truncate() in C (POSIX): Resizing Files Without Opening Them

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truncate() in C (POSIX): Resizing Files Without Opening Them

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Lesson format11 sections1 video4 code blocks1 practice itemUpdated Aug 27, 2026

truncate sets a file to a specified length. If the new length is shorter, data beyond that point is discarded. If longer, the file is extended with zero bytes. The POSIX version in unistd.h operates on a path. ftruncate operates on a file descriptor. Neither requires you to open the file with fopen. This is a system call wrapper, not a standard C library function, so it is not available on Windows without POSIX compatibility layers.

truncate() Function To Truncate A File (POSIX Library)

#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>

int main(void) {
    FILE *fp = fopen("trunc_test.txt", "w");
    fputs("This is a long line of text that will be shortened.\n", fp);
    fclose(fp);

    struct stat st;
    stat("trunc_test.txt", &st);
    printf("Before: %ld bytes\n", (long)st.st_size);

    if (truncate("trunc_test.txt", 10) != 0) {
        perror("truncate");
        return 1;
    }

    stat("trunc_test.txt", &st);
    printf("After:  %ld bytes\n", (long)st.st_size);

    fp = fopen("trunc_test.txt", "r");
    char buf[64];
    size_t n = fread(buf, 1, sizeof(buf) - 1, fp);
    buf[n] = '\0';
    printf("Content: '%s'\n", buf);
    fclose(fp);

    return 0;
}

Run truncate_demo

root@vulnarex:~#gcc -Wall truncate_demo.c -o truncate_demo && ./truncate_demo

The file went from 53 bytes to 10. Everything after byte 10 is gone. The content is now "This is a " with no newline. If you truncate to a length larger than the current file size, the gap is filled with null bytes (\0). This is how sparse files are created on filesystems that support them.

#ftruncate: Operating on an Open File Descriptor#link

ftruncate takes a file descriptor (from open, not fopen) and a length. Use it when you already have the file open and want to resize without closing and reopening. The fileno function extracts the file descriptor from a FILE pointer if you need to bridge the two APIs.

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

int main(void) {
    FILE *fp = fopen("trunc_test.txt", "r+");
    if (fp == NULL) {
        perror("fopen");
        return 1;
    }

    int fd = fileno(fp);
    if (ftruncate(fd, 5) != 0) {
        perror("ftruncate");
        fclose(fp);
        return 1;
    }

    printf("Truncated to 5 bytes via ftruncate.\n");
    fclose(fp);
    return 0;
}

Run ftruncate_demo

root@vulnarex:~#gcc -Wall ftruncate_demo.c -o ftruncate_demo && ./ftruncate_demo && cat trunc_test.txt

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quiz BLOCK (★ 20 XP)

What happens when you truncate a file to a length larger than its current size?

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truncate and ftruncate are POSIX tools for file size management. They appear in database engines, log rotators, and file format writers. On Windows, the equivalent is SetEndOfFile via the Win32 API. Next module: recursion, where functions call themselves and the stack becomes both your tool and your enemy.

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ftruncate: Operating on an Open File Descriptor

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August 27, 2026

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Laboratory sanity code

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#include <stdio.h>

#include <unistd.h>

#include <sys/stat.h>

int main(void) {

FILE *fp = fopen("trunc_test.txt", "w");

fputs("This is a long line of text that will be shortened.\n", fp);

fclose(fp);

struct stat st;

stat("trunc_test.txt", &st);

printf("Before: %ld bytes\n", (long)st.st_size);

if (truncate("trunc_test.txt", 10) != 0) {

perror("truncate");

return 1;

}

stat("trunc_test.txt", &st);

printf("After: %ld bytes\n", (long)st.st_size);

fp = fopen("trunc_test.txt", "r");

char buf[64];

size_t n = fread(buf, 1, sizeof(buf) - 1, fp);

buf[n] = '\0';

printf("Content: '%s'\n", buf);

fclose(fp);

return 0;

}

#include <stdio.h>

#include <unistd.h>

int main(void) {

FILE *fp = fopen("trunc_test.txt", "r+");

if (fp == NULL) {

perror("fopen");

return 1;

}

int fd = fileno(fp);

if (ftruncate(fd, 5) != 0) {

perror("ftruncate");

fclose(fp);

return 1;

}

printf("Truncated to 5 bytes via ftruncate.\n");

fclose(fp);

return 0;

}