man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

fread(3C)

Name

fread - binary input

Synopsis

#include <stdio.h>

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

Description

The fread() function reads 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 are made to the fgetc(3C) 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) is 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.

The fread() function may mark the st_atime field of the file associated with stream for update. The st_atime field will be marked for update by the first successful execution of fgetc(3C), fgets(3C), fgetwc(3C), fgetws(3C), fread(), fscanf(3C), getc(3C), getchar(3C), getdelim(3C), getline(3C), gets(3C), or scanf(3C) using stream that returns data not supplied by a prior call to ungetc(3C) or ungetwc(3C).

Return Values

Upon successful completion, fread() returns 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() returns 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 is set and errno is set to indicate the error.

Errors

Refer to fgetc(3C).

Examples

Example 1 Reading from a Stream

The following example reads a single element 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, sizeof(buf), 1, fp);
...

Usage

The ferror() or feof() functions must be used to distinguish between an error condition and end-of-file condition. See ferror(3C).

Because of possible differences in element length and byte ordering, files written using fwrite(3C) are application-dependent, and possibly cannot be read using fread() by a different application or by the same application on a different processor.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
MT-Safe
Standard

See Also

read(2), fclose(3C), ferror(3C), fopen(3C), getc(3C), getdelim(3C), getline(3C), gets(3C), printf(3C), putc(3C), puts(3C), attributes(5), standards(5)