C Programming FAQs(英文原版).pdf

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comp.lang.c Frequently Asked Questions

Declarations and Initializations

Question 1.1

Question 1.4

Question 1.7

Question 1.11

Question 1.12

Question 1.14

Question 1.21

Question 1.22

Question 1.25

Question 1.30

Question 1.31

Question 1.32

Question 1.34

Structures, Unions, and Enumerations

Question 2.1

Question 2.2

Question 2.3

Question 2.4

Question 2.6

Question 2.7

Question 2.8

Question 2.9

Question 2.10

Question 2.11

Question 2.12

Question 2.13

Question 2.14

Question 2.15

Question 2.18

Question 2.20

Question 2.22

Question 2.24

Expressions

Question 3.1

Question 3.2

Question 3.3

Question 3.4

Question 3.5

Question 3.8

Question 3.9

Question 3.12

Question 3.14

Question 3.16

Pointers

Question 4.2

Question 4.3

Question 4.5

Question 4.8

Question 4.9

Question 4.10

Question 4.11

Question 4.12

Null Pointers

Question 5.1

Question 5.2

Question 5.3

Question 5.4

Question 5.5

Question 5.6

Question 5.9

Question 5.10

Question 5.12

Question 5.13

Question 5.14

Question 5.15

Question 5.16

Question 5.17

Question 5.20

Arrays and Pointers

Question 6.1

Question 6.2

Question 6.3

Question 6.4

Question 6.7

Question 6.8

Question 6.9

Question 6.11

Question 6.12

Question 6.13

Question 6.14

Question 6.15

Question 6.16

Question 6.17

Question 6.18

Question 6.19

Question 6.20

Question 6.21

Memory Allocation

Question 7.1

Question 7.2

Question 7.3

Question 7.5

Question 7.6

Question 7.7

Question 7.8

Question 7.14

Question 7.16

Question 7.17

Question 7.19

Question 7.20

Question 7.21

Question 7.22

Question 7.23

Question 7.24

Question 7.25

Question 7.26

Question 7.27

Question 7.30

Question 7.31

Question 7.32

Characters and Strings

Question 8.1

Question 8.2

Question 8.3

Question 8.6

Question 8.9

Boolean Expressions and Variables

Question 9.1

Question 9.2

Question 9.3

C Preprocessor

Question 10.2

Question 10.3

Question 10.4

Question 10.6

Question 10.7

Question 10.8

Question 10.9

Question 10.11

Question 10.12

Question 10.13

Question 10.14

Question 10.15

Question 10.16

Question 10.18

Question 10.19

Question 10.20

Question 10.22

Question 10.23

Question 10.25

Question 10.26

ANSI/ISO Standard C

Question 11.1

Question 11.2

Question 11.3

http://www.eskimo.com/~scs/C-faq/q11.4.html

Question 11.5

Question 11.8

Question 11.9

Question 11.10

Question 11.12

Question 11.13

Question 11.14

Question 11.15

Question 11.16

Question 11.17

Question 11.18

Question 11.19

Question 11.20

Question 11.21

Question 11.22

Question 11.24

Question 11.25

Question 11.26

Question 11.27

Question 11.29

Question 11.30

Question 11.31

Question 11.32

Question 11.33

Question 11.34

Question 11.35

Stdio

Question 12.1

Question 12.2

Question 12.4

Question 12.5

Question 12.6

Question 12.9

Question 12.10

Question 12.11

Question 12.12

Question 12.13

Question 12.15

Question 12.17

Question 12.18

Question 12.19

Question 12.20

Question 12.21

Question 12.23

Question 12.24

Question 12.25

Question 12.26

Question 12.30

Question 12.33

Question 12.34

Question 12.38

Library Functions

Question 13.1

Question 13.2

Question 13.5

Question 13.6

Question 13.7

Question 13.8

Question 13.9

Question 13.10

Question 13.11

Question 13.12

Question 13.13

Question 13.14

Question 13.15

Question 13.16

Question 13.17

Question 13.18

Question 13.20

Question 13.24

Question 13.25

Question 13.26

Question 13.28

Floating Point

Question 14.1

Question 14.2

Question 14.3

Question 14.4

Question 14.5

Question 14.6

Question 14.7

Question 14.8

Question 14.9

Question 14.11

Question 14.12

Question 14.13

Variable-Length Argument Lists

Question 15.1

Question 15.2

Question 15.3

Question 15.4

Question 15.5

Question 15.6

Question 15.7

Question 15.8

Question 15.9

Question 15.10

Question 15.11

Question 15.12

Question 15.13

Strange Problems

Question 16.3

Question 16.4

Question 16.5

Question 16.6

Question 16.8

comp.lang.c Frequently Asked Questions comp.lang.c Frequently Asked Questions This collection of hypertext pages is Copyright 1995 by Steve Summit. Content from the book ``C Programming FAQs: Frequently Asked Questions'' (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by permission of the author and the publisher as a service to the community. It is intended to complement the use of the published text and is protected by international copyright laws. The content is made available here and may be accessed freely for personal use but may not be published or retransmitted without written permission. This page is the top of an HTML version of the Usenet comp.lang.c Frequently Asked Questions (FAQ) list. An FAQ list is a collection of questions commonly asked on Usenet, together with presumably definitive answers, provided in an attempt to keep repeated questions on the newsgroup down to a low background drone so that discussion can move on to more interesting matters. Since they distill knowledge gleaned from many sources and answer questions which are demonstrably Frequent, FAQ lists serve as useful references outside of their originating Usenet newsgroups. This list is, I dare to claim, no exception, and the HTML version you're looking at now, as well as other versions referenced just below, are intended to be useful to C programmers everywhere. Several other versions of this FAQ list are available, including a book-length version published by Addison-Wesley. (The book, though longer, also has a few more errors; I've prepared an errata list.) See also question 20.40. Like so many web pages, this is very much a ``work in progress.'' I would, of course, like it if it were perfect, but it's been two years or so since I first started talking about putting this thing on the web, and if I were to wait until all the glitches were worked out, you might never see it. Each page includes a ``mail feedback'' button, so you can help me debug it. (At first, you don't have to worry about reporting minor formatting hiccups; many of these result from lingering imperfections in the programs that generate these pages, or from the fact that I have not exhaustively researched how various browsers implement the HTML tags I'm using, or from the fact that I haven't gone the last yard in trying to rig up HTML that looks good in spite of the fact that HTML doesn't have everything you need to make things look good.) These pages are synchronized with the posted Usenet version and the Addison-Wesley book version. Since not all questions appear in all versions, the question numbers are not always contiguous. [Note to web authors, catalogers, and bookmarkers: the URL is the right way to link to these pages. All other URL's implementing this collection are subject to change.] You can browse these pages in at least three ways. The table of contents below is of the list's major sections; these links lead to sub-lists of the questions for those sections. The ``all questions'' link leads to a list of all the questions; each question is (obviously) linked to its answer. Finally, the ``read http://www.eskimo.com/~scs/C-faq/top.html (1 of 3) [2003-07-28 ÿÿ€ÿˆÿ„ 9:05:16]

comp.lang.c Frequently Asked Questions sequentially'' link leads to the first question; you can then follow the ``next'' link at the bottom of each question's page to read through all of the questions and answers sequentially. Steve Summit scs@eskimo.com 1. Declarations and Initializations 2. Structures, Unions, and Enumerations 3. Expressions 4. Pointers 5. Null Pointers 6. Arrays and Pointers 7. Memory Allocation 8. Characters and Strings 9. Boolean Expressions and Variables 10. C Preprocessor 11. ANSI/ISO Standard C 12. Stdio 13. Library Functions 14. Floating Point 15. Variable-Length Argument Lists 16. Strange Problems http://www.eskimo.com/~scs/C-faq/top.html (2 of 3) [2003-07-28 ÿÿ€ÿˆÿ„ 9:05:16]

comp.lang.c Frequently Asked Questions 17. Style 18. Tools and Resources 19. System Dependencies 20. Miscellaneous Bibliography Acknowledgements All Questions Read Sequentially http://www.eskimo.com/~scs/C-faq/top.html (3 of 3) [2003-07-28 ÿÿ€ÿˆÿ„ 9:05:16]

Declarations and Initializations 1. Declarations and Initializations 1.1 How do you decide which integer type to use? 1.4 What should the 64-bit type on new, 64-bit machines be? 1.7 What's the best way to declare and define global variables? 1.11 What does extern mean in a function declaration? 1.12 What's the auto keyword good for? 1.14 I can't seem to define a linked list node which contains a pointer to itself. 1.21 How do I declare an array of N pointers to functions returning pointers to functions returning pointers to characters? 1.22 How can I declare a function that returns a pointer to a function of its own type? 1.25 My compiler is complaining about an invalid redeclaration of a function, but I only define it once and call it once. 1.30 What can I safely assume about the initial values of variables which are not explicitly initialized? 1.31 Why can't I initialize a local array with a string? 1.32 What is the difference between char a[] = "string"; and char *p = "string"; ? 1.34 How do I initialize a pointer to a function? top http://www.eskimo.com/~scs/C-faq/s1.html [2003-07-28 ÿÿ€ÿˆÿ„ 9:05:35]

Question 1.1 Question 1.1 How do you decide which integer type to use? If you might need large values (above 32,767 or below -32,767), use long. Otherwise, if space is very important (i.e. if there are large arrays or many structures), use short. Otherwise, use int. If well- defined overflow characteristics are important and negative values are not, or if you want to steer clear of sign-extension problems when manipulating bits or bytes, use one of the corresponding unsigned types. (Beware when mixing signed and unsigned values in expressions, though.) Although character types (especially unsigned char) can be used as ``tiny'' integers, doing so is sometimes more trouble than it's worth, due to unpredictable sign extension and increased code size. (Using unsigned char can help; see question 12.1 for a related problem.) A similar space/time tradeoff applies when deciding between float and double. None of the above rules apply if the address of a variable is taken and must have a particular type. If for some reason you need to declare something with an exact size (usually the only good reason for doing so is when attempting to conform to some externally-imposed storage layout, but see question 20.5), be sure to encapsulate the choice behind an appropriate typedef. References: K&R1 Sec. 2.2 p. 34 K&R2 Sec. 2.2 p. 36, Sec. A4.2 pp. 195-6, Sec. B11 p. 257 ANSI Sec. 2.2.4.2.1, Sec. 3.1.2.5 ISO Sec. 5.2.4.2.1, Sec. 6.1.2.5 H&S Secs. 5.1,5.2 pp. 110-114 Read sequentially: next up top This page by Steve Summit // Copyright 1995 // mail feedback http://www.eskimo.com/~scs/C-faq/q1.1.html [2003-07-28 ÿÿ€ÿˆÿ„ 9:05:55]

Question 1.4 Question 1.4 What should the 64-bit type on new, 64-bit machines be? Some vendors of C products for 64-bit machines support 64-bit long ints. Others fear that too much existing code is written to assume that ints and longs are the same size, or that one or the other of them is exactly 32 bits, and introduce a new, nonstandard, 64-bit long long (or __longlong) type instead. Programmers interested in writing portable code should therefore insulate their 64-bit type needs behind appropriate typedefs. Vendors who feel compelled to introduce a new, longer integral type should advertise it as being ``at least 64 bits'' (which is truly new, a type traditional C does not have), and not ``exactly 64 bits.'' References: ANSI Sec. F.5.6 ISO Sec. G.5.6 Read sequentially: prev next up top This page by Steve Summit // Copyright 1995 // mail feedback http://www.eskimo.com/~scs/C-faq/q1.4.html [2003-07-28 ÿÿ€ÿˆÿ„ 9:06:13]

Question 1.7 Question 1.7 What's the best way to declare and define global variables? First, though there can be many declarations (and in many translation units) of a single ``global'' (strictly speaking, ``external'') variable or function, there must be exactly one definition. (The definition is the declaration that actually allocates space, and provides an initialization value, if any.) The best arrangement is to place each definition in some relevant .c file, with an external declaration in a header (``.h'') file, which is #included wherever the declaration is needed. The .c file containing the definition should also #include the same header file, so that the compiler can check that the definition matches the declarations. This rule promotes a high degree of portability: it is consistent with the requirements of the ANSI C Standard, and is also consistent with most pre-ANSI compilers and linkers. (Unix compilers and linkers typically use a ``common model'' which allows multiple definitions, as long as at most one is initialized; this behavior is mentioned as a ``common extension'' by the ANSI Standard, no pun intended. A few very odd systems may require an explicit initializer to distinguish a definition from an external declaration.) It is possible to use preprocessor tricks to arrange that a line like DEFINE(int, i); need only be entered once in one header file, and turned into a definition or a declaration depending on the setting of some macro, but it's not clear if this is worth the trouble. It's especially important to put global declarations in header files if you want the compiler to catch inconsistent declarations for you. In particular, never place a prototype for an external function in a .c file: it wouldn't generally be checked for consistency with the definition, and an incompatible prototype is worse than useless. See also questions 10.6 and 18.8. References: K&R1 Sec. 4.5 pp. 76-7 K&R2 Sec. 4.4 pp. 80-1 ANSI Sec. 3.1.2.2, Sec. 3.7, Sec. 3.7.2, Sec. F.5.11 ISO Sec. 6.1.2.2, Sec. 6.7, Sec. 6.7.2, Sec. G.5.11 Rationale Sec. 3.1.2.2 H&S Sec. 4.8 pp. 101-104, Sec. 9.2.3 p. 267 CT&P Sec. 4.2 pp. 54-56 http://www.eskimo.com/~scs/C-faq/q1.7.html (1 of 2) [2003-07-28 ÿÿ€ÿˆÿ„ 9:06:18]

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