How to Build a Real‑Time Console Progress Bar in C

Carriage Return vs Newline

In C, \n moves the cursor to the next line and returns to column 0, while \r only returns to the beginning of the current line. Using \n therefore creates a new line; \r can be used to overwrite the current line.

Standard Output Buffering

The C standard library provides three buffering modes for stdout:

Unbuffered : data is written directly to the device.

Line buffered : the buffer is flushed when a newline ( \n) is encountered.

Fully buffered : the buffer is flushed only when it becomes full.

Because stdout is line‑buffered on a terminal, a newline forces an immediate display. Without a newline the output remains in the buffer until the program terminates or the buffer is flushed manually.

#include <stdio.h>
#include <unistd.h>
int main(){
    printf("I am a proc
"); // prints immediately
    sleep(3);
    return 0;
}

#include <stdio.h>
#include <unistd.h>
int main(){
    printf("I am a proc"); // no newline → buffered
    sleep(3);
    return 0;
}

Countdown Example

Using \r after each number returns the cursor to the start of the line, creating a countdown effect. Because the output lacks a newline, fflush(stdout) must be called to force the buffer to the terminal.

#include <stdio.h>
#include <unistd.h>
int main(){
    int count = 3;
    while(count >= 0){
        printf("%d\r", count--);
        fflush(stdout);
        sleep(1);
    }
    return 0;
}

For multi‑digit counts, a width specifier (e.g., %2d) ensures the field occupies a fixed number of characters, preventing leftover digits from remaining on screen.

#include <stdio.h>
#include <unistd.h>
int main(){
    int count = 10;
    while(count >= 0){
        printf("%2d\r", count--);
        fflush(stdout);
        sleep(1);
    }
    return 0;
}

Full Progress Bar Implementation

The program below builds a 100‑character progress bar. It maintains a character array proc, prints the bar with left‑aligned filling using the format "[%-100s] [%d%%]\r", flushes the output, updates the array with ‘#’, and sleeps 100 ms between updates.

#include <stdio.h>
#include <string.h>
#include <unistd.h>

void ProcBar(){
    int i = 0;
    char proc[102];
    memset(proc, '\0', sizeof(proc));
    while(i <= 100){
        printf("[%-100s] [%d%%]\r", proc, i);
        fflush(stdout);
        proc[i] = '#';
        usleep(100000); // 100 ms
        i++;
    }
    printf("
");
}

int main(){
    ProcBar();
    return 0;
}