fread - binary input
size_t fread(void *restrict ptr, size_t size, size_t nitems,
FILE *restrict stream);
[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.1-2017 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(), gets(), or scanf() using stream that returns data not supplied by a prior call to ungetc().
Upon successful completion, fread() shall return the number of elements successfully read which is less than nitems only if a read error or end-of-file is encountered. If size or nitems is 0, fread() shall return 0 and the contents of the array and the state of the stream remain unchanged. Otherwise, if a read error occurs, the error indicator for the stream shall be set, [CX] and errno shall be set to indicate the error.
Refer to fgetc.
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; 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; 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.
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.
Standard I/O Streams, feof, ferror, fgetc, fopen, fscanf, getc, gets
First released in Issue 1. Derived from Issue 1 of the SVID.
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.
Changes are made related to support for finegrained timestamps.
POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0178  and XSH/TC1-2008/0179  are applied.
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0129  is applied.
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