How to handle a warning from the clang compiler?

Question

I'd like my program to compile with clang with no warnings. The function appears to work when compiled but why? How can I handle the warning?

$ clang cpu-disk-info.c
cpu-disk-info.c:108:17: warning: implicit declaration of function 'read' is
      invalid in C99 [-Wimplicit-function-declaration]
    while ((n = read(0, buf, betterSIZE)) > 0)
                ^
cpu-disk-info.c:109:5: warning: implicit declaration of function 'write' is
      invalid in C99 [-Wimplicit-function-declaration]
    write(1, buf, n);
    ^

Code

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define SIZE 1024
#define betterSIZE 4*SIZE /* read a better size at a time */

int main(int argc, char **argv)
{

    /* copy(); */

    /* make the names known */

    void info(char file_name[]);
    void buffered(char file_name[]);
    void better_buffered(char file_name[]);

   /* test */

    clock_t toc;
    clock_t tic = clock();
    info("coreutils_8.13.orig.tar.gz"); 
    info("coreutils_8.13.orig.tar.gz"); 
    info("coreutils_8.13.orig.tar.gz"); 
    info("coreutils_8.13.orig.tar.gz"); 
    info("coreutils_8.13.orig.tar.gz"); 
    toc = clock();
    printf("Unbuffered: %f seconds\n", (double)(toc - tic) / CLOCKS_PER_SEC);
    tic = clock();    
    buffered("coreutils_8.13.orig.tar.gz"); 
    buffered("coreutils_8.13.orig.tar.gz"); 
    buffered("coreutils_8.13.orig.tar.gz"); 
    buffered("coreutils_8.13.orig.tar.gz");
    buffered("coreutils_8.13.orig.tar.gz"); 
    toc = clock();
    printf("Buffered: %f seconds\n", (double)(toc - tic) / CLOCKS_PER_SEC);
    tic = clock();    
    better_buffered("coreutils_8.13.orig.tar.gz"); 
    better_buffered("coreutils_8.13.orig.tar.gz"); 
    better_buffered("coreutils_8.13.orig.tar.gz"); 
    better_buffered("coreutils_8.13.orig.tar.gz");
    better_buffered("coreutils_8.13.orig.tar.gz"); 
    toc = clock();
    printf("Better buffered: %f seconds\n", (double)(toc - tic) / CLOCKS_PER_SEC);
    return 0;
}

void info(char file_name[])
{
    int ch;
    FILE *fp;
    fp = fopen(file_name,"r");
    // read mode
    if (fp == NULL)
    {
        perror(file_name);
        exit(EXIT_FAILURE);
    }
    while ((ch = fgetc(fp)) != EOF)
    {
        //putchar(ch);
    }
    fclose(fp);
}

void buffered(char file_name[])
{
    char buf[SIZE];
    FILE *fp;
    size_t nread;
    fp = fopen(file_name, "r");
    if (fp) {
        while ((nread = fread(buf, 1, sizeof buf, fp)) > 0)
    {
            //fwrite(buf, 1, nread, stdout);
    }
        if (ferror(fp)) {
            /* to do: deal with error */
        }
        fclose(fp);
    }
}


void better_buffered(char file_name[])
{
    char buf[betterSIZE];
    FILE *fp;
    size_t nread;
    fp = fopen(file_name, "r");
    if (fp) {
        while ((nread = fread(buf, 1, sizeof buf, fp)) > 0)
    {
            //fwrite(buf, 1, nread, stdout);
    }
        if (ferror(fp)) {
            /* to do: deal with error */
        }
        fclose(fp);
    }
}

int copy() /* input2output ie anything to anything */
{
    char buf[betterSIZE];
    int n;
    while ((n = read(0, buf, betterSIZE)) > 0)
    write(1, buf, n);
    return 0;
}

I'm using ubuntu (and would like the code to work also on solaris and bsd).

Test

$ ./benchm 
Unbuffered: 0.670000 seconds
Buffered: 0.040000 seconds
Better buffered: 0.020000 seconds

Answer 1

For read and write, you have to

#include <unistd.h>

they're not declared in stdio.h.