C file input/output: Difference between revisions

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{{Use dmy dates|date=February 2022}}

{{C Standard Library}}

The [[C (programming language)|C programming language]] provides many [[standard library]] [[subroutine|functions]] for [[computer file|file]] [[input/output|input and output]]. These functions make up the bulk of the [[C standard library]] [[header file|header]] {{mono|<'''stdio.h'''>}}.<ref>{{cite book |title=ISO/IEC 9899:1999 specification |at=p. 274, § 7.19 |language=en-US}}</ref> The functionality descends from a "portable I/O package" written by [[Mike Lesk]] at [[Bell Labs]] in the early 1970s,<ref>{{cite book |last1=Kernighan |first1=Brian |author-link1=Brian Kernighan |last2=Pike |first2=Rob |author-link2=Rob Pike |title=[[The UNIX Programming Environment]] |publisher=[[Prentice Hall]] |___location=[[Englewood Cliffs]] |year=1984 |page=200|bibcode=1984upe..book.....K }}</ref> and officially became part of the [[Unix]] operating system in [[Version 7 Unix|Version 7]].<ref name="reader">{{cite tech report |first1=M. D. |last1=McIlroy |author-link1=Doug McIlroy |year=1987 |url=http://www.cs.dartmouth.edu/~doug/reader.pdf |title=A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 |series=CSTR |number=139 |institution=Bell Labs}}</ref>

The I/O functionality of C is fairly low-level by modern standards; C abstracts all file operations into operations on [[stream (computing)|stream]]s of [[byte]]s, which may be "input streams" or "output streams". Unlike some earlier programming languages, C has no direct support for [[random access|random-access]] data files; to read from a record in the middle of a file, the programmer must create a stream, [[fseek|seek]] to the middle of the file, and then read bytes in sequence from the stream.

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! Description

|-

! rowspan=79 | File access

| colspan=2 style="text-align:center;font-family:monospace" | {{anchor|fopen}}[https://en.cppreference.com/w/c/io/fopen fopen]

| Opens a file (with a non-Unicode filename on Windows and possible UTF-8 filename on Linux)

|-

| colspan=2 style="text-align:center;font-family:monospace" | {{anchor|popen}}[https://man7.org/linux/man-pages/man3/popen.3.html popen]

| opens a process by creating a pipe, forking, and invoking the shell

|-

| colspan=2 style="text-align:center;font-family:monospace" | {{anchor|freopen}}[https://en.cppreference.com/w/c/io/freopen freopen]

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| colspan=2 style="text-align:center;font-family:monospace" | {{anchor|fclose}}[https://en.cppreference.com/w/c/io/fclose fclose]

| Closes a file

|-

| colspan=2 style="text-align:center;font-family:monospace" | {{anchor|pclose}}[https://man7.org/linux/man-pages/man3/pclose.3p.html pclose]

| closes a stream

|-

| colspan=2 style="text-align:center;font-family:monospace" | {{anchor|setbuf}}[https://en.cppreference.com/w/c/io/setbuf setbuf]

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int main(void) {

char buffer[5];

size_t len;

FILE* fp = fopen("myfile", "rb");

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}

if ((len = fread(buffer, 1, 5, fp)) < 50) {

fclose(fp);

fputs("An error occurred while reading the file.\n", stderr);

return EXIT_FAILURE;

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printf("The bytes read were: ");

for (int i = 0; i < 5len; ++i) {

printf("%02X ", buffer[i]);

}

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==Alternatives to stdio{{anchor|Sfio}}==

{{Redirect|Sfio|<ref></ref>other uses of "SFIO"|SFIO (disambiguation)}}

Several alternatives to {{mono|stdio}} have been developed. Among these is theare [[Input/output (C++)|C++ {{mono|iostream}}I/O]] libraryheaders <code><iostream></code> and <code><print></code>, part of the [[ISO C++|ISO C++ standard]]. ISO C++ still requires the {{mono|stdio}} functionality.

Other alternatives include the Sfio<ref>{{Cite web |url=http://akpublic.research.att.com/sw/tools/sfio/ |title=Sfio: A Safe/Fast I/O Library |access-date=16 March 2021 |archive-date=11 February 2006 |archive-url=https://web.archive.org/web/20060211021834/http://akpublic.research.att.com/sw/tools/sfio/ |url-status=bot: unknown }}</ref> (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 {{mono|stdio}}. Among its features is the possibility to insert [[callback function]]s into a stream to customize the handling of data read from or written to the stream.<ref>{{cite conference |title=SFIO: Safe/Fast String/File IO |first1=David G. |last1=Korn |author-link=David Korn (computer scientist) |first2=Kiem-Phong |last2=Vo |conference=Proc. Summer USENIX Conf. |year=1991 |citeseerx=10.1.1.51.6574}}</ref> It was released to the outside world in 1997, and the last release was 1 February 2005.<ref>{{cite conference |first1=Glenn S. |last1=Fowler |first2=David G. |last2=Korn |first3=Kiem-Phong |last3=Vo |title=Extended Formatting with Sfio |conference=Proc. Summer USENIX Conf. |year=2000}}</ref>