, by ptr.
int fgetc(FILE *stream)
Gets the next character (an unsigned char) from the specified stream and advances the position indicator for the stream.
char fgets(char str, int n, FILE *stream)
read line to str. stop when newline or eof is read
int fputc(int char, FILE *stream)
Writes a character specified by the argument char to the specified stream and advances the position indicator for the stream.
int fputs(­const char str, FILE stream)
Writes a string to the specified stream up to but not including the null character.

Reading and writing binary files

#include <stdio.h>
#define SIZE 20
struct Person{
char name[SIZE];
long id;
float age;
} typedef person_t;

void main(){
person_t p1={"momo", 1111, 23.5}, p2 = {"gogo", 2222, 24.8}, p3, p4;

FILE* f = fopen("persons.bin", "wb");

fwrite(&p1, sizeof(person_t), 1, f);
fwrite(&p2, sizeof(person_t), 1, f);

fclose(f);

f = fopen("persons.bin", "rb");
fread(&p3, sizeof(person_t), 1, f);
fread(&p4, sizeof(person_t), 1, f);

fclose(f);

printf("p3: name: %s\t id: %ld\t age: %.2f\n", p3.name, p3.id, p3.age);
printf("p4: name: %s\t id: %ld\t age: %.2f\n", p4.name, p4.id, p4.age);
}
 

Reading file into struct

typedef struct person {
    char name[20];
    int age;
} person;

int main() {
    person *people = NULL;
    char name[20];
    int size, age = 0;
    FILE *f = fopen("people.txt", "r");
    while (!feof(f)){
        scanf("%s %d", name, &age);
        people = realloc(people, sizeof(person)(++age)size);
        strcpy((people+size-1)->name, name);
        (people+size-1)->age = age;
    }
    fclose(f);
}
assume people.txt is formatted like this:
name1 age
name2 age
  

Writing to file

#include <stdio.h>

void main()
{
FILE* f = fopen("myFile.txt", "w");
int res;

if (f == NULL){
printf("Failed opening the file. Exiting!\n");
return;
}
fputs("Hello World!\n", f);
fclose(f);

f = fopen("myFile.txt", "a");
fputs("And Good Morning!\n", f);
fclose(f);
}