英文:
read from stdin, and write to stdout, why my read() function alwasy return 1?
问题
我有一个简单的代码来测试read()和write()系统调用，如下所示：```c#include<stdlib.h>#include<unistd.h>#include<stdio.h>int main(){#define BUFFSIZE 4096int n;char buf[BUFFSIZE];while ((n = read(STDIN_FILENO, buf, BUFFSIZE)) > 0) {if (write(STDOUT_FILENO, buf, n) != n) {printf("write err.\n");}}if (n < 0) {printf("read error.\n");}return 0;}
我像这样运行它：
$ ./a.outhelloh
我使用gdb并发现read()始终返回1。
<details><summary>英文:</summary>I have a simple code to test read() and write() system call, as below```c#include<stdlib.h>#include<unistd.h>#include<stdio.h>int main(){#define BUFFSIZE 4096int n;char buf[BUFFSIZE];while (n=read(STDIN_FILENO, buf, BUFFSIZE) > 0) {if (write(STDOUT_FILENO, buf, n) != n) {printf("write err.\n");}}if (n < 0) {printf("read error.\n");}return 0;}
and I run it like this:
$ ./a.outhelloh
I use gdb and find read() always returns 1.
答案1
得分: 3
The statement:
while (n=read(STDIN_FILENO, buf, BUFFSIZE) > 0)
is parsed as:
while ( n = (read (STDIN_FILENO, buf, BUFSIZE) > 0))
The comparison operator > has higher precedence than the assignment operator =. Comparison and logical operators return the value false or true, which in this statement gets stored in the variable n. You need parentheses around the assignment to n in order to secure the order of operations like so:
while ( (n = read (STDIN_FILENO, buf, BUFSIZE)) > 0)
Alternatively, the loop can be rewritten in this manner:
for (;;) {n = read (STDIN_FILENO, buf, BUFSIZE);if (n <= 0) {break;}if (write (STDOUT_FILENO, buf, n) != n) {fputs ("write error.\n", stderr);}}
Note that read() returns a ssize_t, not an int.
英文:
The statement:
while (n=read(STDIN_FILENO, buf, BUFFSIZE) > 0)
is parsed as:
while ( n = (read (STDIN_FILENO, buf, BUFSIZE) > 0))
The comparison operator > has higher precedence than the assignment operator =. Comparison and logical operators return the value false or true, which in this statement gets stored in the variable n. You need parentheses around the assignment to n in order to secure order of operations like so:
while ( (n = read (STDIN_FILENO, buf, BUFSIZE)) > 0)
Alternatively, the loop can be rewritten in this manner:
for (;;) {n = read (STDIN_FILENO, buf, BUFSIZE);if (n <= 0) {break;}if (write (STDOUT_FILENO, buf, n) != n) {fputs ("write error.\n", stderr);}}
Note that read() returns a ssize_t, not an int.
答案2
得分: 2
你必须在while条件中加上括号，否则 n 是 read(STDIN_FILENO, buf, BUFFSIZE)) > 0 的结果。
#include <stdlib.h>#include <unistd.h>#include <stdio.h>int main(){#define BUFFSIZE 4096int n;char buf[BUFFSIZE];while ((n=read(STDIN_FILENO, buf, BUFFSIZE)) > 0) {if (write(STDOUT_FILENO, buf, n) != n) {printf("write err.\n");}}if (n < 0) {printf("read error.\n");}return 0;}
英文:
You must put parentheses in the while condition, otherwise n is the result of read(STDIN_FILENO, buf, BUFFSIZE)) > 0.
#include<stdlib.h>#include<unistd.h>#include<stdio.h>int main(){#define BUFFSIZE 4096int n;char buf[BUFFSIZE];while ((n=read(STDIN_FILENO, buf, BUFFSIZE)) > 0) {if (write(STDOUT_FILENO, buf, n) != n) {printf("write err.\n");}}if (n < 0) {printf("read error.\n");}return 0;}
答案3
得分: 0
高级问题
节省时间。启用所有警告。比在SO上发帖更有效。
while (n=read(STDIN_FILENO, buf, BUFFSIZE) > 0) {警告：建议在用作真值的赋值周围加上括号[-Wparentheses]
以及其他。
英文:
Higher level problem
Save time. Enable all warnings. More efficient than posting on SO.
while (n=read(STDIN_FILENO, buf, BUFFSIZE) > 0) {warning: suggest parentheses around assignment used as truth value [-Wparentheses]
and others.
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