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Reading bytes from IFS stream file using ifs C api read() in RPGLE

Reading bytes from IFS stream file using ifs C api read() in RPGLE
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Reading bytes from IFS stream file using C API read in RPGLE

The read() API

The read() API is just the opposite of the write() API. It reads bytes of data from a IFS stream file, and stores them into the area of memory(buffer) that we point it to.

the read() function reads n number of bytes of input into the memory area indicated by buffer.

Prototype of read() api in C

int read(int fildes, void *buf, size_t nbyte);
  1. int: It tells what type of value this read() api returns. The int data type in C is a 32 bits signed integer and is equivalent to "10 I 0" in RPGLE. It returns the actual number of bytes being read than asked to read.
  2. the read() function reads nbyte bytes of input into the memory area indicated by buf.
  3. int fildes:The first parameter is an integer named File descriptor which is the return value from the open() api and identifies the file being opened.
  4. void *buf:read() api will store bytes in buf that it loads from the stream file being read. This is a void pointer which means any no. of bytes can be read and any type of data can be loaded into buf whether its character, numeric or data structure.
  5. size_t nbyte: This is the number of bytes to read. The read() api will read the no. of byets specified in nbytes paramater from the stream file. However, if we are at the end of the stream file we may end up in reading fewer bytes than expected by the read() api. Here, "size_t" is user-defined type. Its defined same variable for which we have used LIKE keyword in RPGLE to define it. The byte size is stored idifferently in different platforms. The size_t is defined inside header file "sys/types.h". In IBM i, size_t is defined as 32 bit unsigned integer i.e. RPG "10U 0". If nbyte is zero, read() returns a value of zero without attempting any other action.

Prototype for read() API in RPGLE

RPG Code in Fixed format for Using read() api in RPGLE to read bytes from IFS stream files.
     HDFTACTGRP(*NO)                                                                                 
     D read            PR            10i 0 extproc('read')                      * 32 bit, no. of byt 
     D fileds                        10i 0 value                                *file descriptor     
     D buffer                          *   value                                * pointer to byte    
     D noofbytes                     10U 0 value                                * 32 bits            
                                                                                                     
     D open            PR            10I 0 extproc('open')                                           
     D ifspath                         *   value options(*string)               *ifs path            
     D oflag                         10I 0 value                                *string of 32 bits   
     D mode                          10U 0 value options(*nopass)               * 9 bits             
     D codepage                      10U 0 value options(*nopass)               *                    
                                                                                                     
      * <-----oflag---->                                                                            
     D O_readonly      C                   1                                                         
     D O_writeonly     C                   2                                                         
     D O_readwrite     C                   4                                                         
     D O_createfileifnotexist...                                                                     
     D                 C                   8                                                         
     D O_exclusivecreate...                                                                          
     D                 C                   16                          
     D O_truncateto0bytes...                                           
     D                 C                   64                          
     D O_appendtofile  C                   256                         
     D O_converttextbycodepage...                                      
     D                 C                   8388608                     
     D O_openintextmode...                                             
     D                 C                   16777216                    
      *                                                                
      * <-----mode---->                                                
      * owner,group,other (RWX)                                        
      *                                         owner authority        
     D M_readowner     C                   256                         
     D M_writeowner    C                   128                         
     D M_executeowner  C                   64                          
      *                                         group authority        
     D M_readgroup     C                   32                          
     D M_writegroup    C                   16                          
     D M_executegroup  C                   8                           
      *                                         other people           
     D M_readother     C                   4                              
     D M_writeother    C                   2                              
     D M_executeother  C                   1                              
      *                                                                   
     Difspath          s            512a                                  
     Doflag            s             10I 0                                
     Dmode             s             10U 0                                
     Dcodepage         s             10U 0                                
     Dfiledescriptor   s             10i 0                                
                                                                          
      *                                                                   
     Difsdata          s            500a                                  
     Dreturn_read      s             10i 0 inz                            
     C                   EVAL      ifspath = '/home/easyclass/openfile1'  
     C                   EVAL      oflag =  O_readonly                    
     C                   EVAL      filedescriptor = open(%trim(ifspath):  
     C                             oflag)                                 
     C                   IF        filedescriptor < 0                     
     C                   RETURN                                           
     C                   ENDIF                                            
     C                   EVAL      return_read = read(filedescriptor: 
     C                             %addr(ifsdata):%size(ifsdata))     
     C                   IF        return_read < 1                    
     C     'EOF'         DSPLY                                        
     C                   ENDIF                                        
     C                   EVAL      *INLR = *ON                        
     C                   RETURN                                                                                                              
RPG Code in /Free and /End-Free format for Using read() api in RPGLE to read bytes from IFS stream files.
     HDFTACTGRP(*NO)                                                                                 
     D read            PR            10i 0 extproc('read')                      * 32 bit, no. of byt 
     D fileds                        10i 0 value                                *file descriptor     
     D buffer                          *   value                                * pointer to byte    
     D noofbytes                     10U 0 value                                * 32 bits            
                                                                                                     
     D open            PR            10I 0 extproc('open')                                           
     D ifspath                         *   value options(*string)               *ifs path            
     D oflag                         10I 0 value                                *string of 32 bits   
     D mode                          10U 0 value options(*nopass)               * 9 bits             
     D codepage                      10U 0 value options(*nopass)               *                    
                                                                                                     
      * <-----oflag---->                                                                             
     D O_readonly      C                   1                                                         
     D O_writeonly     C                   2                                                         
     D O_readwrite     C                   4                                                         
     D O_createfileifnotexist...                                                                     
     D                 C                   8                                                         
     D O_exclusivecreate...                                                                          
     D                 C                   16                      
     D O_truncateto0bytes...                                       
     D                 C                   64                      
     D O_appendtofile  C                   256                     
     D O_converttextbycodepage...                                  
     D                 C                   8388608                 
     D O_openintextmode...                                         
     D                 C                   16777216                
      *                                                            
      * <-----mode---->                                            
      * owner,group,other (RWX)                                    
      *                                         owner authority    
     D M_readowner     C                   256                     
     D M_writeowner    C                   128                     
     D M_executeowner  C                   64                      
      *                                         group authority    
     D M_readgroup     C                   32                      
     D M_writegroup    C                   16                      
     D M_executegroup  C                   8                       
      *                                         other people       
     D M_readother     C                   4        
     D M_writeother    C                   2        
     D M_executeother  C                   1        
      *                                             
     Difspath          s            512a            
     Doflag            s             10I 0          
     Dmode             s             10U 0          
     Dcodepage         s             10U 0          
     Dfiledescriptor   s             10i 0                               
                                                    
      *                                             
     Difsdata          s            500a            
     Dreturn_read      s             10i 0 inz      
      /free                                         
       ifspath = '/home/easyclass/openfile1';       
                                                    
       oflag = O_readonly ;                         
       filedescriptor = open(%trim(ifspath):                                 
                             oflag);                                         
                                                                             
       if filedescriptor < 0;                                                
         return;                                                             
       endif;                                                                
                                                                             
       return_read  = read(filedescriptor:%addr(ifsdata):%size(ifsdata));    
       if return_read < 1;                                                   
         DSPLY 'EOF';                                                        
       endif;                                                                
       *inlr = *on;                                                          
       return;                                                               
      /end-free                                                                                                                                                                       
RPG Code in Fully Free format for Using read() api in RPGLE to read bytes from IFS stream files.
**FREE                                      
CTL-OPT DFTACTGRP(*NO);                     
DCL-PR read int(10) EXTPROC('read');        
  fileds  int(10) VALUE;                    
  buffer  pointer VALUE;                    
  noofbytes uns(10) VALUE;                  
END-PR;                                     
                                            
DCL-PR open int(10) EXTPROC('open');        
  ifspath pointer VALUE options(*string);   
  oflag   int(10) VALUE;                    
  mode uns(10) VALUE options(*nopass);      
  codepage uns(10) VALUE options(*nopass);  
END-PR;                                     
// * <-----oflag---->                      
DCL-C O_readonly 1;                         
DCL-C O_writeonly 2;                        
DCL-C O_readwrite 4;                        
DCL-C O_createfileifnotexist 8;             
DCL-C O_exclusivecreate 16;                                      
DCL-C O_truncateto0bytes 64;                                     
DCL-C O_appendtofile  256;                                       
DCL-C O_converttextbycodepage 8388608;                           
DCL-C O_openintextmode 16777216;                                 
//    * <-----mode---->                                         
//    * owner,group,other (RWX)                                  
//    *                                         owner authority  
DCL-C M_readowner 256;                                           
DCL-C M_writeowner 128;                                          
DCL-C M_executeowner 64;                                         
//    *                                         group authority  
DCL-C M_readgroup 32;                                            
DCL-C M_writegroup 16;                                           
DCL-C M_executegroup 8;                                          
//    *                                         other people     
DCL-C M_readother 4;                                             
DCL-C M_writeother 2;                                            
DCL-C M_executeother 1;                                          
                                                                 
DCL-S ifspath CHAR(512);                                                 
DCL-S oflag int(10);                                                     
DCL-S mode  uns(10);                                                     
DCL-S codepage uns(10);                                                  
DCL-S filedescriptor int(10);                                            
                                                                         
DCL-S ifsdata char(500) inz;                                             
DCL-S return_read int(10) inz;                                           
       ifspath = '/home/easyclass/openfile1';                            
                                                                         
       oflag = O_readonly ;                                              
                                                                         
       filedescriptor = open(%trim(ifspath):                             
                             oflag);                                     
                                                                         
       if filedescriptor < 0;                                            
         return;                                                         
       endif;                                                            
                                                                         
       return_read  = read(filedescriptor:%addr(ifsdata):%size(ifsdata));
       if return_read < 1;               
         DSPLY 'EOF';                    
       endif;                            
       *inlr = *on;                      
       return;                           

Brief Explanation of the RPGLE code using READ() ifs C api

  • First, we initialize the ifs path variable with the path name for the stream file to be opened at IFS. Here, file named "openfile1" will be opened present in in user current home directory.
  • Initialize the oflag parameter of the OPEN() api with flag O_readonly which means open it in read only mode.
  • Call the OPEN() api by passing ifspath variable, flag variable. Once the OPEN() api got executed it will return the file descriptor in variable named "filedescriptor".
  • Check whether filedescriptor is less than zero then return from the program which means file was not opened by the OPEN() api and any error occurs during file open. This filedescriptor is associated with the file being opened and is unique for each file being opened by the OPEN() api. It is later used by other IFS C api to know which file has to be operated.
  • If file descriptor is equals or greater than zero then that means file was opened successfully by the open() api.
  • Now call the read() api by passing the file descriptor being returned from the open() api and address of the variable ifsdata and size of the data to be read from disk i.e. ifs stream file. Once read() api got executed it will return us the integer value that tells actual number of bytes being read from ifs stream file and loads the data in variable ifsdata. The return value can be lesser than the size of data to be read when we are at the end of the ifs stream file.
  • Check if return value from read() api i.e. actual no. of bytes read from disk is lesser than 1 then it means read() api has read all the data in ifs file. Therefore, display End of File reached.
  • Compile-Run-Output of the above program

  • Compile the above RPGLE program by either using command CRTSQLRPGI or option 14 on source member of type SQLRPGLE.
  • Call the program using call command CALL from the IBM i command line.
  • For output, Please debug the program and check the data being loaded in variable ifsdata.
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