CARACTERÍSTICAS ADICIONALES DE C

CARACTERÍSTICAS ADICIONALES DE C

1- ENUMERACIONES

Una enumeración es tipo de datos, similar a la estructura o la unión. Sus miembros son constantes que están escritas como identificadores y, sin embargo, tienen asignados valores numéricos. Estas constantes representan valores que pueden ser asignados a variables de enumeración. En general, una enumeración se define como:

enum nombre ;

Una vez que la enumeración ha sido definida, las correspondientes variables de enumeración pueden ser definidas como:

tipo_almacenamiento enum nombre variable1, variable2, , variableK;

La definición de la enumeración puede ser combinada con la declaración de variables de este tipo. En este caso el nombre es opcional:

tipo_almacenamiento enum nombre

variable1, variable2, , variableK;

Ej: enum colores ;

enum colores primerplano, fondo;

enum primerplano, fondo;

typedef enum color;

color primerplano, fondo;

Las constantes de enumeración tienen automáticamente asignados valores enteros, empezando por 0 para la primera constante e incrementándose en 1 para las sucesivas.

Ej: En el caso anterior: negro=0, blanco=1, rojo=2,

Estas asignaciones automáticas pueden ser modificadas dentro de la definición de la enumeración mediante expresiones ordinarias de asignación. Las constantes que no tengan asignados valores explícitos se les asignarán automáticamente valores que se obtienen incrementando sucesivamente en 1 el valor de la última asignación explícita. Esto puede producir que dos o más constantes de enumeración tengan el mismo valor entero.

Ej: enum colores ;

negro=-1, blanco=0, rojo=1, verde=2, azul=2

Las variables de enumeración pueden ser tratadas como cualquier otra variable entera (se les puede asignar valores, se pueden comparar, etc). Pero hay ciertas restricciones en el manejo de las enumeraciones. En particular, una constante de enumeración no puede ser leída por el ordenador y asignada a una variable de enumeración. (Es posible leer un entero y asignarlo a una variable de enumeración pero no se suele hacer.) Además, solo el valor entero de una enumeración puede ser escrito por el ordenador.

Ej: enum colores primerplano, fondo;

primerplano=blanco;

fondo=azul;

if (fondo==azul)

primerplano=verde;

else

primerplano=rojo;

Una variable de enumeración puede ser inicializada en la definición, asignándole una constante de enumeración o un valor entero (lo normal es usar constantes de enumeración).

Ej: enum colores ;

enum colores primerplano=blanco, fondo=negro;

2.- PARÁMETROS DE LA LÍNEA DE ÓRDENES

Los paréntesis que suelen estar vacíos en la función main, pueden contener argumentos especiales que permiten pasarle parámetros a main desde el sistema operativo. La mayoría de las versiones de C permiten dos tipos de argumentos:

- argc. Es una variable entera que indica el número de parámetros pasados.

- argv. Es un array de punteros a carácter, un array de cadenas de caracteres.

Cada cadena del array representa un parámetro.

La ejecución de un programa se hace, normalmente, especificando el nombre del archivo ejecutable que contiene el programa en el nivel del sistema operativo. Para pasar uno o más parámetros cuando se inicia la ejecución, los parámetros deben seguir al nombre de programa en la línea de órdenes:

nombre_programa parámetro1 parámetro2 parámetroN

El nombre del programa será almacenado como primer elemento de argv, seguido por cada uno de los parámetros. Por tanto, si el nombre del programa es seguido por n parámetros, habrá (n+1) entradas en argv. Además, argc tendrá asignado automáticamente el valor (n+1).

Ej: Dado el siguiente programa almacenado en un archivo llamado muestra:

#include <stdio.h>

main(int argc, char *argv[])

Si la línea de órdenes que inicia la ejecución es:

muestra rojo blanco azul

El resultado de la ejecución será:

argc = 4

argv[0] = muestra.exe

argv[1] = rojo

argv[2] = blanco

argv[3] = azul

Si la línea de órdenes que inicia la ejecución es:

muestra rojo 'blanco azul'

El resultado de la ejecución será:

argc = 3

argv[0] = muestra.exe

argv[1] = rojo

argv[2] = blanco azul

3.- MACROS

La sentencia #define puede ser usada para definir constantes y para definir macros, que son indicadores equivalentes a expresiones, sentencias o grupos de sentencias. Suelen definirse al principio pero pueden ir por el medio. El ámbito de una macro va desde el punto donde se define hasta el final del archivo donde ha sido definida.

Ej: #define area longitud*anchura a = area; /* a = longitud*anchura; */

#define raiz(a, b) sqrt (a*a + b*b)

#define media(a,b)

4.- EL PREPROCESADOR DE C

El preprocesador de C es una colección de sentencias especiales, llamadas directivas, que son ejecutadas al principio ( o antes, según se mire) del proceso de compilación. Pueden aparecer en cualquier parte del programa pero suelen ir al principio (la directiva sólo tiene aplicación en la porción de programa que sigue a su aparición).

Las directivas son: #if, #elif, #else, #endif, #ifdef, #ifndef, #line y #undef.

También hay tres operadores especiales: defined, # y ##.

Ej: #if FONDO == 7

#define PRIMERPLANO 0

#elif FONDO == 6

#define PRIMERPLANO 1

#else

#define PRIMERPLANO 6

#endif

#if defined(FONDO)

#define PRIMERPLANO 0

#else

#define FONDO 7

#define PRIMERPLANO 0

#endif

main( )

LA SALA DE ORDENADORES

Terminal

Connect nombre-host, co nombre-host o telnet nombre-host

Login y password

Pc

Arrancar desde disco “especial”

Telnet nombre-host.

Login y password.

Name

ftp - file transfer program

Syntax

ftp [-d] [[-g] -i] [-n] [-v] [host]

Description

The ftp command is the user interface to the ARPANET standard File Transfer Protocol. The program allows you to transfer files to and from a remote network site.

If you specify the client host on the command line, ftp immediately attempts to establish a connection to an FTP server on that host; other­wise, ftp enters its command interpreter and awaits instructions. While ftp is awaiting commands, it displays the prompt ftp>.

Options

-d Enables debugging.

-g Disables file name expansion.

-i Disables interactive prompting during multiple file transfers.

-n Disables autologin during an initial connection. If auto-login is enabled, ftp will check the .netrc file in the user's home directory for an entry describing an account on the remote machine. If no entry exists, ftp will use the login name on the local machine as the user identity on the remote machine, prompt for a password and, optionally, an account with which to log in.

-v Displays all responses from the remote server as well as all data transfer statistics.

Commands

The commands listed in this section are recognized by ftp. Command arguments that have embedded spaces may be quoted with quotation. This commands are:

marks (') to retain the spaces.

? [ command ] a synonym for help.

! Invokes a shell on the local machine.

$ macro-name [ args ] executes the macro macro-name that was defined with the macdef command. Arguments are passed to the macro unglobbed.

account [ passwd ] supplies a supplemental password required by a remote system for access to resources once a login has been successfully completed. If you do not supply a passwd argument, you are prompted for an account password in a non-echoing input mode.

append local-file [ remote-file ] appends a local file to a file on the remote machine. If remote-file is not specified, the local file name is used in naming the remote file. File transfer uses the current settings for type, format, mode, and structure.

ascii sets the file transfer type to network ASCII. This is the default type.

bell sounds a bell after each file transfer command is completed.

binary sets the file transfer type to support binary image transfer.

bye terminates the FTP session with the remote server and exits ftp.

case toggles the remote computer's file name case mapping during mget commands. When case is on (the default is off), the remote computer's file names are written in the local directory with all letters in uppercase mapped to lowercase.

cd remote-directory changes the working directory on the remote machine to remote-directory.

cdup changes the remote machine working directory to the parent of the current remote machine working directory.

close terminates the FTP session with the remote server and returns to the command interpreter.

cr toggles the carriage return stripping during ascii type file retrieval. Records are denoted by a carriage return/linefeed sequence during ascii type file transfer. When cr is on (the default), carriage returns are stripped from this sequence to conform with the UNIX single-linefeed record delimiter. Records on non-UNIX remote systems can contain single linefeeds; when an ascii type transfer is made, these linefeeds can be distinguished from a record delimiter only when cr is off.

debug [ debug-value ] toggles the debugging mode. If an optional debug-value is specified, it is used to set the debugging level. When debug­ging is on, ftp prints each command sent to the remote machine, preceded by the string --->.

delete remote-file deletes the file remote-file on the remote machine.

dir [ remote-directory ] [ local-file ] prints a listing of the contents of the directory remote-directory, and optionally places the output in local-file. If you do not specify a directory, ftp uses the current working directory. If you do not specify a local file, ftp displays the listing on the terminal.

disconnect a synonym for close.

form format sets the file transfer form to format. The default format is file.

get remote-file [ local-file ] copies the remote-file to the local machine. If you do not specify a local file name, ftp uses the same name it has on the remote machine. The current settings for type, form, mode, and structure are used while transferring the file.

glob toggles file name expansion for mdelete, mget, and mput. If globbing (file name substitution) is turned off with glob, the file name arguments are taken literally and not expanded. Globbing for mput is done as in csh(1). For mdelete and mget, each remote file name is expanded separately on the remote machine and the lists are not merged. Expansion of a direc­tory name is likely to be different from expansion of the name of an ordinary file. The exact result depends on the remote operating system and ftp server, and can be previewed by entering: mls remote-files. Neither mget nor mput is meant to transfer entire directory subtrees of files. That can be done by transferring a tar(1) archive of the subtree (in binary mode).

hash toggles the number-sign (#) printing for each data block transferred. The size of a data block is 1024 bytes.

lcd [ directory ] changes the working directory on the local machine. If you do not specify a local directory directory, ftp uses your home directory.

ls [ remote-directory ] [ local-file ] prints an abbreviated listing of the contents of the remote directory. If remote-directory is left unspecified, ftp uses your current working directory. If you do not specify a local file, the listing is displayed on the terminal.

macdef macro-name defines a macro. Subsequent lines are stored as the macro macro-name; a null line (consecutive newline characters in a file or carriage returns from the terminal) terminates macro input mode. There is a limit of 16 macros and 4096 total characters in all defined macros. Macros remain defined until a close command is executed. The macro processor interprets dollar signs ($) and backslashes () as special characters. A dollar sign ($) followed by a number (or numbers) is replaced by the corresponding argument on the macro invocation command line. A dollar sign ($) followed by an i signals the macro processor that the executing macro is to be looped. On the first pass, $i is replaced by the first argument on the macro invocation command line. On the second pass it is replaced by the second argu­ment, and so on. A backslash () followed by any character is replaced by that character. Use the backslash () to prevent special treatment of the dollar sign ($).

mdelete remote-files deletes the specified files on the remote machine. If globbing (file name substitution) is enabled, the wildcard specification of remote files is first expanded with ls.

mdir remote-files local-file obtains a directory listing of multiple files on the remote machine and places the result in local-file.

mget remote-files retrieves the specified files from the remote machine and places them in the current local directory. If globbing is enabled, the specification of remote files is first expanding with ls.

mkdir directory-name makes a directory on the remote machine.

mls remote-files local-file obtains an abbreviated listing of multiple files on the remote machine and places the result in local-file.

mode [ mode-name ] sets the file transfer mode to mode-name. The default mode is the stream mode.

mput local-files transfers multiple local files from the current local directory to the current working directory on the remote machine.

nmap [ inpattern outpattern ] sets or disables the file name mapping mechanism. If no arguments are specified, the file name mapping mechanism is disabled. If arguments are specified, remote file names are mapped during mput commands and put commands that are issued without a specified remote target file name. If arguments are specified, local file names are mapped during mget commands and get commands that are issued without a specified local target file name. This command is useful when connecting to a non-UNIX remote computer with different file naming conventions or practices. The mapping follows the pattern set by inpattern and outpattern. The inpattern is a template for incoming file names (which may have already been processed according to the ntrans and case settings). Specify variable templates by including the sequences $1, $2, , $9 in inpattern. Use a backslash () to prevent this special treatment of the dollar sign ($) character. All other characters are treated literally, and are used to determine the nmap inpattern variable values. For example, given inpattern $1.$2 and the remote file name mydata.data, $1 has the value mydata, and $2 has the value data. The outpattern determines the resulting mapped file name. The sequences $1, $2, ., $9 are replaced by any value resulting from the inpattern template. The sequence $0 is replace by the original file name. Additionally, the sequence [seq1,seq2] is replaced by seq1 if seq1 is not a null string; otherwise it is replaced by seq2. For example, the command nmap $1.$2.$3 [$1,$2].[$2,file] yields the output file name myfile.data for input file names myfile.data and myfile.data.old, myfile.file for the input file name myfile, and myfile.myfile for the input file name .myfile. Spaces can be included in outpattern, as in the example: nmap $1 |sed 's/ *$//' > $1 use the backslash () to prevent special treatment of the dollar sign ($), left bracket ([), right bracket (]), and comma (,).

ntrans [ inchars [ outchars ] ] sets or disables the file name character translation mechanism. If no arguments are specified, the file name character translation mechanism is disabled. If arguments are specified, characters in remote file names are translated during mput commands and put commands that are issued without a specified remote target file name. If arguments are specified, characters in local file names are translated during mget commands and get commands that are issued without a specified local target file name. This command is useful when connecting to a non-UNIX remote computer with different file naming conventions or practices. Characters in a file name matching a character in inchars are replaced with the corresponding character in outchars. If the character's position in inchars is longer than the length of outchars, the character is deleted from the file name.

open host [ port ] establishes a connection to the specified host ftp server. If an optional port number is supplied, ftp attempts to contact an ftp server at that port. If the auto-login option is on (the default), ftp automatically attempts to log the user in to the ftp server (see the description of the .netrc file).

prompt toggles interactive prompting. Interactive prompting occursduring multiple file transfers to allow the user to retrieve or store files selectively. If prompting is turned off (the default), any mget or mput transfers all files.

proxy ftp-command executes an ftp command on a secondary control connection. This command allows simultaneous connection to two remote ftp servers for transferring files between the two servers. The first proxy command should be an open, to establish the secondary control connection. Type the command proxy ? to see other ftp commands executable on the secondary connection. The following commands behave differently when prefaced by proxy:

1) open does not define new macros during the auto-login process.

2) close does not erase existing macro definitions.

3) get and mget transfer files from the host on the primary control connection to the host on the secondary control connection.

4) put, mput and append transfer files from the host on the secondary control connection to the host on the primary control connection. Third-party file transfers depend upon sup­port of the FTP PASV command by the server on the secondary control connection.

put local-file [ remote-file ] copies a local file to the remote machine. If you do not specify a remote-file, the local file name is used to name the remote file. File transfer uses the current settings for type, format, mode, and structure.

pwd prints the name of the current working directory on the remote machine.

quit a synonym for bye.

quote arg1 arg2 sends the arguments that are specified, verbatim, to the remote ftp server. A single FTP reply code is expected in return.

recv remote-file [ local-file ] a synonym for get.

remotehelp [ command-name ] requests help from the remote ftp server. If a command name is specified, it is supplied to the server as well.

rename [ from ] [ to ] renames the file from on the remote machine to the file to.

reset clears the reply queue. This command resynchronizes command/reply sequencing with the remote ftp server. If the remote server violates FTP, resynchronization may be necessary.

rmdir directory-name deletes a directory on the remote machine.

runique toggles storing of files on the local system with unique file names. If a file already exists with a name equal to the target local file name for a get or mget command, a .1 is appended to the name. If the resulting name matches another existing file, a .2 is appended to the original name. If this process continues up to .99, an error message is printed, and the transfer does not take place. The generated unique file name will be reported. Note that runique will not affect local files generated from a shell command (see below). The default value is off.

send local-file [ remote-file ] a synonym for put.

sendport toggles the use of PORT commands. By default, ftp attempts to use a PORT command when establishing a connection for each data transfer. If the PORT command fails, ftp uses the default data port. When the use of PORT commands is disabled, no attempt is made to use PORT commands for each data transfer. This is useful for certain FTP implementations that do ignore PORT commands but, incorrectly, indicate that the commands have been accepted.

status shows the current status of ftp.

struct [ struct-name ] sets the file transfer structure to struct-name. By default the file structure is used.

sunique toggles storing of files on a remote machine under unique filenames. The remote ftp server must support the FTP STOU command for successful completion of this command. The remote server reports the unique name. The default value is off.

tenex sets the file transfer type to that needed to talk to TENEX machines.

trace toggles packet tracing.

type [ type-name ] sets the file transfer type to type-name. If no type is specified, the current type is printed. The default type is network ASCII.

user user-name [ password ] [ account ] identifies you to the remote ftp server. If the password is not specified and the server requires it, ftp disables the local echo and then prompts you for it. If an account field is not specified, and the ftp server requires it, you are prompted for it also. Unless ftp is invoked with auto-login disabled, this process is done automatically on initial connection to the ftp server.

verbose toggles the verbose mode. In verbose mode, all responses from the ftp server are displayed. In addition, if verbose is on, statistics regarding the efficiency of a file transfer are reported when the transfer is complete. By default, verbose is on.

Aborting a File Transfer to abort a file transfer, use the terminal interrupt key (usually CTRL/C). Sending transfers are halted immediately. Receiving transfers are halted by sending an FTP ABOR command to the remote server, and discarding any further data received. The speed at which this is accomplished depends on the remote server's support for ABOR processing. If the remote server does not support the ABOR command, an ftp> prompt appears when the remote server has completed sending the requested file. The terminal interrupt key sequence is ignored when ftp has completed any local processing and is awaiting a reply from the remote server. A long delay in this mode may result from ABOR processing, or from unexpected behavior by the remote server, including violations of the FTP. If the delay results from unexpected remote server behavior, the local ftp program must be killed by hand.

File-Naming Conventions files specified as arguments to ftp commands are processed according to the following rules:

1) Standard input is used for reading and standard output is used for writing when the file name is specified as a minus sign (-).

2) If the first character of the file name is a vertical bar (|), the remainder of the argument is interpreted as a shell command. The ftp command then forks a shell, using popen(3) with the argument supplied, and reads or writes from standard output or standard input respectively. If the shell command includes spaces, the argument must be quoted, as in '|ls -lt'. A particularly useful example of this mechanism is: 'dir |more'.

3) If globbing is enabled, local file names are expanded according to the rules used in the csh(1) (compare to the glob command). If the ftp command expects a single local file, such as put, only the first file name generated by the globbing operation is used.

4) For mget commands and get commands with unspecified local file names, the local file name is the remote file name and can be altered by a case, ntrans, or nmap setting. The resulting file name may then be altered if runique is on.

5) For mput commands and put commands with unspecified remote file names, the remote file name is the local file name and can be altered by a ntrans or nmap setting. The resulting file name can then be altered by the remote server if sunique is on.

File Transfer Parameters many parameters can affect a file transfer. The type can be ascii, image (binary), ebcdic, or local byte size. The ftp command supports the ascii and image types of file transfer and local byte size 8 for tenex mode transfers. The ftp command supports only the default values for the remaining file transfer parameters: mode, form, and struct.

The .netrc File the .netrc file contains login and initialization information used by the ftp auto-login process. It resides in your home directory. See netrc(5) for a description and example of a .netrc file. The following tokens in the .netrc file are recognized; they can be separated by spaces, tabs, or new-lines.

Machine name identifies a remote machine name. The auto-login process searches the .netrc file for a machine token that matches the remote machine specified on the ftp command line or as an open command argument. Once a match is made, the subsequent .netrc tokens are processed, stopping when the end of file is reached or another machine token is encountered.

Login name identifies a user on the remote machine. If this token is present, the auto-login process initiates a login using the specified name.

Password string supplies a password. If this token is present, the auto-login process supplies the specified string if the remote server requires a password as part of the login process. Note that if this token is present in the .netrc file, and if the .netrc is readable by anyone other than the user, ftp aborts the auto-login process.

Account string supplies an additional account password. When this token is present, the auto-login process supplies the remote server with an additional account password if the remote server requires it. If it does not, the auto-login process initiates an ACCT command.

Macdef name defines a macro. This token functions like the ftp macdef command. A macro is defined with a specified name; its contents begin with the next .netrc line and continue until a null line (consecutive new-line characters) is encountered. If a macro named init is defined, it is automatically executed as the last step in the auto-login process.

Restrictions

Correct execution of many commands depends on proper behavior by the remote server. The ftpd server prevents the unauthorized users listed in the /etc/ftpusers file from transferring files.

An error in the treatment of carriage returns in the 4.2BSD UNIX asciitype transfer code has been corrected. This correction may result in incorrect transfers of binary files to and from 4.2BSD servers using the ascii type. Avoid this problem by using the binary image type.

Files

/etc/ftpusers contains the list of unauthorized users.

See Also

netrc(5), services(5), ftpd(8c), inetd(8c), syslog(8)