how to properly flush the input stream (stdin)

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By jephthah on Jun 10th, 2009 11:19 am

too many new and intermediate users of C attempt to use the command fflush(stdin) to "flush the input buffer". this is patently wrong.

the rule is (and click if you don't believe me) : NEVER USE "FFLUSH()" ON INPUT STREAMS SUCH AS "STDIN"

here is one method of properly flushing extra (and unwanted) characters from the stdin input stream. the code is written as a macro, and requires a character array to collect excess (junk) input. the macro will also ensure the newline character is stripped.

the intended use is in conjunction with fgets. for example:

char userInput[8]; // change array size to any arbitrary value

    printf("input a string and hit <enter>.  A maximum of %d characters will be recorded.\n",sizeof(userInput)-1);

    fgets(userInput,sizeof(userInput),stdin);
    FLUSH_STDIN(userInput);    // strip newline, flush extra chars

    printf("the first %d characters are \"%s\", any others have been discarded.\n",strlen(userInput),userInput);

#define FLUSH_STDIN(x) {if(x[strlen(x)-1]!='\n'){do fgets(Junk,16,stdin);while(Junk[strlen(Junk)-1]!='\n');}else x[strlen(x)-1]='\0';}

char Junk[16]; // buffer for discarding excessive user input, 
               // used by "FLUSH_STDIN" macro

2 Years Ago

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wow, dude, this snippet rocks! A+++ would read again!

jephthah

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2 Years Ago

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Rofl :]

William Hemsworth