File Pointers

• The ability to read/write data to files is the primary way of storing persistent data.
• Persistent Data: Data that doesn’t disappear when your program stops running
• The abstraction of files that C provides is implemented in a data structure known as a FILE.
• File manipulation functions live in stdio.h, which all accept FILE* as one of their parameters, except fopen().

Important stdio.h Functions

fopen()

• Opens a file and returns a file pointer to it.
• Always check the return value to make sure you don’t get back NULL.

FILE* ptr1 = fopen("read.txt", "r");
FILE* ptr2 = fopen("write.txt", "w");
FILE* ptr3 = fopen("append.txt", "a");

fclose()

• Closes a file pointed to by the given file pointer.

fclose(ptr1);

fgetc()

• Reads and returns the next character from the file pointed to.
• File must be fopened in reading ("r") mode.

char ch = fgetc(ptr1);

// `cat` command: read all the characters from a file and print them to the screen, one-by-one
char ch;
while ((ch = fgetc(ptr)) != EOF)
    printf("%c", ch);

fputc()

• Writes or appends a character to the pointed-to file.
• File must be fopened in write or append more.

fputc('A', ptr2);
fputc('!', ptr2);

// `cp` command: copy contents of ptr1 to ptr2
char ch;
while ((ch = fgetc(ptr)) != EOF)
    fputc(ch, ptr2);

fread()

• Reads <qty> units of size <size> from the file pointed-to and store them in memory in a buffer (usually an array) pointed-to by <buffer>
  • Can be thought of as a wrapped for fgetc()
• File must be fopened in reading ("r") mode.

// Read 10 integers from ptr into arr
int arr[10];
fread(arr, sizeof(int), 10, ptr);

// Store 80 doubles in a array of doubles size 80
double* arr2 = malloc(sizeof(double) * 80);
feead(arr2, sizeof(double), 80, ptr);

// Work like fgetc()
char c;
fread(&c, sizeof(char), 1, ptr);

fwrite()

• Writes <qty> units of size <size> to the file pointed-to by reading them from a buffer (e.g., an array) pointed to by <buffer>.
• File must be fopened in write or append more.

// Write ten elements an array of ints size 10 into ptr
int arr[10];
fwrite(arr, sizeof(int), 10, ptr);

// Write 80 doubles from an array of 80 doubles into ptr
double* arr2 = malloc(sizeof(double) * 80);
fwrite(arr2, sizeof(double), 80, ptr);

// Work like fputc()
char c;
fwrite(&c, sizeof(char), 1, ptr);

Other Common Functions

• fgets(): Get string from file
• fputs(): Write string into file
• fprintf(): Print formatted string to file
• fseek(): Set position in stream
• ftell(): Tell position in stream
• feof(): Tells whether read to end of a file
• ferror(): Indicates whether an error has occurred in working with a file