C standard library (libc) |
---|
General topics |
Miscellaneous headers |
TheC programming language provides manystandard libraryfunctions forfileinput and output. These functions make up the bulk of theC standard libraryheader<stdio.h>.[1] The functionality descends from a "portable I/O package" written byMike Lesk atBell Labs in the early 1970s,[2] and officially became part of theUnix operating system inVersion 7.[3]
The I/O functionality of C is fairly low-level by modern standards; C abstracts all file operations into operations onstreams ofbytes, which may be "input streams" or "output streams". Unlike some earlier programming languages, C has no direct support forrandom-access data files; to read from a record in the middle of a file, the programmer must create a stream,seek to the middle of the file, and then read bytes in sequence from the stream.
The stream model of file I/O was popularized by Unix, which was developed concurrently with the C programming language itself. The vast majority of modern operating systems have inherited streams from Unix, and many languages in theC programming language family have inherited C's file I/O interface with few if any changes (for example,PHP).
This library uses what are called streams to operate with physical devices such as keyboards, printers, terminals or with any other type of files supported by the system. Streams are an abstraction to interact with these in a uniform way. All streams have similar properties independent of the individual characteristics of the physical media they are associated with.[4]
Most of the C file input/output functions are defined in<stdio.h> (or in theC++ headercstdio, which contains the standard C functionality but in thestdnamespace).
Byte character | Wide character | Description | |
---|---|---|---|
File access | fopen | Opens a file (with a non-Unicode filename on Windows and possible UTF-8 filename on Linux) | |
popen | opens a process by creating a pipe, forking, and invoking the shell | ||
freopen | Opens a different file with an existing stream | ||
fflush | Synchronizes an output stream with the actual file | ||
fclose | Closes a file | ||
pclose | closes a stream | ||
setbuf | Sets the buffer for a file stream | ||
setvbuf | Sets the buffer and its size for a file stream | ||
fwide | Switches a file stream between wide-character I/O and narrow-character I/O | ||
Direct input/output | fread | Reads from a file | |
fwrite | Writes to a file | ||
Unformatted input/output | fgetc getc | fgetwc getwc | Reads a byte/wchar_t from a file stream |
fgets | fgetws | Reads a byte/wchar_t line from a file stream | |
fputc putc | fputwc putwc | Writes a byte/wchar_t to a file stream | |
fputs | fputws | Writes a byte/wchar_t string to a file stream | |
getchar | getwchar | Reads a byte/wchar_t from stdin | |
— | Reads a byte string from stdin until a newline or end of file is encountered (deprecated in C99, removed from C11) | ||
putchar | putwchar | Writes a byte/wchar_t to stdout | |
puts | — | Writes a byte string to stdout | |
ungetc | ungetwc | Puts a byte/wchar_t back into a file stream | |
Formatted input/output | scanf fscanf sscanf | wscanf fwscanf swscanf | Reads formatted byte/wchar_t input from stdin, a file stream or a buffer |
vscanf vfscanf vsscanf | vwscanf vfwscanf vswscanf | Reads formatted input byte/wchar_t from stdin, a file stream or a buffer using variable argument list | |
printf fprintf sprintf snprintf | wprintf fwprintf swprintf | Prints formatted byte/wchar_t output to stdout, a file stream or a buffer | |
vprintf vfprintf vsprintf vsnprintf | vwprintf vfwprintf vswprintf | Prints formatted byte/wchar_t output to stdout, a file stream, or a buffer using variable argument list | |
perror | — | Writes a description of thecurrent error to stderr | |
File positioning | ftell ftello | Returns the current file position indicator | |
fseek fseeko | Moves the file position indicator to a specific location in a file | ||
fgetpos | Gets the file position indicator | ||
fsetpos | Moves the file position indicator to a specific location in a file | ||
rewind | Moves the file position indicator to the beginning in a file | ||
Error handling | clearerr | Clears errors | |
feof | Checks for the end-of-file | ||
ferror | Checks for a file error | ||
Operations on files | remove | Erases a file | |
rename | Renames a file | ||
tmpfile | Returns a pointer to a temporary file | ||
tmpnam | Returns a unique filename |
Constants defined in the<stdio.h> header include:
Name | Notes |
---|---|
EOF | A negative integer of typeint used to indicate end-of-file conditions |
BUFSIZ | An integer which is the size of the buffer used by thesetbuf() function |
FILENAME_MAX | The size of achar 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 eight |
_IOFBF | An abbreviation for "input/output fully buffered"; it is an integer which may be passed to thesetvbuf() function to requestblock 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 thesetvbuf() function to requestline 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 thesetvbuf() function to requestunbuffered input and output for an open stream |
L_tmpnam | The size of achar array which is large enough to store a temporary filename generated by thetmpnam() function |
NULL | A macro expanding to thenull pointer constant; that is, a constant representing a pointer value which is guaranteednot to be a valid address of an object in memory |
SEEK_CUR | An integer which may be passed to thefseek() function to request positioning relative to the current file position |
SEEK_END | An integer which may be passed to thefseek() function to request positioning relative to the end of the file |
SEEK_SET | An integer which may be passed to thefseek() function to request positioning relative to the beginning of the file |
TMP_MAX | The maximum number of unique filenames generable by thetmpnam() function; will be at least 25 |
Variables defined in the<stdio.h> header include:
Name | Notes |
---|---|
stdin | A pointer to aFILE which refers to the standard input stream, usually a keyboard. |
stdout | A pointer to aFILE which refers to the standard output stream, usually a display terminal. |
stderr | A pointer to aFILE which refers to the standard error stream, often a display terminal. |
Data types defined in the<stdio.h> header include:
ThePOSIX standard defines several extensions tostdio in its Base Definitions, among which are areadline function that allocates memory, thefileno andfdopen functions that establish the link betweenFILE objects andfile descriptors, and a group of functions for creatingFILE objects that refer to in-memory buffers.[5]
The following C program opens a binary file calledmyfile, reads five bytes from it, and then closes the file.
#include<stdio.h>#include<stdlib.h>intmain(void){charbuffer[5];FILE*fp=fopen("myfile","rb");if(fp==NULL){perror("Failed to open file\"myfile\"");returnEXIT_FAILURE;}if(fread(buffer,1,5,fp)<5){fclose(fp);fputs("An error occurred while reading the file.\n",stderr);returnEXIT_FAILURE;}fclose(fp);printf("The bytes read were: ");for(inti=0;i<5;++i){printf("%02X ",buffer[i]);}putchar('\n');returnEXIT_SUCCESS;}
Several alternatives tostdio have been developed. Among these is theC++iostream library, part of theISO C++ standard. ISO C++ still requires thestdio functionality.
Other alternatives include the Sfio[6] (A Safe/Fast I/O Library) library fromAT&T Bell Laboratories. This library, introduced in 1991, aimed to avoid inconsistencies, unsafe practices and inefficiencies in the design ofstdio. Among its features is the possibility to insertcallback functions into a stream to customize the handling of data read from or written to the stream.[7] It was released to the outside world in 1997, and the last release was 1 February 2005.[8]
stdio.h
– Base Definitions Reference,The Single UNIX Specification, Version 4 fromThe Open Group{{cite web}}
: CS1 maint: bot: original URL status unknown (link)