help me understand setvbuf function

This is a discussion on help me understand setvbuf function within the C Programming forums, part of the General Programming Boards category; Code: #include <stdio.h> #include <stdlib.h> #include <string.h> #define BUFSIZE 1024 #define SLEN 81 void append(FILE *source, FILE *dest); int main(void) ...

  1. #1
    Registered User
    Join Date
    Sep 2008

    help me understand setvbuf function

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #define BUFSIZE 1024
    #define SLEN 81
    void append(FILE *source, FILE *dest);
    int main(void)
        FILE *fa, *fs;// fa for append file, fs for source file
        int files = 0;  // number of files appended
        char file_app[SLEN];  // name of append file
        char file_src[SLEN];  // name of source file
        puts("Enter name of destination file:");
        if ((fa = fopen(file_app, "a")) == NULL)
            fprintf(stderr, "Can't open %s\n", file_app);
        if (setvbuf(fa, NULL, _IOFBF, BUFSIZE) != 0)
            fputs("Can't create output buffer\n", stderr);
        puts("Enter name of first source file (empty line to quit):");
        while (gets(file_src) && file_src[0] != '\0')
            if (strcmp(file_src, file_app) == 0)
                fputs("Can't append file to itself\n",stderr);
            else if ((fs = fopen(file_src, "r")) == NULL)
                fprintf(stderr, "Can't open %s\n", file_src);
                if (setvbuf(fs, NULL, _IOFBF, BUFSIZE) != 0)
                    fputs("Can't create input buffer\n",stderr);
                append(fs, fa);
                if (ferror(fs) != 0)
                    fprintf(stderr,"Error in reading file %s.\n",
                if (ferror(fa) != 0)
                    fprintf(stderr,"Error in writing file %s.\n",
                printf("File %s appended.\n", file_src);
                puts("Next file (empty line to quit):");
        printf("Done. %d files appended.\n", files);
        return 0;
    void append(FILE *source, FILE *dest)
        size_t bytes;
        static char temp[BUFSIZE]; // allocate once
        while ((bytes = fread(temp,sizeof(char),BUFSIZE,source)) > 0)
            fwrite(temp, sizeof (char), bytes, dest);
    can someone explain to me how the setvbuf() function works? and also the append function in this sample program thanks..

  2. #2
    Kernel hacker
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    Jul 2007
    Farncombe, Surrey, England

    If you still have questions, please ask.

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  3. #3
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    Also read the FAQ on why gets() is bad news.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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