Pipe to a Subprocess

偶爾想偷懶的我會在code中使用system()，呼叫shell來執行一些東西，並且將執行完的結果丟回來程式中處理，這時候popen就很好用了，popen - pipe stream to or from a process。
popen允許開啟read或write pipe，不能夠同時開啟為read/write，read可以讓程式從popen中讀回來處理，而write可以將程式中的資料丟到popen處理。

popen(,"r")

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
    char buf[128];
    float a, b, c, d;
    FILE *fp;
    if ((fp = popen("(df -h 2>/dev/null) | tail -n +2", "r")) == NULL) {
        fprintf(stderr, "popen() failed\n");
        return -1;
    }
    while (!feof(fp)) {
        fscanf(fp, "%*[^ ]%f%*[^1-9]%f%*[^1-9]%f%*[^1-9]%f%%%*[^\n]", &a, &b, &c, &d);
        printf("%f/%f/%f/%f\n", a, b, c, d);
    }
    return 0;
}

popen(,"w")

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
    char buf[] = "1 2 3 4 5";
    float a, b, c, d;
    FILE *fp;
    if ((fp = popen("xargs -n 1 > /tmp/xx", "w")) == NULL) {
        fprintf(stderr, "popen() failed\n");
        return -1;
    }
    fprintf(fp, "%s", buf);
    pclose(fp);
    return 0;
}

參考資料：
• The GNU C Library, Chapter 15.2

glibc讀書心得 -- Ch2 Error Reporting

在glibc中會去偵錯並且回傳錯誤值，通常我們都應該去檢查這些回傳值，並且作error-handling，比如開檔 open()/fopen()，並非每次都會開成功，所以，open()/fopen()通常都會去檢查return value是不是有正確。

2.1 Checking for Errors
glibc的return value通常是-1、NULL或是EOF，這只能知道錯誤發生，至於發生何種錯誤，都是存在errno這個變數中，當您要使用errno必須include <errno.h>。這個header file也定義很多error code，都是以E為開頭，這些error code在Ch 2.2中有說明，用到再查就OK啦。

2.2 Error Codes
這個章節主要說明定義的error code，值得注意的是EAGAIN和EWOULDBLOCK的值一樣，所以不能放在同一個case switch中，其實我常用的error code大概只有他列的1/5不到吧，看看就好。

2.3 Error Messages 基本上我們都是希望report的錯誤是文字型態，而不是只有error code，所以glibc有提供一些轉換的function：

char * strerror(int errnum)	[Function]
char * strerror_r(int errnum, char *buf, size t n)	[Function]
void perror(const char *message)	[Function]
char * program_invocation_name	[Variable]
char * program_invocation_short_name	[Variable]
void error (int status, int errnum, const char *format, . . . )	[Function]
error_at_line (int status, int errnum, const char *fname,unsigned int lineno, const char *format, . . . )	[Function]

char * strerror(int errnum)
將errnum轉成相對應的字串

#include <stdio.h>
#include <errno.h>  // EINTR
#include <string.h> // strerror()

int main(int argc, char *argv[])
{
    printf("%s\n", strerror(EINTR));
    // print out "Interrupted system call"
    return 0;
}

char * strerror_r(int errnum, char *buf, size t n)
strerror的reentrant版本，mutli-thread就一定要用這個版本。

#define _GNU_SOURCE
#include <stdio.h>
#include <errno.h>  // EINTR
#include <string.h> // strerror()

int main(int argc, char *argv[])
{
    char buf[32];
    printf("%s\n", strerror_r(EINTR, buf, sizeof(buf)));
    // print out "Interrupted system call"
    return 0;
}

void perror(const char *message)
效果和printf(stderr, "%s:%s", message, strerror(errno))相似，把error message寫到stderr去，接著冒號後面再接errno的message。

#include <stdio.h>
#include <errno.h>  // EINTR
#include <string.h> // strerror()

int main(int argc, char *argv[])
{
    perror("my_perror_msg1");
    // print out "my_perror_msg1: Success"
    errno = EINTR;
    perror("my_perror_msg2");
    // print out "my_perror_msg2: Interrupted system call"
    return 0;
}

char * program_invocation_name / char * program_invocation_short_name
該程式的名稱，和argv[0]的值一樣。

#include <stdio.h>
#include <errno.h>  // EINTR
#include <string.h> // strerror()

int main(int argc, char *argv[])
{
    printf("%s\n", argv[0]);
    // print out "./a.out"
    printf("%s\n", program_invocation_name);
    // print out "./a.out"
    printf("%s\n", program_invocation_short_name);
    // print out "a.out"
    return 0;
}

void error (int status, int errnum, const char *format, . . . )
status不為0時，就等同exit(status)，後面的參數errnum和format用途類似perror()，當errnum為0時，就不會印後面的message，但是perror()會印Success。

#include <stdio.h>
#include <errno.h>  // EINTR
#include <string.h> // strerror()

int main(int argc, char *argv[])
{
    error(0, 0, "%s", "msg");
    error(0, EINTR, "%s", "msg");
    // print out "./a.out: msg: Interrupted system call"
    error(1, EINTR, "%s", "msg");
    // print out "./a.out: msg: Interrupted system call" and exit(1)
    return 0;
}

error_at_line(int status, int errnum, const char *fname, unsigned int lineno, const char *format, ...)
error_at_line()和error()很像，只是多了fname和lineno印出檔案名稱和行號。

#include <stdio.h>
#include <errno.h>  // EINTR
#include <string.h> // strerror()

int main(int argc, char *argv[])
{
    error_at_line(0, 0, __FILE__, __LINE__, "%s", "msg");
    // print out "./a.out:error_at_line.c:7: msg"
    error_at_line(0, EINTR, __FILE__, __LINE__, "%s", "msg");
    // print out "./a.out:error_at_line.c:8: msg: Interrupted system call"
    error_at_line(1, EINTR, __FILE__, __LINE__, "%s", "msg");
    // print out "./a.out:error_at_line.c:11: msg: Interrupted system call"
    return 0;
}

參考資料：
• The GNU C Library, Chapter 2

glibc讀書心得 -- Ch1 Introduction

1.1 Getting Started
C語言並沒有內建一些常用的操作，如input/output、memory management等等，這些通通被定義在Standard Library中(glibc)。

1.2 Standards and Portability
glibc參考的C Library標準有ISO C(American National Standard X3.159-1989—“ANSI C” and later by the International Standardization Organization (ISO): ISO/IEC 9899:1990, “Programming languages—C”.)和POSIX兩個。以及參考System V和Berkeley UNIX兩個實做而成。

1.3 Using the Library
Library在C主要由兩個部份組成：

1. header files that define types and macros and declare variables and functions
2. the actual library or archive that contains the definitions of the variables and functions.

為了能使用glibc，你必須將所需的header file，用＃include這個preprocessor directive將其引入，嚴格來說，您可以不必include這些header file，只要您可以正確的宣告相關的變數、巨集等等，不過為了效率和能正確的宣告相關變數，我們都會採用header file的形式，將其引入(include)。至於Macro的部份就參考我之前的C preprocessor的文章GCC - C Preprocessor應該會更清楚。
關於Reserved Names個人覺得還蠻重要的，就直接貼出來了：

1. Names beginning with a capital ‘E’ followed a digit or uppercase letter may be used for additional error code names. See Chapter 2 [Error Reporting], page 13.
2. Names that begin with either ‘is’ or ‘to’ followed by a lowercase letter may be used for additional character testing and conversion functions. See Chapter 4 [Character Handling], page 65.
3. Names that begin with ‘LC_’ followed by an uppercase letter may be used for additional macros specifying locale attributes. See Chapter 7 [Locales and Internationalization], page 150.
4. Names of all existing mathematics functions (see Chapter 19 [Mathematics], page 466) suffixed with ‘f’ or ‘l’ are reserved for corresponding functions that operate on float and long double arguments, respectively.
5. Names that begin with ‘SIG’ followed by an uppercase letter are reserved for additional signal names. See Section 24.2 [Standard Signals], page 592.
6. Names that begin with ‘SIG_’ followed by an uppercase letter are reserved for additional signal actions. See Section 24.3.1 [Basic Signal Handling], page 600.
7. Names beginning with ‘str’, ‘mem’, or ‘wcs’ followed by a lowercase letter are reserved for additional string and array functions. See Chapter 5 [String and Array Utilities], page 73.
8. Names that end with ‘_t’ are reserved for additional type names.

參考資料：
1. The GNU C Library, Chapter 1