Standard Input/Output Library - University of Windsor

Content60-256 System Programming:Standard Input/Output Libraryby Dr. B. BoufamaSchool of Computer ScienceUniversity of Windsor–Instructor: Dr. A. Habedadlane@cs.uwindsor.cahttp://cs.uwindsor.ca/∼adlane/60-256Standard Input/Output Library60-256 System Programming

ContentContent1 Introduction2 Streams and FILE objects3 BufferingFully buffered I/OLine buffered I/OUnbuffered I/OANSI C buffering requirementsChanging the default bufferingExamples4 Opening a Stream5 Reading and writing a streamStandard Input/Output Library60-256 System Programming

IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamIntroductionThe Standard Input/Output Library was written by DennisRitchie around 1975.This library is specified by the ANSI C standard because it hasbeen implemented on different operating systems.This library handles details such asbuffer allocationperforming I/O in optimal-sized chunksThe header file of this library is /usr/include/stdio.hStandard Input/Output Library60-256 System Programming

IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamStreams and FILE objectsOpening or creating a file → associating a stream with the file.fopen()The function fopen() returns a pointer to the a FILE object.A FILE object is a structure that contains all neededinformation to manage a stream:the file descriptor: a nonnegative integer used for theactual I/Oa pointer to a buffer for the streamthe size of the buffera count of the characters currently in the bufferan error flagan end-of-file flagNormally, an application software never needs to examine theFILE object.Standard Input/Output Library60-256 System Programming

BufferingIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamFully buffered I/OLine buffered I/OUnbuffered I/OANSI C buffering requirementsChanging the default bufferingExamplesGoal of bufferingMinimize the number of I/O system calls.There are three types of buffering provided:fully buffered,line buffered,unbuffered.Standard Input/Output Library60-256 System Programming

Fully buffered I/OIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamFully buffered I/OLine buffered I/OUnbuffered I/OANSI C buffering requirementsChanging the default bufferingExamplesIn the case of fully buffered I/O, actual I/O take place onlywhen the I/O buffer is full.Disk files are fully buffered by the standard I/O library.The buffer is usually created using malloc the first timeI/O is performed on a stream.Flushing buffersWe can call the function fflush() to flush a stream forcing itsassociated buffer to be written even when it is partially filled.Syntax:int fflush(FILE *fp)When fp is NULL, fflush() causes all output streams to beflushed.Standard Input/Output Library60-256 System Programming

Unbuffered I/OIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamFully buffered I/OLine buffered I/OUnbuffered I/OANSI C buffering requirementsChanging the default bufferingExamplesIn this case, the standard I/O library does not buffer thecharacters.→ each time we print or read a single character, the actual I/Ooperation takes place.For example, the standard error stream is normally unbuffered.Standard Input/Output Library60-256 System Programming

IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamANSI C buffering requirementsFully buffered I/OLine buffered I/OUnbuffered I/OANSI C buffering requirementsChanging the default bufferingExamplesANSI C requires the following buffering characteristics:1 Standard I/O are fully buffered if and only if they do notrefer to an interactive device.2 Standard error is never fully buffered.Standard Input/Output Library60-256 System Programming

IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamChanging the default bufferingFully buffered I/OLine buffered I/OUnbuffered I/OANSI C buffering requirementsChanging the default bufferingExamplesWe can change the default buffering using:void setbuf(FILE *fp, char *buf);If buf is NULL then, buffering is disabled.Otherwise, buf must point to a buffer of length BUFSIZ.void setvbuf(FILE *fp, char *buf, int mode,size t size);This function can specify which buffering we wantdepending on the value of mode:IOFBF: fully bufferedIOLBF: line bufferedIONBF: unbufferedNote that when an unbuffered stream is specified, the bufand size arguments are ignored.Standard Input/Output Library60-256 System Programming

Example 1IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamFully buffered I/OLine buffered I/OUnbuffered I/OANSI C buffering requirementsChanging the default bufferingExamples#include #include // This is needed for sleep()int main(void){ // without fflushint i=0;char line[100]="Hello, my name No-Name\n";while(line[i] != NULL){putchar(line[i++]);sleep(1);}}Standard Input/Output Library60-256 System Programming

Example 2IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamFully buffered I/OLine buffered I/OUnbuffered I/OANSI C buffering requirementsChanging the default bufferingExamples#include #include // This is needed for sleep()int main(void){// forcing the flush with fflushint i=0;char line[100]="Hello, my name No-Name\n";while(line[i] != NULL){putchar(line[i++]);fflush(stdout); // flush std output buffersleep(1);}}Standard Input/Output Library60-256 System Programming

Example 3IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamFully buffered I/OLine buffered I/OUnbuffered I/OANSI C buffering requirementsChanging the default bufferingExamples#include #include // needed for sleep()int main(int argc, char *argv[]){}while(1){printf("Hello, program is runing right now");sleep(1);}Standard Input/Output Library60-256 System Programming

Opening a StreamIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamThree functions can be used to open a standard I/O stream:FILE *fopen(const char *f, const char *t)this is the most used one.Example: file = fopen(‘‘./data.txt’’, ‘‘r’’)FILE *freopen(const char *f, const char *t,FILE *fp)Opens a specified file on a specified stream, closing thestream first if it was already open.FILE *fdopen(int filedesc, const char *t)This function associates a standard I/O stream with anexisting file descriptor (the filedesc argument).Standard Input/Output Library60-256 System Programming

File descriptorsIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamA file descriptor is a nonnegative integer used by thekernel to specify any open file.A file descriptor is typically returned by the system callopen() that opens/creates a file, a pipe or, a networkcommunication channel.The function fopen() cannot be used to open a pipe or anetwork communication channel→ we use the system call open() to get a file descriptor fora pipe or a channel, then we use fdopen() to associate itwith a standard stream.Standard Input/Output Library60-256 System Programming

Argument typesIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamThe argument type, const char *t, specifies how a streamis to be opened.The possible values of type are :r for read, w for write, a for append at the end of the file, r+ forread and write, w+ for read and write and, a+ for read and writeat the end of the file.SummaryRestrictions r w a r+ w+ a+File must exist already ∗ ∗Previous contents of file lost ∗ ∗Stream can be read ∗ ∗ ∗ ∗Stream can be written ∗ ∗ ∗ ∗ ∗Stream can be written only ∗ ∗at endStandard Input/Output Library60-256 System Programming

RestrictionsIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamWhen a file is open for read and write, the following restrictionsapply:Output cannot be directly followed by input without anintervening fflush, fseek, fsetpos, or rewind.Input cannot be directly followed by output without anintervening fseek, fsetpos, rewind or, an inputoperation that encounters an EOF.int fclose(FILE *fp)int fclose(FILE *fp) closes any opened stream. Inparticular:Any buffered output data is flushed,Any buffered input data is discarded,Any allocated buffer is released.Note that when a process terminates normally, all openstandard I/O streams are closed.Standard Input/Output Library60-256 System Programming

Unformatted I/OIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamThere are 3 types of unformatted I/O:Single-character I/OLine I/O: to read or write a line at a timeDirect I/O: also called binary I/O. Useful when dealing withstructures and binary information.Standard Input/Output Library60-256 System Programming

IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamSingle-character Input functionsint getc(FILE *), int fgetc(FILE *) andint getchar(void).We have:getchar(void) is equivalent to getc(stdin).getc() can be implemented as a macro whereasfgetc() cannot.→ getc is more efficient,→ the address of fgetc can be passed as a parameterunlike getc (macros do not have addresses).Important issues about these functionsThey return the next character as an unsigned charconverted into an int. Reason: the high order bit is set withoutcausing the return value to be negative.→ we can read all possible 255 different values of a byte. Inparticular, we will never get a character with -1(EOF) as itsvalue.→ The return value from these functions can’t be stored in achar variable then compared to EOFStandard Input/Output Library60-256 System Programming

Example...IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a stream#include int main(int argc, char *argv[]){FILE *fd;char ch;int fileSize=-1;}fd = fopen(argv[1], "r");do{ch=getc(fd);fileSize++;} while( ch != EOF);printf("Size of %s is %d\n", argv[1], fileSize);./size size.c → Size of size.c is 242 (correct size)Standard Input/Output Library60-256 System Programming

...ExampleIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamThis can be solved simply by using int instead of char.#include int main(int argc, char *argv[]){FILE *fd;int ch;int fileSize=-1;//ÿfd = fopen(argv[1], "r");do{ch=getc(fd);fileSize++;} while( ch != EOF);printf("Size of %s is %d\n", argv[1], fileSize);}./size size.c → Size of size.c is 259 (correct size)Standard Input/Output Library60-256 System Programming

IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamNote that these 3 functions return -1 whether an error or theend-of-file occurs.How to differentiate between the 2 situations?For each stream, two flags are maintained in the FILE object:an error flag and an end-of-file flag.The following three functions should be used to access theseflags:int ferror(FILE*)returns nonzero for true, 0 otherwiseint feof(FILE*)returns nonzero for true, 0 otherwisevoid clearerr(FILE*)clears both the error and the end-of-file flags.Standard Input/Output Library60-256 System Programming

IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamPushing back a characterint ungetc(int c, FILE *fp)The function int ungetc(int c, FILE *fp)Return c if OK, -1 otherwise.This function is used to push back a character after readingfrom a stream.This is useful when we need to peek at the next character todetermine what to do next.Note that:The pushed-back character does not have to be the sameas the one that was read.We cannot push back EOFWe can push back a character after end-of-file is reached.Then, we can read that character because ungetc() clearsthe end-of-file flag of the stream.Standard Input/Output Library60-256 System Programming

IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamSingle-character Output functionsint putc(int c, FILE *fp): print a single characterint fputc(int c, FILE *fp): same as putc()except that, putc() can be implemented as a macrowhereas fputc() cannot.int putchar(int c): equivalent to putc(c, stdin).All these functions return the printed character c as an intwhen successful and -1 otherwise.Standard Input/Output Library60-256 System Programming

ExampleIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a stream#include int main(int argc, char *argv[]){FILE *f;char c;}f=fopen(argv[1], "w");while((c=getchar()) != EOF)fputc(c, f);Question: what happens if we exit with a CTRL-C?Standard Input/Output Library60-256 System Programming

ExampleIntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamIf this is a serious issue, then we should modify our code to:#include int main(int argc, char *argv[]){FILE *f;char c;f=fopen(argv[1], "w");setbuf(f, NULL);while((c=getchar()) != EOF)fputc(c, f);}or to#include int main(int argc, char *argv[]){FILE *f;char c;f=fopen(argv[1], "w");while((c=getchar()) != EOF){fputc(c, f);fflush(f);}}Standard Input/Output Library60-256 System Programming

IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamString/Line Input functionsInput functions:char *fgets(char *dest, int n, FILE *fp)Reads up through, including, the next newline, but no morethan n-1 characters. Then NULL added to whatever hasbeen read.char *gets(char *dest)Reads from stdin in a similar way as fgets(), but hassome serious side effects. In particular, it allows buffer tooverflow. It should never be used!Note that dest is the address of the array to read thecharacters into it. Both functions return dest if OK, NULLotherwise (end-of-file or error).Standard Input/Output Library60-256 System Programming

IntroductionStreams and FILE objectsBufferingOpening a StreamReading and writing a streamString/Line Output functionsOutput functions:fputs(const char *str, FILE *fp)Write the NULL terminated string, stored in str, to astream.puts(const char *str)Write the NULL terminated string, stored in str, to stdoutand add a newline character at the end.Note that NULL character at the end is not written by bothfunctions.Standard Input/Output Library60-256 System Programming