The C programming language provides many standard library functions for file input and output. These functions make up the bulk of the C standard library header .[1] The functionality descends from a "portable I/O package" written by Mike Lesk at Bell Labs in the early 1970s,[2] and officially became part of the Unix operating system in Version 7.[3]
The I/O functionality of C is fairly low-level by modern standards; C abstracts all file operations into operations on streams of bytes, which may be "input streams" or "output streams". Unlike some earlier programming languages, C has no direct support for random-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 the C 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 (or in the C++ header, which contains the standard C functionality but in the namespace).
| 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) | |
| freopen | Opens a different file with an existing stream | ||
| fflush | Synchronizes an output stream with the actual file | ||
| fclose | Closes a file | ||
| 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/ from a file stream |
| fgets | fgetws | Reads a byte/ line from a file stream | |
| fputc putc | fputwc putwc | Writes a byte/ to a file stream | |
| fputs | fputws | Writes a byte/ string to a file stream | |
| getchar | getwchar | Reads a byte/ 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/ to stdout | |
| puts | Writes a byte string to stdout | ||
| ungetc | ungetwc | Puts a byte/ back into a file stream | |
| Formatted input/output | scanf fscanf sscanf | wscanf fwscanf swscanf | Reads formatted byte/ input from stdin, a file stream or a buffer |
| vscanf vfscanf vsscanf | vwscanf vfwscanf vswscanf | Reads formatted input byte/ from stdin, a file stream or a buffer using variable argument list | |
| printf fprintf sprintf snprintf | wprintf fwprintf swprintf | Prints formatted byte/ output to stdout, a file stream or a buffer | |
| vprintf vfprintf vsprintf vsnprintf | vwprintf vfwprintf vswprintf | Prints formatted byte/ output to stdout, a file stream, or a buffer using variable argument list | |
| perror | Writes a description of the current 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 header include:
| Name | Notes | |
|---|---|---|
| EOF | A negative integer of type used to indicate end-of-file conditions | |
| BUFSIZ | An integer which is the size of the buffer used by the function | |
| FILENAME_MAX | The size of a 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 the 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 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 function to request unbuffered input and output for an open stream | |
| L_tmpnam | The size of a array which is large enough to store a temporary filename generated by the 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 function to request positioning relative to the current file position | |
| SEEK_END | An integer which may be passed to the function to request positioning relative to the end of the file | |
| SEEK_SET | An integer which may be passed to the function to request positioning relative to the beginning of the file | |
| TMP_MAX | The maximum number of unique filenames generable by the function; will be at least 25 |
Variables defined in the header include:
| Name | Notes | |
|---|---|---|
| stdin | A pointer to a which refers to the standard input stream, usually a keyboard. | |
| stdout | A pointer to a which refers to the standard output stream, usually a display terminal. | |
| stderr | A pointer to a which refers to the standard error stream, often a display terminal. |
Data types defined in the header include:
The POSIX standard defines several extensions to in its Base Definitions, among which are a function that allocates memory, the and functions that establish the link between objects and file descriptors, and a group of functions for creating objects that refer to in-memory buffers.
The following C program opens a binary file called myfile, reads five bytes from it, and then closes the file.
int main(void)
Several alternatives to have been developed. Among these is the C++ library, part of the ISO C++ standard. ISO C++ still requires the functionality.
Other alternatives include the Sfio[5] (A Safe/Fast I/O Library) library from AT&T Bell Laboratories. This library, introduced in 1991, aimed to avoid inconsistencies, unsafe practices and inefficiencies in the design of . Among its features is the possibility to insert callback functions into a stream to customize the handling of data read from or written to the stream.[6] It was released to the outside world in 1997, and the last release was 1 February 2005.[7]