Input/Output

Files
file	stdio.h

Functions
int	printf (const char *format,...)
	Print formatted data to stdout More...
int	fprintf (FILE *stream, const char *format,...)
	Print formatted data to stream. More...
int	fgetc (FILE *stream)
	Read a character. More...
char *	fgets (char *s, int size, FILE *stream)
	Read a string. More...
int	fputc (int c, FILE *stream)
	Write a character. More...
int	fputs (const char *s, FILE *stream)
	Write a string. More...
void	perror (const char *s)
	Print an error message. More...
int	feof (FILE *stream)
	Test end-of-file indicator of a file stream. More...
int	ferror (FILE *stream)
	Test error indicator of a file stream. More...

Detailed Description

This code snippet illustrates the usage of fgetc() and fputc():

int c;
while ((c = fgetc(stdin)) != EOF) {
    if (fputc((unsigned char) c, stdout) == EOF) {
        perror("fputc");
        exit(EXIT_FAILURE);
    }
}
if (ferror(stdin)) {
    // error
}
  
// no error; end of file reached

This code snippet illustrates the usage of fgets() and fputs():

char buffer[1024];
char *check;
  
// reads at most 1023 characters per iteration
while ((check = fgets(buffer, 1024, stdin)) != NULL) {
    if (fputs(buffer, stdout) == EOF) {
        perror("fputs");
        exit(EXIT_FAILURE);
    }
}
if (ferror(stdin)) {
    // error
}
  
// alternative to ferror():
// if (feof(stdin)) {
//     // no error; end of file reached
// } else {
//     // error
// }

Function Documentation

printf()

int printf	(	const char *	format,
			...
	)

The printf() (print formatted) function produces output according to a format string as specified in format and writes it stdout. The format string defines the structure of the output (i.e., how many and which additional arguments) and the additional parameter of the printf() function are used to replace the arguments in the format string with actual data. It is important that the number of additional parameter matches the number of arguments of the format string.

An argument in the format string starts with a %. The following table shows some important arguments, however, there are several more options described in the manpage of printf() (man 3 printf):

Type	Description
i, d	integer
u	unsigned integer
f	floating point
p	pointer
s	string
c	character

Example:

int i = 5;
char c = 'a';
void *p = &i;
  
printf("Hello World!\n");                     // no arguments
printf("$> i: %i\n$> c: %c\n$> f: %f\n \
        $> p: %p\n", i, c, 3.14, p);          // including arguments

// produces:
Hello world!
$> i: 5
$> c: a
$> f: 3.140000
$> p: 0x7ffebe5dbe34

We do not expect error handling for printf().

Parameters

format	format string
...	arguments

Return values

>=0	number of characters printed
<0	on error, errno is set

fprintf()

int fprintf	(	FILE *	stream,
		const char *	format,
			...
	)

The fprintf() (file stream print formatted) function is very similar to printf(), except that it does not write to stdout, but to stream.

For more details see printf().

// both lines write to stdout
fprintf(stdout, "Hello world!\n");
printf("Hello world!\n");
  
// write to stderr
fprintf(stderr, "Error in file '%s' at line %u\n", __FILE__, __LINE__);

We do not expect error handling for fprintf().

Parameters

stream	file stream to write
format	format string, see printf()
...	arguments

Return values

>=0	number of characters printed
<0	on error, errno is set

fgetc()

int fgetc

(

FILE *

stream

)

The fgetc() (file stream get character) function returns an unsigned char casted to an int read from a stream or returns EOF.

fgetc() returns EOF on error or when the end of the file is reached. To distinguish these two events the feof() or ferror() function can be used. Be aware, that the return value of fgetc() must be saved in an int (and not in an unsigned char) in order to distinguish EOF and 0xFF (which is the character ÿ when interpreted as ISO-8859-1).

Parameters

stream

file stream to read

Return values

EOF	on error or end of file, errno is set on error
!=EOF	unsigned char read from stream

fgets()

char* fgets	(	char *	s,
		int	size,
		FILE *	stream
	)

The fgets() (file stream get string) function reads at most size-1 characters from stream and saves them in the buffer pointed to by s. It terminates the string in s with a null byte (\0).

fgets() reads in characters until it finds a newline character (\n) or it has read size-1 characters. If an error occurred or the end of file was reached it returns NULL. The ferror() and feof() functions can be used to distinguish these two events.

Parameters

s	buffer to write to
size	size of the buffer
stream	file stream to read

Return values

NULL	on error or end of file, errno is set on error
!=NULL	pointer to s

fputc()

int fputc	(	int	c,
		FILE *	stream
	)

The fputc() (file stream put character) function writes the character c to stream.

We do not expect error handling when using fputc().

Parameters

c	char to print (casted to an int)
stream	file stream to write

Return values

!=EOF	printed char
EOF	on error, errno is set

fputs()

int fputs	(	const char *	s,
		FILE *	stream
	)

The fputs() (file stream put string) function writes the string pointed to by s to stream.

We do not expect error handling when using fputs().

Parameters

s	string to be printed
stream	file stream to write

Return values

>=0	on success
EOF	on error, errno is set

perror()

void perror

(

const char *

s

)

The perror() (print error) function produces an error message on stderr printing the string in s followed by a human-readable description of the value in errno. This is especially helpful after a failed call to a system or library function, which sets the errno variable (e.g., malloc()).

Parameters

s	string printed before the actual error message

feof()

int feof

(

FILE *

stream

)

The feof() (file stream end of file) function tests the EOF indicator of stream.

Parameters

stream

file stream to test

Return values

0	end-of-file indicator is not set
!=0	end-of-file indicator is set

ferror()

int ferror

(

FILE *

stream

)

The ferror() (file stream error) function tests the error indicator of stream.

Parameters

stream

file stream to test

Return values

0	error indicator is not set
!=0	error indicator is set