Apue Note - Standard I/O Library

Notes of APUE

With the standard I/O library, the discussion centers on streams. When we open or create a file with the standard I/O library, we say that we have associated a stream with the file.


The goal of the buffering provided by the standard I/O library is to use the minimum number of read and write calls.

With setbuf, we can turn buffering on or off.

#include <stdio.h>
void setbuf(FILE *restrict fp, char *restrict buf);

Opening a Stream

#include <stdio.h>
FILE *fopen(const char *restrict pathname, const char *restrict type);
FILE *freopen(const char *restrict pathname, const char *restrict type,
              FILE *restrict fp);
FILE *fdopen(int fd, const char *type);

Reading and Writing a Stream

Three functions allow us to read one character at a time.

#include <stdio.h>
int getc(FILE *fp);
int fgetc(FILE *fp);
int getchar(void);

The difference between getc and fgetc is that getc can be implemented as a macro, whereas fgetc cannot be implemented as a macro. This means three things.

  • The argument to getc should not be an expression with side effects, because it could be evaluated more than once.
  • Since fgetc is guaranteed to be a function, we can take its address. This allows us to pass the address of fgetc as an argument to another function.
  • Calls to fgetc probably take longer than calls to getc, as it usually takes more time to call a function.

Output Functions

#include <stdio.h>
int putc(int c, FILE *fp);
int fputc(int c, FILE *fp);
int putchar(int c);

Line-at-a-Time I/O

Line-at-a-time input is provided by the two functions, fgets and gets. Line-at-a-time output is provided by fputs and puts.

Standard I/O Efficiency

#include <stdio.h>
int main()
    int c;
    while ((c = getc(stdin)) != EOF)
        putc(c, stdout);

with reads/writes lines

#include <stdio.h>
#define MAXLINE 4096

int main()
    char buf[MAXLINE];
    while (fgets(buf, MAXLINE, stdin) != NULL)
        fputs(buf, stdout);

After testing, we’ve learned that the standard I/O library is not much slower than calling the read and write functions directly.

Binary I/O

The following two functions are provided for binary I/O.

#include <stdio.h>
size_t fread(void *restrict ptr, size_t size, size_t nobj,
             FILE *restrict fp);
size_t fwrite(const void *restrict ptr, size_t size, size_t nobj,
              FILE *restrict fp);

These functions have two common uses:

/* to write elements 2 through 5 of a floating-point array */
float data[10];
fwrite(&data[2], sizeof(float), 4, fp);

/* Read or write a structure */
struct {
    short  count;
    long   total;
    char   name[NAMESIZE];
} item;
fwrite(&item, sizeof(item), 1, fp);

A fundamental problem with binary I/O is that it can be used to read only data that has been written on the same system.

Positioning a Stream