If other files have been opened, pipe assigns the first available file descriptor for writing and the next available file descriptor for reading. In this case, the pipe does not necessarily use adjacent file descriptors. For example, assume that two files have been opened and assigned to file descriptors 3 and 4 and the first file is then closed. If pipe is called at this point, file descriptor 3 is assigned for writing and file descriptor 5 is assigned for reading. Element 0 of the array will contain 5 and element 1 will contain 3.

In large applications that do large amounts of I/O, it gets more difficult to predict which file descriptors are going to be assigned to a pipe; and, unless the child knows which file descriptors are being used, it will not be able to read and write successfully from and to the pipe.

One way to be sure that the correct file descriptors are being used is to use the following procedure:

1. Choose two descriptor numbers that will be known to both the parent and the child. The numbers should be high enough to account for any I/O that might be done before the pipe is created.
2. Call pipe in the parent at some point before calling an exec function.
3. In the parent, use dup2 to assign the file descriptors returned by pipe to the file descriptors you chose. This now reserves those file descriptors for the pipe; any subsequent I/O will not interfere with the pipe.

You can read and write through the pipe using the UNIX I/O functions read and write , specifying the appropriate file descriptors. As an alternative, you can issue fdopen calls to associate file pointers with these file descriptors so that you can use the Standard I/O functions ( fread and fwrite ).

Two separate file descriptors are used for reading from and writing to the pipe, but only one mailbox is used so some I/O synchronization is required. For example, assume that the parent writes a message to the pipe. If the parent is the first process to read from the pipe, then it will read its own message back as shown in Figure REF-1.

Figure REF-1 Reading and Writing to a Pipe

Return Values

0	Indicates success.
--1	Indicates an error.

Initiates a pipe to a process.

Format

#include <stdio.h>

FILE *popen (const char *command, const char *type);

Arguments

command

A pointer to a null-terminated string containing a shell command line.

type

A pointer to a null-terminated string containing an I/O mode. Because open files are shared, you can use a type r command as an input filter and a type w command as an output filter. Specify one of the following values for the type argument:

• r -- the calling program can read from the standard output of the command by reading from the returned file stream.
• w -- the calling program can write to the standard input of the command by writing to the returned file stream.

Description

This function creates a pipe between the calling program and a shell command awaiting execution. It returns a pointer to a FILE structure for the stream.

Note When you use the popen function to invoke an output filter, beware of possible deadlock caused by output data remaining in the program buffer. You can avoid this by either using the setvbuf function to ensure that the output stream is unbuffered, or the fflush function to ensure that all buffered data is flushed before calling the pclose function.

See also fflush , pclose , and setvbuf in this section.

Return Values

x	A pointer to the FILE structure for the opened stream.
NULL	Indicates an error. Unable to create files or processes.

Returns the first argument raised to the power of the second argument.

Format

#include <math.h>

double pow (double x, double y);

float powf (float x, float y); (ALPHA ONLY)

long double powl (long double x, long double y); (ALPHA ONLY)

Arguments

x

A floating-point base to be raised to an exponent y.

y

The exponent to which the base x is to be raised.

Description

The pow functions raise a floating-point base x to a floating-point exponent y. The value of pow(x,y) is computed as e**(y ln(x)) for positive x.

If x is 0 and y is negative, -HUGE_VAL is returned, and errno is set to EDOM.

Return Values

x	The result of the first argument raised to the power of the second.
1.0	The base is zero and the exponent is zero.
HUGE_VAL	The result overflowed; errno is set to ERANGE.
-HUGE_VAL	The base is zero and the exponent is negative; errno is set to EDOM.

Example

#include <stdio.h> #include <math.h> #include <errno.h> main() { double x; errno = 0; x = pow(-3.0, 2.0); printf("%d, %f\n", errno, x); }

This example program outputs the following:

0, 9.000000

Performs formatted output from the standard output (stdout). See Chapter 2 for information on format specifiers.

Format

#include <stdio.h>

int printf (const char *format_spec, ...);

Arguments

format_spec

Characters to be written literally to the output or converted as specified in the ... arguments.

...

Optional expressions whose resultant types correspond to conversion specifications given in the format specification.

If no conversion specifications are given, you may omit the output sources. Otherwise, the function call must have exactly as many output sources as there are conversion specifications, and the conversion specifications must match the types of the output sources.

Conversion specifications are matched to output sources in left-to-right order. Excess output pointers, if any, are ignored.

Return Values

x	The number of bytes written.
Negative value	Indicates that an output error occurred. The function sets errno . For a list of errno values set by this function, see fprintf in this section.

Perform a printf in the specified window, starting at the current position of the cursor. The printw function acts on the stdscr window.

Format

#include <curses.h>

printw (char *format_spec, ...);

int wprintw (WINDOW *win, char *format_spec, ...);

Arguments

win

A pointer to the window.

format_spec

A pointer to the format specification string.

...

Optional expressions whose resultant types correspond to conversion specifications given in the format specification.

If no conversion specifications are given, you may omit the output sources. Otherwise, the function call must have exactly as many output sources as there are conversion specifications, and the conversion specifications must match the types of the output sources.

Conversion specifications are matched to output sources in left-to-right order. Excess output pointers, if any, are ignored.

Description

The formatting specification (format_spec) and the other arguments are identical to those used with the printf function.

The printw and wprintw functions format and then print the resultant string to the window using the addstr function. For more information, see the printf and scrollok functions in this section. See Chapter 2 for information on format specifiers.

Return Values

OK	Indicates success.
ERR	Indicates that the function makes the window scroll illegally.

The putc macro writes a single character to a specified file.

Format

#include <stdio.h>

int putc (int character, FILE *file_ptr);

Arguments

character

An object of type int .

file_ptr

A file pointer.

Description

Since putc is a macro, a file pointer argument with side effects (for example, putc (ch, *f++) ) might be evaluated incorrectly. In such a case, use the fputc function instead. See also fputc in this section.

Return Values

x	The character written to the file. Indicates success.
EOF	Indicates output errors.

Writes a single character to the standard output (stdout) and returns the character.

Format

#include <stdio.h>

int putchar (int character);

Argument

character

An object of type int .

Description

This function is identical to fputc (character, stdout).

Return Values

character	Indicates success.
EOF	Indicates output errors.

Sets an environmental variable.

Format

#include <stdlib.h>

int putenv (const char *string);

Arguments

string

A pointer to a name=value string.

Description

This function sets the value of an environment variable by altering an existing variable or by creating a new one. The string argument points to a string of the form name=value, where name is the environment variable and value is the new value for it.

The string pointed to by string becomes part of the environment, so altering the string changes the environment. When a new string-defining name is passed to putenv , the space used by string is no longer used.

Note The putenv function manipulates the environment pointed to by the environ external variable, and can be used with getenv . However, the third argument to the main function (the environment pointer), is not changed. The putenv function uses the malloc function to enlarge the environment. A potential error is to call putenv with an automatic variable as the argument, then exit the calling function while string is still part of the environment.

Return Values

0	Indicates success.
--1	Indicates an error. errno is set to ENOMEM---Not enough memory available to expand the environment list.

Restriction

This function cannot take a 64-bit address. See Section 1.10.

Writes a character string to the standard output (stdout) followed by a new-line character.

Format

#include <stdio.h>

int puts (const char *str);

Argument

str

A pointer to a character string.

Description

This function does not copy the terminating null character to the output stream.

Return Values

Nonnegative value	Indicates success.
EOF	Indicates output errors.

Writes characters to a specified file.

Format

#include <stdio.h>

int putw (int integer, FILE *file_ptr);

Arguments

integer

An object of type int or long .

file_ptr

A file pointer.

Description

This function writes four characters to the output file as an int . No conversion is performed.

Return Values

integer	Indicates success.
EOF	Indicates output errors.

Converts a wide character to its corresponding multibyte value, and writes the result to a specified file.

Format

#include <wchar.h>

wint_t putwc (wint_t wc, FILE *file_ptr);

Arguments

wc

An object of type wint_t .

file_ptr

A file pointer.

Description

Since putwc might be implemented as a macro, a file pointer argument with side effects (for example putwc (wc, *f++) ) might be evaluated incorrectly. In such a case, use the fputwc function instead. See also fputwc in this section.

Return Values

x	The character written to the file. Indicates success.
WEOF	Indicates an output error. The function sets errno . For a list of the errno values set by this function, see fputwc in this section.

Writes a wide character to the standard output (stdout) and returns the character.

Format

#include <wchar.h>

wint_t putwchar (wint_t wc);

Arguments

wc

An object of type wint_t .

Description

This function is identical to fputwc (wc, stdout).

Return Values

x	The character written to the file. Indicates success.
WEOF	Indicates an output error. The function sets errno . For a list of the errno values set by this function, see fputwc in this section.

Returns the absolute value of an integer as an __int64 . llabs is a synonym for qabs .

Format

#include <stdlib.h>

__int64 qabs (__int64 j);

__int64 llabs (__int64 j);

Argument

j

A value of type __int64 .

Returns the quotient and the remainder after the division of its arguments. lldiv is a synonym for qdiv .

Format

#include <stdlib.h>

qdiv_t qdiv (__int64 numer, __int64 denom);

lldiv_t lldiv (__int64 numer, __int64 denom);

Arguments

numer

A numerator of type __int64 .

denom

A denominator of type __int64 .

Description

The types qdiv_t and lldiv_t are defined in the <stdlib.h> header file as follows:

typedef struct { __int64 quot, rem; } qdiv_t, lldiv_t;

Sorts an array of objects in place. It implements the quick-sort algorithm.

Format

#include <stdlib.h>

void qsort (void *base, size_t nmemb, size_t size, int (*compar) (const void *, const void *));

Arguments

base

A pointer to the first member of the array. The pointer should be of type pointer-to-element and cast to type pointer-to-character.

nmemb

The number of objects in the array.

size

The size of an object, in bytes.

compar

A pointer to the comparison function.

Description

Two arguments are passed to the comparison function pointed to by compar. The two arguments point to the objects being compared. Depending on whether the first argument is less than, equal to, or greater than the second argument, the comparison function returns an integer less then, equal to, or greater than 0.

The comparison function compar need not compare every byte, so arbitrary data might be contained in the objects in addition to the values being compared.

The order in the output of two objects that compare as equal is unpredictable.

Generates a specified software signal. Generating a signal causes the action routine established by the signal , ssignal , or sigvec function to be invoked.

Format

#include <signal.h>

int raise (int sig); (ANSI C)

int raise (int sig[, int sigcode]); (COMPAQ C EXTENSION)

Arguments

sig

The signal to be generated.

sigcode

An optional signal code, available only when not compiling in strict ANSI C mode. For example, signal SIGFPE---the arithmetic trap signal---has 10 different codes, each representing a different type of arithmetic trap.

The signal codes can be represented by mnemonics or numbers. The arithmetic trap codes are represented by the numbers 1 to 10; the SIGILL codes are represented by the numbers 0 to 2. The code values are defined in the <signal.h> header file. See Tables 4-4 and 4-5 for a list of signal mnemonics, codes, and corresponding OpenVMS exceptions.

Description

Calling this function has one of the following results:

• If raise specifies a sig argument that is outside the range defined in the <signal.h> header file, then the raise function returns 0, and the errno variable is set to EINVAL.
• If signal , ssignal , or sigvec establishes SIG_DFL (default action) for the signal, then the functions do not return. The image is exited with the OpenVMS error code corresponding to the signal.
• If signal , ssignal , or sigvec establishes SIG_IGN (ignore signal) as the action for the signal, then raise returns its argument, sig.
• signal , ssignal , or sigvec must establish an action function for the signal. That function is called and its return value is returned by raise .

See Chapter 4 for more information on signal processing.

See also gsignal , signal , ssignal , and sigvec in this section.

Return Values

0	If successful.
nonzero	If unsuccessful.