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Input/output functionality in the C programming language
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 <stdio.h>.[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 <stdio.h> (or in the C++ header cstdio, which contains the standard C functionality but in the std namespace).
ByteConstants 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 eight _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 25Variables defined in the <stdio.h> header include:
Name Notes stdin A pointer to a FILE which refers to the standard input stream, usually a keyboard. stdout A pointer to a FILE which refers to the standard output stream, usually a display terminal. stderr A pointer to a FILE which refers to the standard error stream, often a display terminal.Data types defined in the <stdio.h> header include:
The POSIX standard defines several extensions to stdio in its Base Definitions, among which are a readline function that allocates memory, the fileno and fdopen functions that establish the link between FILE objects and file descriptors, and a group of functions for creating FILE objects that refer to in-memory buffers.[5]
The following C program opens a binary file called myfile, reads five bytes from it, and then closes the file.
#include <stdio.h>
#include <stdlib.h>
int main(void) {
char buffer[5];
FILE* fp = fopen("myfile", "rb");
if (fp == NULL) {
perror("Failed to open file \"myfile\"");
return EXIT_FAILURE;
}
if (fread(buffer, 1, 5, fp) < 5) {
fclose(fp);
fputs("An error occurred while reading the file.\n", stderr);
return EXIT_FAILURE;
}
fclose(fp);
printf("The bytes read were: ");
for (int i = 0; i < 5; ++i) {
printf("%02X ", buffer[i]);
}
putchar('\n');
return EXIT_SUCCESS;
}
Alternatives to stdio[edit]
Several alternatives to stdio have been developed. Among these is the C++ iostream library, part of the ISO C++ standard. ISO C++ still requires the stdio functionality.
Other alternatives include the Sfio[6] (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 stdio. 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.[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 5 from The Open Group{{cite web}}: CS1 maint: bot: original URL status unknown (link)RetroSearch is an open source project built by @garambo | Open a GitHub Issue
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