Printf not printing output to console in musl toolchain

For Developers
1

Hi all, I am using openwrt with musl toolchain. Here I am facing some strange behavior wrt to fprintf and fscanf.

For the following example c program

#include <stdio.h>
#include <string.h>
int
prompt_yesNo(char *promptStr)
{
char inbuf[8] = {0};
int ret = 0;
fprintf(stdout, "%s\b", promptStr);

while(fgets(inbuf, sizeof(inbuf), stdin)) {
    if (!strchr(inbuf, '\n'))
        while(fgetc(stdin)!='\n');

    if ((0 == strcasecmp(inbuf, "y\n")) || (0 == strcasecmp(inbuf, "yes\n"))) {
        ret = 1;
        goto out;
    } else if ((0 == strcasecmp(inbuf, "n\n")) || (0 == strcasecmp(inbuf, "no\n"))) {
        ret = 0;
        goto out;
    }
    printf("Invalid input. Please enter the choice(y/n) again :");
}

out :
return ret;
}

int main()
{
int a;
char bufPrompt[128] = {0};
sprintf(bufPrompt, "Do you want to enter a size (y/n) : ");
if (prompt_yesNo(bufPrompt))
{
fprintf(stdout, "Please enter the size : ");
fscanf(stdin, "%u", &a);
fprintf(stdout,"entired size is %d",a);
}
return 0;
}

The expected output should be

Do you want to enter a size (y/n): y [given by me]
Please enter the size: 8 [given by me]
entired size is 8

But for me when I give input as y, console is hanging and not printing "Please enter the size" it is expecting for a input i.e when we provide input 8 it is completing the rest of code and shows me output entered size is 8.

In online there have been few solutions like use setvbuf(stdout, NULL, _IONBF, 0); in every main function or use fflush(stdout) after printf/fprintf of all packages to overcome issues like this, but this won't be an appropriate way. Can we modify definition of printf or something in toolchain level to overcome this issue?
Or are there any other solutions for the same?

3

Did I miss something here?

4

nah, just don't think my reply was helpful ...

1 Like
5

Buffered output is a feature you will have to deal with (using fflush(3)) when you want to program C. There's no way around it.

6

Ok, Thanks for the response, I will check that, can you please help me providing any documentation link for the same.

7

https://pubs.opengroup.org/onlinepubs/9699919799/functions/fflush.html

1 Like
8

I have two builds one with uclibc toolchain in which same c program executes fine whereas musl is failing as such, So I am just wondering what caused the difference.

9

Probably just minor differences in implementation that govern when to do an implicit flush.