CODEWITHSUNDEEP
ccharprintf
dkm
dkm

Reputation: 15

why \n in printf prints no value or old value of char array?

I was trying to test named pipe when i encountered this error. I have a file named client.c who writes to a named pipe. And I have a file named server.c who reads from a named pipe and print its value. Looks like there is some issue with server.c

Client code.

//-------CLIENT-----------
#include<stdio.h>
#include<sys/stat.h>
#include<sys/types.h>
#include<fcntl.h>
#include<unistd.h>
#include<string.h>
#define FIFO_FILE "MYFIFO"
int main()
{
    int fd, ch;
    char readbu[80];
    int write_byte;
    fd=open(FIFO_FILE,O_WRONLY);
    while(1)
    {
        printf("Enter choice \n1.Pepsi\n2.Coke\n3.Limca\n4.Maza\n");
        scanf("%d",&ch);
        switch(ch)
        {
            case 1: strcpy(readbu,"Pepsi\0");  
            write_byte=write(fd,readbu,sizeof(readbu));
            break;
            case 2: strcpy(readbu,"Coke\0");  
            write_byte=write(fd,readbu,sizeof(readbu));
            break;
            case 3: strcpy(readbu,"Limca\0");  
            write_byte=write(fd,readbu,sizeof(readbu));
            break;
            case 4: strcpy(readbu,"Maza\0");  
            write_byte=write(fd,readbu,sizeof(readbu));
            break;
            default:printf("Invalid");
            break;
        }
    }
    return 0;
}

Server code: 

// Server code    
#include<sys/stat.h>
#include<sys/types.h>
#include<fcntl.h>
#include<unistd.h>
#include<string.h>
#define FIFO_FILE "MYFIFO"
int main()
{
    int fd;
    char readbuf[80];
    char end[10];
    int to_end;
    int read_bytes;
    mknod(FIFO_FILE,S_IFIFO|0640,0);
    strcpy(end,"end");
    while(1)
    {
        fd=open(FIFO_FILE,O_RDONLY);
        read_bytes=read(fd,readbuf,sizeof(readbuf));
        readbuf[read_bytes]='\0';
        printf("Rec str = %s of len %d", readbuf,(int)strlen(readbuf));
        to_end=strcmp(readbuf,end);
        if(to_end==0)
        { close(fd);
            break;
        }`enter code here`
    }
    return 0;
}

At server side above code does not print any output.

If i change the printf statements to below then I observe that first iteration it prints blank line and then for other iteration it print old values. 

printf("\nRec str = %s of len %d", readbuf,(int)strlen(readbuf));

If i change the printf statements to below then I observe that it prints correct values. 

printf("Rec str = %s of len %d\n", readbuf,(int)strlen(readbuf));

I am completely confused how the \n makes so much difference.

I tried to check the value in readbuf using gdb and it actually contains the correct values. Kindly help me understand the trick here.

Upvotes: 0

Views: 63

Answers (1)

Clifford
Clifford

Reputation: 93476

The output buffer is not normally flushed until it is either full, a newline is inserted, or an explicit fflush( stdout ) call.

When you have the newline at the start of the output, it flushes the previously buffered data, and buffers the following data.

The following will resolve the issue:

printf("\nRec str = %s of len %d", readbuf,(int)strlen(readbuf));
fflush( stdout ) ;

Upvotes: 2

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