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Input/Output and Standard C Library

Jan Faigl

Department of Computer Science

Faculty of Electrical Engineering

Czech Technical University in Prague

Lecture 06

BE5B99CPL - C Programming Language

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Overview of the Lecture

Part 1 - Input and Output

File Operations

Character Oriented I/O

Text Files

Block Oriented I/O

Non-Blocking I/O

Terminal I/OK. N. King: chapters 22

Part 2 - Selected Standard Libraries

Standard library - Selected Functions

Error HandlingK. N. King: chapters 21, 23, 24, 26, and 27

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O

Part I

Input and Output

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O Text vs Binary FilesThere is not significant difference between text and binary files from the machine processing perspectiveText files are oriented to be a human readable In text files, bytes represent charactersThe content is usually organized into lines

Different markers for theend-of-lineare used (1 or 2 bytes)There can be a special marker for theend-of-file(Ctrl-Z)

It is from CP/M and later used in DOS. It is not widely used in Unix like systems.For parsing text files, we can use

Character oriented functions -putchar(),getchar(),putc(),getc()Functions for formatted i/o -printf()andscanf()as shortcuts for the

fprintf()andfscanf()with thestdinandstdoutstreamsLine oriented functions -puts(),gets()and variantsfputs(),fgets()Text files can be considered as a sequence of bytes

Numeric values as text need to be parsed and formatted in writing Numbers in binary files may deal with byte ordering

E.g., ARM vs x86

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O

File openFunctions for input/output are defined in the standard libraryThe file access is through using a pointer to a file (stream)FILE*File can be opened usingfopen()

FILE*fopen(constchar * restrictpath,const char * restrictmode); Notice, therestrictkeywordOperations with the files are

Stream oriented - sequential reading/writing

Thecurrent p ositionin the file is lik ea cursorAt the opening the file, the cursor is set to the beginning of the file

The mode of the file operations is specified in themodeparameter"r"- reading from the file

The program (user) needs to have sufficient rights for reading from the file."w"- writing to the file

A new file is created if it does not exists; otherwise the content of the

file is cleared."a"- append to the file - the cursor is set to the end of the fileThe modes can be combined, e.g.,"r+"open the file for reading

and writingSeeman fopen

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O fopen(),fclose(), andfeof()Test the file has been opened

1char*fname = " file.txt";

2

3if((f = f open(fname," r")) == NULL) {

4fprintf(stderr," Error:open f ile"% s"\n", fname);

5}Close file -int fclose(FILE *stream);

1if(fclose(f) == EOF) {

2fprintf(stderr," Error:close f ile"% s"\n", fname);

3}Test of reaching the end-of-file (EOF) -int feof(FILE *stream);

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O File PositioningEvery stream has the cursor, i.e., an associated file position

The position can be set usingoffsetrelatively towhenceint fseek(FILE *stream, long offset, int whence);

wherewhenceSEEK_SET- set the position from the beginning of fileSEEK_CUR- relatively to the current file positionSEEK_END- relatively to the end of file

If the position is successfully set,fseek()returns 0void rewind(FILE *stream);sets the position to the beginning

of fileThe position can be stored and set by the functions int fgetpos(FILE * restrict stream, fpos_t * restrict pos); int fsetpos(FILE *stream, const fpos_t *pos);

Seeman fseek,man rewind, etc

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O File Stream ModesModes in thefopen()can be combined

FILE*fopen(constchar * restrictpath,const char * restrictmode);"r"open for reading"w"Open for writing (file is created if it does not exist)"a"open for appending (set cursor to the end of file or create a

new file if it does not exists)"r+"open for reading and writing (starts at beginning)"w+"open for reading and writing (truncate if file exists)"a+"open for reading and writing (append if file exists)There are restrictions for the combined modes with"+"We cannot switch from reading to writing without calling a file-

positioning function or reaching the end of fileWe cannot switch from writing to reading without callingfflush()

or calling a file-positioning function.

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O Temporary FilesFILE* tmpfile(void);- creates a temporary file that exists until it is closed or the program existschar* tmpnam(char *s);- generates a name for a temporary fileIfsisNULL, it creates a name and store it in a static variable and return a pointer to itOtherwise it copies the string into the provided character array (s) and returns the pointer to the first character of the array

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O

File Bufferingint fflush(FILE *stream);- flushes buffer for the givenstreamfflush(NULL);- flushes all buffers (all output streams)Change the buffering mode, size, and location of the buffer

int setvbuf(FILE * restrict stream, char * restrict buf, int mode, size_t size);

Themodecan be one of the following macros

_IOFBF- full buffering. Data are read from the stream when buffer is empty and written to the stream when it is full _IOLBF- line buffering. Data are read or written from/to the stream one line at a time _IONBF- no buffer. Direct reading and writing without buffer define

BUFFER _SIZE512

char buffer[BU FFER_SIZE];

setvbuf(stream, buffer, _IOFBF, BUFFER_SIZE);void setbuf(FILE * restrict stream, char * restrict buf);

- similar tosetvbuf()but with default mode

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O

Detecting End-of-File and Error ConditionsThree possible errors can occur during reading data (e.g.,fscanf)End-of-file- w ereach the end of file

Or, the stream is closed, e.g.,stdinRead error- the read function is unable to read data from the stream

Matching failure- the read data do esnot match the requeste dfo rmat

Each stream (FILE*) has two indicators:error indicator- indicates that a read or write error occursend-of-file indicator- is set when the end of file is reachedThe indicators can be read (tested if the indicator is set or not) and

clear the error and eof indicatorsint ferror(FILE *stream); void clearerr(FILE *stream); int feof(FILE *stream);

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O

Reading and Writing Single ByteBasic function for reading fromstdinandstdoutaregetchar()andputchar()Both function returnintvalue, to indicate an error (EOF)The written and read values are converted tounsigned charThe variants of the function for the specific stream are

int getc(FILE *stream);and

int putc(int c, FILE *stream);getchar()is equivalent togetc(stdin)putchar()is equivalent toputc()with thestdoutstreamReading byte-by-byte (unsigned char) can be also used to read

binary data, e.g., to construct 4 bytes lengthintfrom the four byte (char) values

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O Example - Copy usinggetc()andputc()1/2Simple copy program based on reading bytes fromstdinand writing them tostdout

1intc;

2intbytes = 0;

3while((c = getc(stdin)) != EOF) {

4if(putc(c, stdout) == EOF) {

5fprintf(stderr," Errori nputc ");

6break;

7}

8bytes += 1;

9} lec06/copy-getc_putc.c

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O

Example - Copy usinggetc()andputc()2/2We can count the number of bytes and need time to copy the bytes

1#include

2...3

4structtimeva lt1, t2;

5gettimeofday(&t1, NULL);

6

7...// cop ythe stdin -> stdout

8

9gettimeofday(&t2, NULL);

10doubledt = t2.tv_sec - t1.tv_sec + ((t2.tv_usec - t1.

tv_usec) / 1000000.0);

11doublemb = bytes / (1024 * 1024);

12fprintf(stderr," %.2lfMB /sec\n", mb / dt);lec06/copy-getc_putc.cExample of creating random file and using the program

clang -O2 copy-getc_putc.c dd bs=512m count=1 if =/dev/random of=/tmp/rand1.dat1+0 recordsin 1+0 records out

536870912 bytes transferred

in

7.89722 7secs (67982205 bytes/s ec)

./a.out < /tmp/rand1.dat >/tmp/rand2.dat

326.10 MB/sec

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O Line Oriented I/OA whole (text) line get to be read by char* gets(char *str);

char* fgets(char * restrict str, int size, FILE * restrict stream);gets()cannot be used securely due to lack of bounds checkingA line can be written byfputs()anputs()puts()write the given string and anewline cha racterto the

stdoutstreamputs()andfputs()return a non-negative integer on success and

EOFon error

Seeman fgets,man fputs

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O Formatted I/O -fscanf()int fscanf(FILE *file, const char *format, ...); It return number of read items, e.g., for the input record 1 13.4The statement int r = f scanf(f, s d lf n , str, &i, &d);sets (in the case of success) the variable r to the value 3 r == 3For reading strings, it is necessary to respect the size of the allocated memory , e.g., by using the limited length of the read string char str[10]; int r = fs canf(f, %9 s d lf n , str, &i, &d); lec06/file_scanf.c

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O Formatted I/O -fprintf()int fprintf(FILE *file, const *format, ...); int main( int a rgc, char *argv[]) {char*fname = argc > 1 ? argv[ 1]: " out.txt";FILE *f;if((f = fopen(fname, " w")) == NULL) {

fprintf(stderr," Error:O penfile "% s"\n", fname);return-1; }fprintf(f," Programarguments argc :% d\n", argc);

for int i = 0; i < argc; + +i){ fprintf(f, argv d s n , i, argv[i]); }if(fclose(f) == EOF) { fprintf(stderr," Error:C losefile "% s"\n", fname);return-1; }return0; }lec06/file_printf.c

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O Block Read/WriteWe can usefread()andfwrite()to read/write a block of data size_t fread( void restrict ptr, size_t size, size_t nmemb,

FILE *

restrict stream); size_t fwrite( const void restrict ptr, size_t size, size_t nmemb,

FILE *

restrict stream); Useconstto indicate (ptr) is used only for reading

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O Block Read/Write - Example 1/4Program to read/write a given (as#define BSIZE) number of

intvalues using#define BUFSIZElength bufferWriting is enabled by the optional program argument-wFile for reading/writing is a mandatory program argument

1#include

2#include

3#include

4#include

5#include

6#include

7

8#include

9

10#ifndefBUFSIZE

11#defineBUFSIZE 32768

12#endif

13

14#ifndefBSIZE

15#defineBSIZE 4098

16#endif17intmain( intargc, char *argv[])

18{

19intc = 0;

20_Bool read = true;

21constchar *fnam e= NULL;

22FILE *file;

23constchar *mode = " r";

24while(argc-- > 1) {

25fprintf(stderr," DEBUG:argc :% d"% s"\n", argc, argv[argc]);

26if(strcmp(argv[argc], " -w") == 0) {

27fprintf(stderr," DEBUG:enable writting \n");

28read = false;// enable writting

29mode =" w";

30}else {

31fname = argv[argc];

32}

33}// end w hilelec06/demo-block_io.c

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O

Block Read/Write - Example 2/4

34file = fopen(fname, mode);

35if(!file) {

36fprintf(stderr," ERROR:Cannot open file "% s",error % d- % s\n", fname, errno, strerror(errno));

37return-1;

38}

39int*data = ( int*)malloc(BSIZE *sizeof (int));

40assert(data);

41structtimeval t1, t2;

42gettimeofday(&t1, NULL);

43if(read) { /* READ FILE */

44fprintf(stderr," INFO:Read from the file "% s"\n", fname);

45c = fread(data,sizeof (int), BSIZE, file);

46if(c != BSI ZE){

47fprintf(stderr," WARN:Read only % iobjects ( int)\n", c);

48}else {

49fprintf(stderr," DEBUG:Read % iobjects ( int)\n", c);

50}

51}else { /* WRITE FILE */

52charbuffer[BUFSIZE];

53if(setvbuf(file, buffer, _IOFBF, BUFSIZE)) { /* SET BUFFER */

54fprintf(stderr," WARN:Cannot set buffer ");

55}

56fprintf(stderr," INFO:Write to the file "% s"\n", fname);

57c = fwrite(data,sizeof (int), BSIZE, file);

58if(c != BSI ZE){

59fprintf(stderr," WARN:Write only % iobjects ( int)\n", c);

60}else {

61fprintf(stderr," DEBUG:Write % iobjects ( int)\n", c);

62}

63fflush(file);

64}

65gettimeofday(&t2, NULL);lec06/demo-block_io.c

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O

Block Read/Write - Example 3/4

66doubledt = t 2.tv_sec- t1.tv_sec + ((t2.tv_us ec- t1.tv_usec) / 1000000.0);

67doublemb = ( sizeof(int) * c)/ (1024 * 1024);

68fprintf(stderr," DEBUG:feof :% iferror :% i\n", feof(file), ferror(file));

69fprintf(stderr," INFO:% s% luMB \n", (read ?" read": " write"),sizeof (int)*BSIZE/(1024 * 1024));

70fprintf(stderr," INFO:%.2 lfMB /sec\n", mb / dt);

71free(data);

72returnEXIT_SUCCESS;

73}DefaultBUFSIZE(32 kB) to write/read 108integer values (480 MB)

clang -DBSIZE=100000000 demo-block_io.c && ./a.out -w a 2>&1 | grep INFO

INFO: Write to the file "a"

INFO: write 381 MB

INFO: 10.78 MB/sec

./a.out a 2>&1 | grep INFO

INFO: Read from the file "a"

INFO: read

381 MB

INFO: 1683.03 MB/secTry to read more elements results infeof(), but not inferror() clang -DBSIZE=200000000 demo-block_io.c && ./a.out a

DEBUG: argc: 1 "a"

INFO: Read from the file "a"

WARN: Read only 100000000 objects (int)

DEBUG: feof: 1 ferror: 0

INFO: read

762 MB

INFO: 1623.18 MB/seclec06/demo-block_io.c

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O Block Read/Write - Example 4/4Increased write bufferBUFSIZE(128 MB) improves writing performance clang -DBSIZE=100000000 -DBUFSIZE=134217728 demo-block_io.c && ./a.out -w aa 2>&1 | grep INFO

INFO: Write to the file "aa"

INFO: write 381 MB

INFO: 325.51 MB/secBut does not improve reading performance, which relies on the standard size of the buffer clang -DBSIZE=100000000 -DBUFSIZE=134217728 demo-block_io.c && ./a.out aa 2>&1 | grep INFO

INFO: Read from the file "aa"

INFO: read

38 1MB

INFO: 1693.39 MB/seclec06/demo-block_io.c

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O

Blocking and Non-Blocking I/O OperationsUsually I/O operations are considered asblocking requestedSystem call does not return control to the application until the re-

quested I/O is completedIt is motivated that we need all the requested data and I/O opera- tions are usually slower than the other parts of the program.

We have to wait for the data anywayIt is also calledsynchronousprogrammingNon-Blockingsystem calls do not wait for unrelated I/O to

complete, and thus do not block the applicationIt is suitable for network programming, multiple clients, graphical

user interface, or when we need to avoid "deadlock" or too long

waiting due to slow or not reliable communicationCall for reading requested data will read (and "return") only data

that are available in the input bufferAsynchronousprogramming withnon-blockingcallsReturn control to the application immediately

Data are transferred to/from buffer "on the background"

Call back, triggering a signal, etc.

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File OperationsCha racterOriented I/O T extFiles Blo ckOriented I/O Non-Blo ckingI/O T erminalI /O Non-Blocking I/O Operations - ExampleSetting the file stream (file descriptor) to theO_NONBLOCKmode Also for socket descriptorFor reading from regular files it does not too much sense to use non-blocking operationsReading from block devices such as serial port, e.g.,/dev/ttyS10 may be more suitableWe can setO_NONBLOCKflag for a file descriptor usingfcntl() include POSIX open fil eby the open() syste mcall that return a file descriptor int fd = open(quotesdbs_dbs6.pdfusesText_11