stdin, stdout, stderr

Three text streams are predefined. These streams are implicitly opened and unoriented at program startup.

What constitutes an interactive device is implementation-defined.

These macros are expanded to expressions of type FILE * .

Notes

Although not mandated by POSIX, the UNIX convention is that stdin and stdout are line-buffered if associated with a terminal and stderr is unbuffered.

These macros may be expanded to modifiable lvalues. If any of these FILE * lvalue is modified, subsequent operations on the corresponding stream result in unspecified or undefined behavior.

Example

#include <stdarg.h>
#include <stdio.h>
 
int my_printf(const char * restrict fmt, ...)
{
    va_list vl;
    va_start(vl, fmt);
    int ret = vfprintf(stdout, fmt, vl);
    va_end(vl);
    return ret;
}
 
int main(void)
{
    my_printf("Rounding:\t%f %.0f %.32f\n", 1.5, 1.5, 1.3);
    my_printf("Padding:\t%05.2f %.2f %5.2f\n", 1.5, 1.5, 1.5);
    my_printf("Scientific:\t%E %e\n", 1.5, 1.5);
    my_printf("Hexadecimal:\t%a %A\n", 1.5, 1.5);
}

Rounding:       1.500000 2 1.30000000000000004440892098500626
Padding:        01.50 1.50  1.50
Scientific:     1.500000E+00 1.500000e+00
Hexadecimal:    0x1.8p+0 0X1.8P+0

References

See also