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diff --git a/share/doc/psd/04.uprog/p3 b/share/doc/psd/04.uprog/p3 new file mode 100644 index 000000000000..45ad20646054 --- /dev/null +++ b/share/doc/psd/04.uprog/p3 @@ -0,0 +1,466 @@ +.\" Copyright (C) Caldera International Inc. 2001-2002. All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions are +.\" met: +.\" +.\" Redistributions of source code and documentation must retain the above +.\" copyright notice, this list of conditions and the following +.\" disclaimer. +.\" +.\" Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" +.\" All advertising materials mentioning features or use of this software +.\" must display the following acknowledgement: +.\" +.\" This product includes software developed or owned by Caldera +.\" International, Inc. Neither the name of Caldera International, Inc. +.\" nor the names of other contributors may be used to endorse or promote +.\" products derived from this software without specific prior written +.\" permission. +.\" +.\" USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA +.\" INTERNATIONAL, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR +.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED +.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE +.\" DISCLAIMED. IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE +.\" FOR ANY DIRECT, INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR +.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF +.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR +.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, +.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE +.\" OR OTHERWISE) RISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN +.\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. +.\" +.\" +.NH +THE STANDARD I/O LIBRARY +.PP +The ``Standard I/O Library'' +is a collection of routines +intended to provide +efficient +and portable +I/O services +for most C programs. +The standard I/O library is available on each system that supports C, +so programs that confine +their system interactions +to its facilities +can be transported from one system to another essentially without change. +.PP +In this section, we will discuss the basics of the standard I/O library. +The appendix contains a more complete description of its capabilities. +.NH 2 +File Access +.PP +The programs written so far have all +read the standard input and written the standard output, +which we have assumed are magically pre-defined. +The next step +is to write a program that accesses +a file that is +.ul +not +already connected to the program. +One simple example is +.IT wc , +which counts the lines, words and characters +in a set of files. +For instance, the command +.P1 +wc x.c y.c +.P2 +prints the number of lines, words and characters +in +.UL x.c +and +.UL y.c +and the totals. +.PP +The question is how to arrange for the named files +to be read \(em +that is, how to connect the file system names +to the I/O statements which actually read the data. +.PP +The rules are simple. +Before it can be read or written +a file has to be +.ul +opened +by the standard library function +.UL fopen . +.UL fopen +takes an external name +(like +.UL x.c +or +.UL y.c ), +does some housekeeping and negotiation with the operating system, +and returns an internal name +which must be used in subsequent +reads or writes of the file. +.PP +This internal name is actually a pointer, +called a +.IT file +.IT pointer , +to a structure +which contains information about the file, +such as the location of a buffer, +the current character position in the buffer, +whether the file is being read or written, +and the like. +Users don't need to know the details, +because part of the standard I/O definitions +obtained by including +.UL stdio.h +is a structure definition called +.UL FILE . +The only declaration needed for a file pointer +is exemplified by +.P1 +FILE *fp, *fopen(); +.P2 +This says that +.UL fp +is a pointer to a +.UL FILE , +and +.UL fopen +returns a pointer to +a +.UL FILE . +.UL FILE \& ( +is a type name, like +.UL int , +not a structure tag. +.PP +The actual call to +.UL fopen +in a program +is +.P1 +fp = fopen(name, mode); +.P2 +The first argument of +.UL fopen +is the +name +of the file, +as a character string. +The second argument is the +mode, +also as a character string, +which indicates how you intend to +use the file. +The only allowable modes are +read +.UL \&"r" ), ( +write +.UL \&"w" ), ( +or append +.UL \&"a" ). ( +.PP +If a file that you open for writing or appending does not exist, +it is created +(if possible). +Opening an existing file for writing causes the old contents +to be discarded. +Trying to read a file that does not exist +is an error, +and there may be other causes of error +as well +(like trying to read a file +when you don't have permission). +If there is any error, +.UL fopen +will return the null pointer +value +.UL NULL +(which is defined as zero in +.UL stdio.h ). +.PP +The next thing needed is a way to read or write the file +once it is open. +There are several possibilities, +of which +.UL getc +and +.UL putc +are the simplest. +.UL getc +returns the next character from a file; +it needs the file pointer to tell it what file. +Thus +.P1 +c = getc(fp) +.P2 +places in +.UL c +the next character from the file referred to by +.UL fp ; +it returns +.UL EOF +when it reaches end of file. +.UL putc +is the inverse of +.UL getc : +.P1 +putc(c, fp) +.P2 +puts the character +.UL c +on the file +.UL fp +and returns +.UL c . +.UL getc +and +.UL putc +return +.UL EOF +on error. +.PP +When a program is started, three files are opened automatically, +and file pointers are provided for them. +These files are the standard input, +the standard output, +and the standard error output; +the corresponding file pointers are +called +.UL stdin , +.UL stdout , +and +.UL stderr . +Normally these are all connected to the terminal, +but +may be redirected to files or pipes as described in +Section 2.2. +.UL stdin , +.UL stdout +and +.UL stderr +are pre-defined in the I/O library +as the standard input, output and error files; +they may be used anywhere an object of type +.UL FILE\ * +can be. +They are +constants, however, +.ul +not +variables, +so don't try to assign to them. +.PP +With some of the preliminaries out of the way, +we can now write +.IT wc . +The basic design +is one that has been found +convenient for many programs: +if there are command-line arguments, they are processed in order. +If there are no arguments, the standard input +is processed. +This way the program can be used stand-alone +or as part of a larger process. +.P1 +#include <stdio.h> + +main(argc, argv) /* wc: count lines, words, chars */ +int argc; +char *argv[]; +{ + int c, i, inword; + FILE *fp, *fopen(); + long linect, wordct, charct; + long tlinect = 0, twordct = 0, tcharct = 0; + + i = 1; + fp = stdin; + do { + if (argc > 1 && (fp=fopen(argv[i], "r")) == NULL) { + fprintf(stderr, "wc: can't open %s\en", argv[i]); + continue; + } + linect = wordct = charct = inword = 0; + while ((c = getc(fp)) != EOF) { + charct++; + if (c == '\en') + linect++; + if (c == ' ' || c == '\et' || c == '\en') + inword = 0; + else if (inword == 0) { + inword = 1; + wordct++; + } + } + printf("%7ld %7ld %7ld", linect, wordct, charct); + printf(argc > 1 ? " %s\en" : "\en", argv[i]); + fclose(fp); + tlinect += linect; + twordct += wordct; + tcharct += charct; + } while (++i < argc); + if (argc > 2) + printf("%7ld %7ld %7ld total\en", tlinect, twordct, tcharct); + exit(0); +} +.P2 +The function +.UL fprintf +is identical to +.UL printf , +save that the first argument is a file pointer +that specifies the file to be +written. +.PP +The function +.UL fclose +is the inverse of +.UL fopen ; +it breaks the connection between the file pointer and the external name +that was established by +.UL fopen , +freeing the +file pointer for another file. +Since there is a limit on the number +of files +that a program may have open simultaneously, +it's a good idea to free things when they are no longer needed. +There is also another reason to call +.UL fclose +on an output file +\(em it flushes the buffer +in which +.UL putc +is collecting output. +.UL fclose \& ( +is called automatically for each open file +when a program terminates normally.) +.NH 2 +Error Handling \(em Stderr and Exit +.PP +.UL stderr +is assigned to a program in the same way that +.UL stdin +and +.UL stdout +are. +Output written on +.UL stderr +appears on the user's terminal +even if the standard output is redirected. +.IT wc +writes its diagnostics on +.UL stderr +instead of +.UL stdout +so that if one of the files can't +be accessed for some reason, +the message +finds its way to the user's terminal instead of disappearing +down a pipeline +or into an output file. +.PP +The program actually signals errors in another way, +using the function +.UL exit +to terminate program execution. +The argument of +.UL exit +is available to whatever process +called it (see Section 6), +so the success or failure +of the program can be tested by another program +that uses this one as a sub-process. +By convention, a return value of 0 +signals that all is well; +non-zero values signal abnormal situations. +.PP +.UL exit +itself +calls +.UL fclose +for each open output file, +to flush out any buffered output, +then calls +a routine named +.UL _exit . +The function +.UL _exit +causes immediate termination without any buffer flushing; +it may be called directly if desired. +.NH 2 +Miscellaneous I/O Functions +.PP +The standard I/O library provides several other I/O functions +besides those we have illustrated above. +.PP +Normally output with +.UL putc , +etc., is buffered (except to +.UL stderr ); +to force it out immediately, use +.UL fflush(fp) . +.PP +.UL fscanf +is identical to +.UL scanf , +except that its first argument is a file pointer +(as with +.UL fprintf ) +that specifies the file from which the input comes; +it returns +.UL EOF +at end of file. +.PP +The functions +.UL sscanf +and +.UL sprintf +are identical to +.UL fscanf +and +.UL fprintf , +except that the first argument names a character string +instead of a file pointer. +The conversion is done from the string +for +.UL sscanf +and into it for +.UL sprintf . +.PP +.UL fgets(buf,\ size,\ fp) +copies the next line from +.UL fp , +up to and including a newline, +into +.UL buf ; +at most +.UL size-1 +characters are copied; +it returns +.UL NULL +at end of file. +.UL fputs(buf,\ fp) +writes the string in +.UL buf +onto file +.UL fp . +.PP +The function +.UL ungetc(c,\ fp) +``pushes back'' the character +.UL c +onto the input stream +.UL fp ; +a subsequent call to +.UL getc , +.UL fscanf , +etc., +will encounter +.UL c . +Only one character of pushback per file is permitted. |