BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  About fflush() function          BorlandTalk.com Forum Index -> C++ Builder (Language C++) View previous topic :: View next topic   Author Message Vladimir Grigoriev Guest Posted: Fri Feb 10, 2006 5:03 pm    Post subject: About fflush() function Hi Look at the code below. It is simple C-program test (BCB 5 console application). #include <stdio.h> #include <stdlib.h> #pragma hdrstop #include <conio.h> //-------------------------------------------------------------------------- - #pragma argsused int main(int argc, char* argv[]) { FILE *fp, *fp2; int rc, rc2, rc3, rc4; fp = fopen( "c:\\file.txt", "w+" ); fprintf( fp, "first record" ); fp2 = fopen( "c:\\file.txt", "r" ); rc = fgetc( fp2 ); printf( "return code is %i\n", rc ); fputc( '\n', fp ); fflush( fp ); rc2 = fgetc( fp2 ); printf( "value is now %c\n", rc2 ); rewind( fp ); fprintf( fp, "some updates\n" ); rc3 = fgetc( fp2 ); printf( "value is now %c\n", rc3 ); fflush( fp ); fflush( fp2 ); rc4 = fgetc( fp2 ); printf( "value is now %c\n", rc4 ); printf( "\nPress any key to exit..." ); getch(); return EXIT_SUCCESS; } The program output is return code is -1 value is now f value is now i value is now r Press any key to exit... My question is why is the last "value is now" "r"? I thought that after fflush( fp ); fflush( fp2 ); the last "value is now" must be "m" as the new string "some updates\n" was written. What is wrong? Vladimir Grigoriev Back to top Alan Bellingham Guest Posted: Fri Feb 10, 2006 5:03 pm    Post subject: Re: About fflush() function "Vladimir Grigoriev" <vlad.moscow (AT) mail (DOT) ru> wrote: Quote: Hi Look at the code below. It is simple C-program test (BCB 5 console application). #include <stdio.h #include <stdlib.h #pragma hdrstop #include <conio.h //-------------------------------------------------------------------------- - #pragma argsused int main(int argc, char* argv[]) { FILE *fp, *fp2; int rc, rc2, rc3, rc4; fp = fopen( "c:\\file.txt", "w+" ); fprintf( fp, "first record" ); fp2 = fopen( "c:\\file.txt", "r" ); An odd way of doing things! I assume this is a test so that you can see whether output flushing works, though. Quote: rc = fgetc( fp2 ); printf( "return code is %i\n", rc ); fputc( '\n', fp ); fflush( fp ); Appends the newline. Quote: rc2 = fgetc( fp2 ); printf( "value is now %c\n", rc2 ); Gets first character of input file. This is the 'f' of 'first record'. Quote: rewind( fp ); fprintf( fp, "some updates\n" ); rc3 = fgetc( fp2 ); Gets second character of input file. This is the 'i' of 'first record'. Quote: printf( "value is now %c\n", rc3 ); fflush( fp ); Flushes the output - so that if 'some updates' isn't already there on disc, it is now. Quote: fflush( fp2 ); A rather pointless call that does nothing. (fflush() flushes data out to a write file. It is documented not to have any defined behaviour for input files.) Quote: rc4 = fgetc( fp2 ); Gets third character of input file. This is the 'r' of 'first record' - at least, it was 'r' when the buffer was read, and even though the buffer doesn't represent the current disc contents, that's what it read and it hasn't seen any reason to update. Quote: My question is why is the last "value is now" "r"? I thought that after fflush( fp ); fflush( fp2 ); the last "value is now" must be "m" as the new string "some updates\n" was written. What is wrong? You have picked up the incorrect idea that fflush() works on an input stream. It does not. You have also come up with the more unusual idea that when working on an input file, fflush() empties the current buffer and causes it to be reread. (This is an interesting idea, and almost worth submitting to the standardisation committees. It's certainly more interesting than the common idea that fflush(stdin) will empty the input stream.) What you need to do to achieve what you appear to want to do is to turn off read buffering. Use setbuf() or setvbuf() on the read file. Check the docs for details. Note that unbuffered reading may well be much, much slower than buffered reading. Alan Bellingham -- ACCU Conference 2006 - 19-22 April, Randolph Hotel, Oxford, UK Back to top Ed Mulroy Guest Posted: Fri Feb 10, 2006 5:03 pm    Post subject: Re: About fflush() function Before you go any further add code to check that the value of the pointer returned by each of the fopen calls is not NULL and report the error to the screen if it is. .. Ed Quote: Vladimir Grigoriev wrote in message news:43ecb9cf (AT) newsgroups (DOT) borland.com... Hi Look at the code below. It is simple C-program test (BCB 5 console application). #include <stdio.h #include <stdlib.h #pragma hdrstop #include <conio.h //-------------------------------------------------------------------------- - #pragma argsused int main(int argc, char* argv[]) { FILE *fp, *fp2; int rc, rc2, rc3, rc4; fp = fopen( "c:\\file.txt", "w+" ); fprintf( fp, "first record" ); fp2 = fopen( "c:\\file.txt", "r" ); rc = fgetc( fp2 ); printf( "return code is %i\n", rc ); fputc( '\n', fp ); fflush( fp ); rc2 = fgetc( fp2 ); printf( "value is now %c\n", rc2 ); rewind( fp ); fprintf( fp, "some updates\n" ); rc3 = fgetc( fp2 ); printf( "value is now %c\n", rc3 ); fflush( fp ); fflush( fp2 ); rc4 = fgetc( fp2 ); printf( "value is now %c\n", rc4 ); printf( "\nPress any key to exit..." ); getch(); return EXIT_SUCCESS; } The program output is return code is -1 value is now f value is now i value is now r Press any key to exit... My question is why is the last "value is now" "r"? I thought that after fflush( fp ); fflush( fp2 ); the last "value is now" must be "m" as the new string "some updates\n" was written. What is wrong? Back to top Vladimir Grigoriev Guest Posted: Mon Feb 13, 2006 10:03 am    Post subject: Re: About fflush() function Hi Ed There is not a big essence to include error checking in this simple test program. I tried to make the program as simple as possible. For me the question about how flushing buffer influences on read/write operations is more important. Vladimir Grigoriev "Ed Mulroy" <dont_email_me (AT) bitbuc (DOT) ket> wrote in message news:43ecc070$1 (AT) newsgroups (DOT) borland.com... Quote: Before you go any further add code to check that the value of the pointer returned by each of the fopen calls is not NULL and report the error to the screen if it is. . Ed Vladimir Grigoriev wrote in message news:43ecb9cf (AT) newsgroups (DOT) borland.com... Hi Look at the code below. It is simple C-program test (BCB 5 console application). #include <stdio.h #include <stdlib.h #pragma hdrstop #include <conio.h //-------------------------------------------------------------------------- - #pragma argsused int main(int argc, char* argv[]) { FILE *fp, *fp2; int rc, rc2, rc3, rc4; fp = fopen( "c:\\file.txt", "w+" ); fprintf( fp, "first record" ); fp2 = fopen( "c:\\file.txt", "r" ); rc = fgetc( fp2 ); printf( "return code is %i\n", rc ); fputc( '\n', fp ); fflush( fp ); rc2 = fgetc( fp2 ); printf( "value is now %c\n", rc2 ); rewind( fp ); fprintf( fp, "some updates\n" ); rc3 = fgetc( fp2 ); printf( "value is now %c\n", rc3 ); fflush( fp ); fflush( fp2 ); rc4 = fgetc( fp2 ); printf( "value is now %c\n", rc4 ); printf( "\nPress any key to exit..." ); getch(); return EXIT_SUCCESS; } The program output is return code is -1 value is now f value is now i value is now r Press any key to exit... My question is why is the last "value is now" "r"? I thought that after fflush( fp ); fflush( fp2 ); the last "value is now" must be "m" as the new string "some updates\n" was written. What is wrong? Back to top Alan Bellingham Guest Posted: Mon Feb 13, 2006 11:03 am    Post subject: Re: About fflush() function "Vladimir Grigoriev" <vlad.moscow (AT) mail (DOT) ru> wrote: Quote: Hi Alan I have taken this example from IBM z/OS C/C++ Programming Guide. Here is a text from the guide Then their example is an extension to Standard C++, probably specific to that operating system only. Standard C++ doesn't work that way. On the other hand, if you want to be reading and writing to the same file from the same program, then the solution is not to open it twice, but only once. I was assuming that you were showing the read and write parts from two different programs, but in a single program to make it synchronous. Alan Bellingham -- ACCU Conference 2006 - 19-22 April, Randolph Hotel, Oxford, UK Back to top Vladimir Grigoriev Guest Posted: Mon Feb 13, 2006 11:03 am    Post subject: Re: About fflush() function Also I have looked through help documentation of BCB 5 and I have found function _flushall(). But this function flushes all buffers of input and output files. It has not a parameter as FILE * to flush a buffer of a specified file. Vladimir Grigoriev "Alan Bellingham" <alanb (AT) episys (DOT) com> wrote in message news:4ffpu156q6dk9on344du59lsu4rqg2fg20 (AT) 4ax (DOT) com... Quote: "Vladimir Grigoriev" <vlad.moscow (AT) mail (DOT) ru> wrote: Hi Look at the code below. It is simple C-program test (BCB 5 console application). #include <stdio.h #include <stdlib.h #pragma hdrstop #include <conio.h //------------------------------------------------------------------------- - - #pragma argsused int main(int argc, char* argv[]) { FILE *fp, *fp2; int rc, rc2, rc3, rc4; fp = fopen( "c:\\file.txt", "w+" ); fprintf( fp, "first record" ); fp2 = fopen( "c:\\file.txt", "r" ); An odd way of doing things! I assume this is a test so that you can see whether output flushing works, though. rc = fgetc( fp2 ); printf( "return code is %i\n", rc ); fputc( '\n', fp ); fflush( fp ); Appends the newline. rc2 = fgetc( fp2 ); printf( "value is now %c\n", rc2 ); Gets first character of input file. This is the 'f' of 'first record'. rewind( fp ); fprintf( fp, "some updates\n" ); rc3 = fgetc( fp2 ); Gets second character of input file. This is the 'i' of 'first record'. printf( "value is now %c\n", rc3 ); fflush( fp ); Flushes the output - so that if 'some updates' isn't already there on disc, it is now. fflush( fp2 ); A rather pointless call that does nothing. (fflush() flushes data out to a write file. It is documented not to have any defined behaviour for input files.) rc4 = fgetc( fp2 ); Gets third character of input file. This is the 'r' of 'first record' - at least, it was 'r' when the buffer was read, and even though the buffer doesn't represent the current disc contents, that's what it read and it hasn't seen any reason to update. My question is why is the last "value is now" "r"? I thought that after fflush( fp ); fflush( fp2 ); the last "value is now" must be "m" as the new string "some updates\n" was written. What is wrong? You have picked up the incorrect idea that fflush() works on an input stream. It does not. You have also come up with the more unusual idea that when working on an input file, fflush() empties the current buffer and causes it to be reread. (This is an interesting idea, and almost worth submitting to the standardisation committees. It's certainly more interesting than the common idea that fflush(stdin) will empty the input stream.) What you need to do to achieve what you appear to want to do is to turn off read buffering. Use setbuf() or setvbuf() on the read file. Check the docs for details. Note that unbuffered reading may well be much, much slower than buffered reading. Alan Bellingham -- ACCU Conference 2006 - 19-22 April, Randolph Hotel, Oxford, UK Back to top Vladimir Grigoriev Guest Posted: Mon Feb 13, 2006 11:03 am    Post subject: Re: About fflush() function Hi Alan I have taken this example from IBM z/OS C/C++ Programming Guide. Here is a text from the guide Reading Updated Records If you have a file open for read at the same time that the file is open for write in thesame application, you will be able to see the new data if you call fflush() torefresh the contents of the input buffer, as in the following example: /* this example demonstrates how updated records are read */ #include <stdio.h> int main(void) { FILE * fp, * fp2; int rc, rc2, rc3, rc4; fp = fopen("a.b","w+"); fprintf(fp,"first record"); fp2 = fopen("a.b","r"); /* Simultaneous Reader */ /* following gets EOF since fp has not completed first line * of output so nothing will be flushed to file yet */ rc = fgetc(fp2); printf("return code is %i\n", rc); fputc('\n', fp); /* this will complete first line */ fflush(fp); /* ensures data is flushed to file */ rc2 = fgetc(fp2); /* this gets 'f' from first record */ printf("value is now %c\n", rc2); rewind(fp); fprintf(fp, "some updates\n"); rc3 = fgetc(fp2); /* gets 'i' ..doesn't know about update */ printf("value is now %c\n", rc3); fflush(fp); /* ensure update makes it to file */ fflush(fp2); /* this updates reader's buffer */ rc4 = fgetc(fp2); /* gets 'm', 3rd char of updated record */ printf("value is now %c\n", rc4); return(0); } So I tried to repeat the test for BCB 5 (console mode) but as you can see the result is different... Vladimir Grigoriev "Alan Bellingham" <alanb (AT) episys (DOT) com> wrote in message news:4ffpu156q6dk9on344du59lsu4rqg2fg20 (AT) 4ax (DOT) com... Quote: "Vladimir Grigoriev" <vlad.moscow (AT) mail (DOT) ru> wrote: Hi Look at the code below. It is simple C-program test (BCB 5 console application). #include <stdio.h #include <stdlib.h #pragma hdrstop #include <conio.h //------------------------------------------------------------------------- - - #pragma argsused int main(int argc, char* argv[]) { FILE *fp, *fp2; int rc, rc2, rc3, rc4; fp = fopen( "c:\\file.txt", "w+" ); fprintf( fp, "first record" ); fp2 = fopen( "c:\\file.txt", "r" ); An odd way of doing things! I assume this is a test so that you can see whether output flushing works, though. rc = fgetc( fp2 ); printf( "return code is %i\n", rc ); fputc( '\n', fp ); fflush( fp ); Appends the newline. rc2 = fgetc( fp2 ); printf( "value is now %c\n", rc2 ); Gets first character of input file. This is the 'f' of 'first record'. rewind( fp ); fprintf( fp, "some updates\n" ); rc3 = fgetc( fp2 ); Gets second character of input file. This is the 'i' of 'first record'. printf( "value is now %c\n", rc3 ); fflush( fp ); Flushes the output - so that if 'some updates' isn't already there on disc, it is now. fflush( fp2 ); A rather pointless call that does nothing. (fflush() flushes data out to a write file. It is documented not to have any defined behaviour for input files.) rc4 = fgetc( fp2 ); Gets third character of input file. This is the 'r' of 'first record' - at least, it was 'r' when the buffer was read, and even though the buffer doesn't represent the current disc contents, that's what it read and it hasn't seen any reason to update. My question is why is the last "value is now" "r"? I thought that after fflush( fp ); fflush( fp2 ); the last "value is now" must be "m" as the new string "some updates\n" was written. What is wrong? You have picked up the incorrect idea that fflush() works on an input stream. It does not. You have also come up with the more unusual idea that when working on an input file, fflush() empties the current buffer and causes it to be reread. (This is an interesting idea, and almost worth submitting to the standardisation committees. It's certainly more interesting than the common idea that fflush(stdin) will empty the input stream.) What you need to do to achieve what you appear to want to do is to turn off read buffering. Use setbuf() or setvbuf() on the read file. Check the docs for details. Note that unbuffered reading may well be much, much slower than buffered reading. Alan Bellingham -- ACCU Conference 2006 - 19-22 April, Randolph Hotel, Oxford, UK Back to top Thomas Maeder [TeamB] Guest Posted: Mon Feb 13, 2006 5:03 pm    Post subject: Re: About fflush() function Please direct your browser at http://info.borland.com/newsgroups/ and read the newsgroup guidelines. One of them asks us not to quote entire posts we are following up to; instead, please trim the quotes to the parts relevant for your reply. Thanks! Back to top Vladimir Grigoriev Guest Posted: Tue Feb 14, 2006 10:03 am    Post subject: Re: About fflush() function O'k Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         BorlandTalk.com Forum Index -> C++ Builder (Language C++) All times are GMT Page 1 of 1   Jump to: Select a forum C++ Builder Newsgroups----------------C++ BuilderC++ Builder (ActiveX)C++ Builder (Announcements)C++ Builder (Command Line Tools)C++ Builder DatabasesC++ Builder Databases (ADO)C++ Builder Databases (Desktop)C++ Builder Databases (InterBase Express)C++ Builder Databases (Multi-Tier)C++ Builder Databases (SQL Servers)C++ Builder (Graphics)C++ Builder (IDE)C++ Builder (Internet)C++ Builder (Internet Socket)C++ Builder (Internet Web)C++ Builder (Jobs)C++ Builder (Language)C++ Builder (Language C++)C++ Builder (Compatibility with MFC)C++ Builder (Native API)C++ Builder (Non-Technical)C++ Builder (Students)C++ Builder (Third Party Tools)C++ Builder (Upgrade)C++ Builder (VCL)C++ Builder (VCL Components Usage)C++ Builder (VCL Components Development)C++ Builder (VCL Usage)C++ Builder (WinAPI)C++ Builder Installation Delphi Newsgroups----------------Delphi (General)Delphi ActiveX Controls UsageDelphi ActiveX Controls DevelopmentDelphi As400Delphi DatabasesDelphi Databases (ADO)Delphi Databases (dbExpress)Delphi Databases (Desktop)Delphi Databases (InterBase Express)Delphi Databases (Multi-Tier)Delphi Databases (SQL Servers)Delphi DeploymentDelphi GraphicsDelphi IDEDelphi InternationalizationDelphi InternetDelphi Internet Isapi-WebBrokerDelphi Internet WebSnapDelphi Internet WinsockDelphi JobsDelphi Language BASMDelphi Language ObjectPascalDelphi Model Driven Architecture (General)Delphi Model Driven Architecture (Third Party)Delphi MultimediaDelphi Native APIDelphi Non-TechnicalDelphi ObjectPascalDelphi OLE AutomationDelphi OO designDelphi OpenToolsAPIDelphi Reporting-ChartingDelphi RTLDelphi StudentsDelphi TeamSourceDelphi Thirdparty ToolsDelphi Thirdparty Tools (General)Delphi Thirdparty Tools (IntraWeb)Delphi Thirdparty Tools (RaveReports)Delphi UpgradeDelphi VCLDelphi VCL ComponentsDelphi VCL Components UsingDelphi VCL Components WritingDelphi WebServices SOAPDelphi WinAPIDelphi XMLDelphi Installationcomp.lang.pascal.delphi.components.misccomp.lang.pascal.delphi.components.usagecomp.lang.pascal.delphi.components.writingcomp.lang.pascal.delphi.databasescomp.lang.pascal.delphi.misccomp.sources.delphi JBuilder Newsgroups----------------JBuilderJBuilder AnnouncementsJBuilder Applet IssuesJBuilder As400JBuilder CompilerJBuilder CORBA/RMIJBuilder DatabasesJBuilder DB SwingJBuilder DebuggerJBuilder DeploymentJBuilder DocumentationJBuilder EnterpriseJBuilder HandheldJBuilder IDEJBuilder JavaJBuilder JavaAPIJBuilder Java LanguageJBuilder Java SwingJBuilder Java BeansJBuilder Java Beans UsageJBuilder Java Beans DevelopmentJBuilder JBCLJBuilder JDataStoreJBuilder JobsJBuilder LinuxJBuilder MacintoshJBuilder & MobileSetJBuilder MultiLingual AppsJBuilder Non-TechnicalJBuilder OpenToolsJBuilder Servlets (JSP)JBuilder StudentsJBuilder Team DevelopmentJBuilder Thirdparty ToolsJBuilder UpgradeJBuilder Installation InterBase Newsgroups----------------InterBaseInterBase Bug DiscussionInterBase GeneralInterBase IBConsoleInterBase OpenSourceInterBase ReplicationInterBase SQLInterBase ThirdpartyInterBase Installation Miscellaneous Borland Newsgroups----------------AppCenterAppServerEnteraVisiBroker  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.