Stream Model of I/O. Basic I/O in C

Transcription

1 Stream Model of I/O 1 A stream provides a connection between the process that initializes it and an object, such as a file, which may be viewed as a sequence of data. In the simplest view, a stream object is simply a serialized view of that other object. input file To be, or not to be? T o b e o r... That is the question. executing process We think of data as flowing in the stream to the process, which can remove data from the stream as desired. The data in the stream cannot be lost by flowing past before the program has a chance to remove it. header file: <stdio.h>

2 C FILE Type Accessing a stream requires a pointer of type FILE. 2 C provides three standard streams, which require no special preparation other than the necessary include directive: stdin standard input keyboard stdout standard output console window stderr standard error console window Alternatively, you can declare FILE pointers and manage their connections to disk files.

3 Output with printf() 3 The Standard Library provides the printf() function which can be used to write formatted output to the standard output stream, stdout. int printf(const char * restrict format,...); The first parameter is a string literal that specifies the formatting to be used. The remaining parameters, if any, specify the values to be written. The return value is the number of characters that were written, or a negative value if an error was encountered.

4 Output Examples The specification of format is mildly complex: 4 int i = 42, j = 17; double x = ; printf("i =%3d j =%3d\n", i, j); printf("x =%7.4f\n", x); i = 42 j = 17 x = There is an excellent discussion of the details in section 22.3 of the King book.

5 More on printf() Format Specifiers 5 The general form of a printf() specifier is: flags min width % # precision L length modifier conversion specifier g flags: optional, more than one allowed - left-justify within field + always show leading sign space precede non-negative numbers with a space # see reference 0 pad with zeros to fill field

6 More on printf() Format Specifiers 6 The general form of a printf() specifier is: flags min width % # precision L length modifier conversion specifier g min width: optional, pad with spaces if field not filled, ignored if insufficient precision: optional, number of digits for integer values, number of digits after decimal point for float/double

7 More on printf() Format Specifiers 7 The general form of a printf() specifier is: flags min width % # precision L length modifier conversion specifier g length modifier: optional, indicates value has a type that's longer or shorter than is normal for a particular conversion specifier (see following) d u hd ld normally used for int values normally used for unsigned int values normally used for short int values normally used for long int values see reference for more Table 22.5

8 More on printf() Format Specifiers 8 The general form of a printf() specifier is: flags min width % # precision L length modifier conversion specifier g conversion specifier: mandatory d, i converts int to decimal form f, F converts double to decimal form; default precison 6 c displays int as unsigned character see reference for more Table 22.6

9 Format Specifiers for <stdint.h> 9 The basic integer format codes will work with int32_t and uint32_t (with compiler warnings), but are not reliable with int64_t and uint64_t. The header file <inttypes.h> provides specialized format codes for the new integer types. Here's a very brief description; see King 27.2 for details. PRIdN for signed integer types, N = 8, 16, 32, 64 PRIuN for unsigned integer types For example: uint64_t K = ; printf("%15"priu64"\n", K); // note use of quotes!!

10 Input with scanf() 10 The Standard Library provides the scanf() function which can be used to write formatted output to the standard output stream, stdin. int scanf(const char * restrict format,...); The first parameter is a string literal that specifies the formatting expected in the input stream. The remaining parameters, if any, specify the variables that will receive the values that are read. The return value is the number of values that were read, or the value of EOF if an input failure occurs.

11 Input Examples 11 Suppose we have an input stream of the following form: int i = 1, j = 1; double x = 1.5; scanf("%d %d %f", &i, &j, &x); printf("%5d %5d %7.4f\n", i, j, x); Suppose we have an input stream of the following form: 17, 42, int i = 1, j = 1; double x = 1.5; scanf("%d, %d, %f", &i, &j, &x); printf("%5d %5d %7.4f\n", i, j, x);

12 More on scanf() Format Specifiers 12 The general form of a scanf() specifier is: % max width length modifier * 12 L g conversion specifier *: optional, read but do not assign value to an object max width: optional, leading whitespace doesn't count

13 More on scanf() Format Specifiers 13 The general form of a scanf() specifier is: % max width length modifier * 12 L g conversion specifier length modifier: optional, indicates object that will receive value has a type that's longer or shorter than is normal for a particular conversion specifier (see following) see reference for more Table conversion specifier: mandatory see reference for more Table 22.12

14 Opening Files 14 File I/O is almost identical to I/O with stdin and stdout. You must make a call in order to associate a FILE pointer with a particular file: FILE *fopen(const char* restrict filename, const char* restrict mode); filename mode path/name of file to be opened "r" "w" "a" "rb" "wb" "ab" "r+" "w+" "a+" see reference for more Return value is valid FILE pointer if successful and NULL otherwise.

15 File I/O 15 File I/O is accomplished using variations of printf() and scanf(): int fprintf(file * restrict stream, const char * restrict format,...); int fscanf(file * restrict stream, const char * restrict format,...); These are used in the same way as their counterparts, aside from taking a FILE*.

16 Closing Files 16 When done with a file, you should close the file: int fclose(file *stream); Any unwritten buffered data will be delivered to the host environment. Any unread buffered data will be discarded. Returns 0 on success and EOF otherwise.