read and write

Learn low-level I/O with read and write.

read and write

read(fd, buf, n) reads up to n bytes from a file into buf. The return value is the number of bytes read; 0 at end of file, -1 on error.

write(fd, buf, n) writes n bytes from buf to a file. The return value is the number of bytes written. Both return ssize_t. Important: read may return fewer bytes than requested; this is not an error. For a full read you need a loop.

Buffer and loop

read and write work on a buffer. A buffer is a memory region that temporarily holds data, usually a char array.

You may not read all data in one call. So you call read in a loop, accumulating the bytes read each time. The same applies to write: bytes written may be fewer than requested. High-level fread does this automatically, but at the POSIX level you manage it.

Exercise write and byte count

Define const char *msg = "Merhaba\n";. Write to stdout with ssize_t n = write(1, msg, 8);. Then print the byte count with printf("%d\n", (int)n);.

Show example solution

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

int main(void) {
    const char *msg = "Merhaba\n";
    ssize_t n = write(1, msg, 8);
    printf("%d\n", (int)n);
    return 0;
}

Test yourself

Match I/O concepts

write

Sends data
read

Receives data
buffer

Temporary buffer
ssize_t

Signed size type
How does read return the number of bytes read?
- As ssize_t
- As void
- As char
- As float
Show answer

As ssize_t — read returns ssize_t, a signed integer.

read call

char buf[8];
ssize_t n = ____(fd, buf, 7);

Show answer

char buf[8];
ssize_t n = read(fd, buf, 7);

Next lesson: Summary - POSIX

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