stdio.h

stdio.h, which stands for "standard input/output header", is the header in the C standard library that contains macro definitions, constants, and declarations of functions and types used for various standard input and output operations. The functionality descends from a "portable I/O package" written by Mike Lesk at Bell Labs in the early 1970s. C++, for compatibility reasons, includes stdio.h as well as an essentially equivalent header, cstdio.

Functions declared in stdio.h are extremely popular, since as a part of the C standard library, they are guaranteed to work on any platform that supports C. Applications on a particular platform might, however, have reasons to use the platform's I/O routines, rather than the stdio.h routines.

Example usage

All functions in C (and its many derivatives) are declared in header files. Thus, programmers have to include the stdio.h header in the source code in order to use the functions declared in it.

#include <stdio.h>
 
int main(void)
{
    int ch;
    while ((ch = getchar()) != EOF)
        putchar(ch);
    putchar('\n');
    return 0;
}

The above program reads all input from standard input and echoes it to standard output, byte by byte, adding a newline at the end of its output.

Member functions

Functions declared in stdio.h can generally be divided into two categories: the functions for file manipulation and the functions for input-output manipulation.

Name	Notes
File manipulation functions
fclose	closes a file associated with the FILE * value passed to it
fopen, freopen, fdopen	opens a file for certain types of reading or writing
remove	removes a file (deletes it)
rename	renames a file
rewind	acts as if fseek(stream, 0L, SEEK_SET) was called for the stream passed, and then its error indicator cleared
tmpfile	creates and open a temporary file, which is deleted when closed with fclose()
Input-output manipulation functions
clearerr	clears end-of-file and error indicators for a given stream
feof	checks whether an end-of-file indicator has been set for a given stream
ferror	checks whether an error indicator has been set for a given stream
fflush	forces any pending buffered output to be written to the file associated with a given stream
fgetpos	stores the file position indicator of the stream associated by its first argument (a FILE *) to its second argument (a fpos_t *)
fgetc	returns one character from a file
fgets	gets a string from the file (ending at newline or end-of-file)
fputc	writes one character to a file
fputs	writes a string to a file
ftell	returns a file-position indicator which can then be passed to fseek
fseek	seeks through a file
fsetpos	sets the file position indicator of a stream associated by its first argument (a FILE *) as stored in its second argument (a fpos_t *)
fread	reads data from a file
fwrite	writes data to a file
getc	reads and returns a character from a given stream and advances the file position indicator; it is allowed to be a macro with the same effects as fgetc, except that it may evaluate the stream more than once
getchar	has the same effects as getc(stdin)
gets	reads characters from stdin until a newline is encountered and stores them in its only argument
printf, vprintf	used to print to the standard output stream
fprintf, vfprintf	used to print to a file
sprintf, snprintf, vsprintf	used to print to a char array (C string)
perror	writes an error message to stderr
putc	writes and returns a character to a stream and advances the file position indicator for it; it is allowed to be a macro with the same effects as fputc, except that it may evaluate the stream more than once
putchar, fputchar	has the same effects as putc(stdout)
scanf, vscanf	used to input from the standard input stream
fscanf, vfscanf	used to input from a file
sscanf, vsscanf	used to input from a char array (e.g., a C string)
setbuf
setvbuf	sets the buffering mode for a given stream
tmpnam	creates a temporary filename
ungetc	pushes a character back onto a stream
puts_(C)	outputs a character string to stdout

Member constants

Constants defined in the stdio.h header include:

Name	Notes
EOF	a negative integer of type int used to indicate end-of-file conditions
BUFSIZ	an integer which is the size of the buffer used by the setbuf() function
FILENAME_MAX	the size of a char array which is large enough to store the name of any file that can be opened
FOPEN_MAX	the number of files that may be open simultaneously; will be at least 8
_IOFBF	an abbreviation for "input/output fully buffered"; it is an integer which may be passed to the setvbuf() function to request block buffered input and output for an open stream
_IOLBF	an abbreviation for "input/output line buffered"; it is an integer which may be passed to the setvbuf() function to request line buffered input and output for an open stream
_IONBF	an abbreviation for "input/output not buffered"; it is an integer which may be passed to the setvbuf() function to request unbuffered input and output for an open stream
L_tmpnam	the size of a char array which is large enough to store a temporary filename generated by the tmpnam() function
NULL	a macro expanding to the null pointer constant; that is, a constant representing a pointer value which is guaranteed not to be a valid address of an object in memory
SEEK_CUR	an integer which may be passed to the fseek() function to request positioning relative to the current file position
SEEK_END	an integer which may be passed to the fseek() function to request positioning relative to the end of the file
SEEK_SET	an integer which may be passed to the fseek() function to request positioning relative to the beginning of the file
TMP_MAX	the maximum number of unique filenames generable by the tmpnam() function; will be at least 25

Member variables

Variables defined in the stdio.h header include:

Name	Notes
stdin	a pointer to FILE which refers to the standard input stream, usually a keyboard.
stdout	a pointer to FILE which refers to the standard output stream, usually a display terminal.
stderr	a pointer to FILE which refers to the standard error stream, often a display terminal.

Member types

Data types defined in the stdio.h header include:

• FILE - a structure containing the information about a file or text stream needed to perform input or output operations on it, including:
  • a file descriptor
  • the current stream position
  • an end-of-file indicator
  • an error indicator
  • a pointer to the stream's buffer, if applicable
• fpos_t - a non-array type capable of uniquely identifying the position of every byte in a file.
• size_t - an unsigned integer type which is the type of the result of the sizeof operator.

See also

• Large file support