附录B 标准库 Appendix B - Standard Library

This appendix is a summary of the library defined by the ANSI standard. The standard library is not part of the C language proper, but an environment that supports standard C will provide the function declarations and type and macro definitions of this library. We have omitted a few functions that are of limited utility or easily synthesized from others; we have omitted multi-byte characters; and we have omitted discussion of locale issues; that is, properties that depend on local language, nationality, or culture.

The functions, types and macros of the standard library are declared in standard headers:

A header can be accessed by

  #include <<em>header>

Headers may be included in any order and any number of times. A header must be included outside of any external declaration or definition and before any use of anything it declares. A header need not be a source file.

External identifiers that begin with an underscore are reserved for use by the library, as are all other identifiers that begin with an underscore and an upper-case letter or another underscore.

B.1 Input and Output:

The input and output functions, types, and macros defined in represent nearly one third of the library.

A stream is a source or destination of data that may be associated with a disk or other peripheral. The library supports text streams and binary streams, although on some systems, notably UNIX, these are identical. A text stream is a sequence of lines; each line has zero or more characters and is terminated by '\n'. An environment may need to convert a text stream to or from some other representation (such as mapping '\n' to carriage return and linefeed). A binary stream is a sequence of unprocessed bytes that record internal data, with the property that if it is written, then read back on the same system, it will compare equal.

A stream is connected to a file or device by openingit; the connection is broken by closing the stream. Opening a file returns a pointer to an object of type FILE, which records whatever information is necessary to control the stream. We will use ``file pointer'' and ``stream'' interchangeably when there is no ambiguity.

When a program begins execution, the three streams stdin, stdout, and stderr are already open.

B.1.1 File Operations

The following functions deal with operations on files. The type size_t is the unsigned integral type produced by the sizeof operator.

B.1.2 Formatted Output

The printf functions provide formatted output conversion.

   int fprintf(FILE *stream, const char *format, ...)

fprintf converts and writes output to stream under the control of format. The return value is the number of characters written, or negative if an error occurred.

The format string contains two types of objects: ordinary characters, which are copied to the output stream, and conversion specifications, each of which causes conversion and printing of the next successive argument to fprintf. Each conversion specification begins with the character % and ends with a conversion character. Between the % and the conversion character there may be, in order:

• Flags (in any order), which modify the specification:
  • -, which specifies left adjustment of the converted argument in its field.
  • +, which specifies that the number will always be printed with a sign.
  • space: if the first character is not a sign, a space will be prefixed.
  • 0: for numeric conversions, specifies padding to the field width with leading zeros.
  • #, which specifies an alternate output form. For o, the first digit will become zero. For x or X, 0x or 0X will be prefixed to a non-zero result. For e, E, f, g, and G, the output will always have a decimal point; for g and G, trailing zeros will not be removed.
• A number specifying a minimum field width. The converted argument will be printed in a field at least this wide, and wider if necessary. If the converted argument has fewer characters than the field width it will be padded on the left (or right, if left adjustment has been requested) to make up the field width. The padding character is normally space, but is 0 if the zero padding flag is present.
• A period, which separates the field width from the precision.
• A number, the precision, that specifies the maximum number of characters to be printed from a string, or the number of digits to be printed after the decimal point for e, E, or f conversions, or the number of significant digits for g or G conversion, or the number of digits to be printed for an integer (leading 0s will be added to make up the necessary width).
• A length modifier h, l (letter ell), or L. ``h'' indicates that the corresponding argument is to be printed as a short or unsigned short; ``l'' indicates that the argument is a long or unsigned long, ``L'' indicates that the argument is a long double.

Width or precision or both may be specified as *, in which case the value is computed by converting the next argument(s), which must be int.

The conversion characters and their meanings are shown in Table B.1. If the character after the % is not a conversion character, the behavior is undefined.

Table B.1 Printf Conversions

B.1.3 Formatted Input

The scanf function deals with formatted input conversion.

int fscanf(FILE *stream, const char *format, ...)

fscanf reads from stream under control of format, and assigns converted values through subsequent arguments, each of which must be a pointer. It returns when format is exhausted. fscanf returns EOF if end of file or an error occurs before any conversion; otherwise it returns the number of input items converted and assigned.

The format string usually contains conversion specifications, which are used to direct interpretation of input. The format string may contain:

• Blanks or tabs, which are not ignored.
• Ordinary characters (not %), which are expected to match the next non-white space character of the input stream.
• Conversion specifications, consisting of a %, an optional assignment suppression character *, an optional number specifying a maximum field width, an optional h, l, or L indicating the width of the target, and a conversion character.

A conversion specification determines the conversion of the next input field. Normally the result is placed in the variable pointed to by the corresponding argument. If assignment suppression is indicated by *, as in %*s, however, the input field is simply skipped; no assignment is made. An input field is defined as a string of non-white space characters; it extends either to the next white space character or until the field width, if specified, is exhausted. This implies that scanf will read across line boundaries to find its input, since newlines are white space. (White space characters are blank, tab, newline, carriage return, vertical tab, and formfeed.)

The conversion character indicates the interpretation of the input field. The corresponding argument must be a pointer. The legal conversion characters are shown in Table B.2.

The conversion characters d, i, n, o, u, and x may be preceded by h if the argument is a pointer to short rather than int, or by l (letter ell) if the argument is a pointer to long. The conversion characters e, f, and g may be preceded by l if a pointer to double rather than float is in the argument list, and by L if a pointer to a long double.

Table B.2 Scanf Conversions

B.1.4 Character Input and Output Functions

B.1.5 Direct Input and Output Functions

B.1.6 File Positioning Functions

B.1.7 Error Functions

Many of the functions in the library set status indicators when error or end of file occur. These indicators may be set and tested explicitly. In addition, the integer expression errno (declared in ) may contain an error number that gives further information about the most recent error.

B.2 Character Class Tests:

The header declares functions for testing characters. For each function, the argument list is an int, whose value must be EOF or representable as an unsigned char, and the return value is an int. The functions return non-zero (true) if the argument c satisfies the condition described, and zero if not.

In the seven-bit ASCII character set, the printing characters are 0x20 (' ') to 0x7E ('-'); the control characters are 0 NUL to 0x1F (US), and 0x7F (DEL).

In addition, there are two functions that convert the case of letters:

If c is an upper-case letter, tolower(c) returns the corresponding lower-case letter, toupper(c) returns the corresponding upper-case letter; otherwise it returns c.

B.3 String Functions:

There are two groups of string functions defined in the header . The first have names beginning with str; the second have names beginning with mem. Except for memmove, the behavior is undefined if copying takes place between overlapping objects. Comparison functions treat arguments as unsigned char arrays.

In the following table, variables s and t are of type char *; cs and ct are of type const char *; n is of type size_t; and c is an int converted to char.

A sequence of calls of strtok(s,ct) splits s into tokens, each delimited by a character from ct. The first call in a sequence has a non-NULL s, it finds the first token in s consisting of characters not in ct; it terminates that by overwriting the next character of s with '\0' and returns a pointer to the token. Each subsequent call, indicated by a NULL value of s, returns the next such token, searching from just past the end of the previous one. strtok returns NULL when no further token is found. The string ct may be different on each call.

The mem... functions are meant for manipulating objects as character arrays; the intent is an interface to efficient routines. In the following table, s and t are of type void *; cs and ct are of type const void *; n is of type size_t; and c is an int converted to an unsigned char.

B.4 Mathematical Functions:

The header declares mathematical functions and macros.

The macros EDOM and ERANGE (found in ) are non-zero integral constants that are used to signal domain and range errors for the functions; HUGE_VAL is a positive double value. A domain error occurs if an argument is outside the domain over which the function is defined. On a domain error, errno is set to EDOM; the return value is implementation-defined. A range erroroccurs if the result of the function cannot be represented as a double. If the result overflows, the function returns HUGE_VAL with the right sign, and errno is set to ERANGE. If the result underflows, the function returns zero; whether errno is set to ERANGE is implementation-defined.

In the following table, x and y are of type double, n is an int, and all functions return double. Angles for trigonometric functions are expressed in radians.

B.5 Utility Functions:

The header declares functions for number conversion, storage allocation, and similar tasks.

void *bsearch(const void *key, const void *base,
              size_t n, size_t size,
              int (*cmp)(const void *keyval, const void *datum))

void qsort(void *base, size_t n, size_t size,
           int (*cmp)(const void *, const void *))

   va_list ap;

   void va_end(va_list ap);

      if (setjmp(env) == 0)
          /* get here on direct call */
      else
          /* get here by calling longjmp */

void (*signal(int sig, void (*handler)(int)))(int)

int raise(int sig)

      Sun Jan  3 15:14:13 1988\n\0

      asctime(localtime(tp)) 

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