如何处理来自clang编译器的警告？

Question

我希望我的程序可以使用clang编译，且无警告。尽管编译后该函数似乎正常运行，但为什么会出现警告？我该如何处理这个警告？

$ 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);
    ^

代码

#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;
}

我正在使用Ubuntu（也希望代码能够在Solaris和BSD上运行）。

测试

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

- Niklas Rosencrantz

Answer 1

对于 read 和 write，您必须

#include <unistd.h>

它们没有在stdio.h中声明。

- Daniel Fischer

---

它在Ubuntu上运行。我希望它也能在Solaris和BSD上运行。谢谢。 - Niklas Rosencrantz

无论如何，应该是一样的，在任何情况下，“man read”告诉您应该包含哪个头文件。 - Daniel Fischer