NAME

fread — binary input

SYNOPSIS

#include <stdio.h>

size_t fread(void *restrict ptr, size_t size, size_t nitems,
       FILE *restrict stream);


DESCRIPTION

^[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The fread() function shall read into the array pointed to by ptr up to nitems elements whose size is specified by size in bytes, from the stream pointed to by stream. For each object, size calls shall be made to the fgetc() function and the results stored, in the order read, in an array of unsigned char exactly overlaying the object. The file position indicator for the stream (if defined) shall be advanced by the number of bytes successfully read. If an error occurs, the resulting value of the file position indicator for the stream is unspecified. If a partial element is read, its value is unspecified.

^[CX] The fread() function may mark the last data access timestamp of the file associated with stream for update. The last data access timestamp shall be marked for update by the first successful execution of fgetc(), fgets(), fread(), fscanf(), getc(), getchar(), getdelim(), getline(), or scanf() using stream that returns data not supplied by a prior call to ungetc().

RETURN VALUE

The fread() function shall return the number of elements successfully read, which shall be less than nitems only if an error or end-of-file is encountered, or size is zero. If size or nitems is 0, fread() shall return 0 and the contents of the array and the state of the stream shall remain unchanged. Otherwise, if an error occurs, the error indicator for the stream shall be set, ^[CX]  and errno shall be set to indicate the error.

ERRORS

Refer to fgetc.

EXAMPLES

Reading from a Stream

The following example transfers a single 100-byte fixed length record from the fp stream into the array pointed to by buf.

#include <stdio.h>
...
size_t elements_read;
char buf[100];
FILE *fp;
...
elements_read = fread(buf, sizeof(buf), 1, fp);
...

If a read error occurs, elements_read will be zero but the number of bytes read from the stream could be anything from zero to sizeof(buf)-1.

The following example reads multiple single-byte elements from the fp stream into the array pointed to by buf.

#include <stdio.h>
...
size_t bytes_read;
char buf[100];
FILE *fp;
...
bytes_read = fread(buf, 1, sizeof(buf), fp);
...

If a read error occurs, bytes_read will contain the number of bytes read from the stream.

APPLICATION USAGE

The ferror() or feof() functions must be used to distinguish between an error condition and an end-of-file condition.

Because of possible differences in element length and byte ordering, files written using fwrite() are application-dependent, and possibly cannot be read using fread() by a different application or by the same application on a different processor.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

2.5 Standard I/O Streams, feof, ferror, fgetc, fopen, fscanf, getc

XBD <stdio.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

Extensions beyond the ISO C standard are marked.

The following changes are made for alignment with the ISO/IEC 9899:1999 standard:

• The fread() prototype is updated.

• The DESCRIPTION is updated to describe how the bytes from a call to fgetc() are stored.

Issue 7

Changes are made related to support for finegrained timestamps.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0178 [232] and XSH/TC1-2008/0179 [14] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0129 [926] is applied.

Issue 8

Austin Group Defect 1196 is applied, clarifying the RETURN VALUE section.

Austin Group Defect 1330 is applied, removing obsolescent interfaces.

Austin Group Defect 1624 is applied, changing the RETURN VALUE section.