C 程序将小写转换为大写以用于标准输出的小问题

Question

I am learning about system calls in linux and I wrote a quick program to copy stdin to stdout.

I want to convert the stdin lowercase letters to uppercase letters before they are written.

I implemented a function with a pointer to the array to capitalize the characters. It only capitalizes the first letter of the write though and I don't understand why. From what I understand, I wouldn't need a for loop because of the read() system call.

#include <unistd.h>
#define SIZE 512
char charToUpper(char *);

int main () {
    int nread;
    char buf[SIZE];
    while (nread = read(0, buf , SIZE)) {
        charToUpper(buf);
        write(1,buf,nread);
    }
    return 0;
}

char charToUpper(char *a){
    if ((*a > 96) && (*a <123)) {
        *a = *a-32;
        return *a;
    }
}

Answer 1

Your charToUpper receives a pointer to a char , and you sent it buf , which decays to a pointer to a char of the first character in buf , hence your result.

Remember in c you do not get the size of the array you are passing for free - you have to pass it on as well. Consider all your operations in charToUpper are on *a , which is of type char , a single character. So to fix this change the declaration to

char charToUpper(char *, unsigned int size);

so know you know how many characters you actually read and need to change. Try and change your program now to suit this. One hint - your return will probably have to move for example.

Answer 2

he program is perfectly fine except for the loop part.

char* charToUpper(char *a){
    char *t=p;
    while(*a!='\0'){
    if ((*a > 96) && (*a <123)) 
        *a = *a-32;
    a++;
    }
    return t;
}

you didn't incremented the loop. do it and u will get

Answer 3

The following proposed code:

1. cleanly compiles
2. properly terminates the input array of characters
3. properly changes all lower case characters to upper case, using the facility: toupper() from the header file: ctype.h
4. performs the desired functionality

and now, the proposed code:

#include <unistd.h>
#include <ctype.h>   // toupper()

#define SIZE 512

void charToUpper(char *);

int main ( void ) 
{
    ssize_t nread;
    char buf[SIZE];
    while ( (nread = read(0, buf , SIZE) ) > 0) 
    {
        // NUL terminate the character string
        buf[nread] = '\0';

        // convert all characters to uppercase
        charToUpper(buf);

        // output the (all uppercase) string to stdout
        write(1,buf, (size_t)nread);
    }
    return 0;
}


void charToUpper( char buf[] )
{
    for( size_t i = 0; buf[i]; i++ )
    {
        buf[i] = (char)toupper( buf[i] );
    }

}