The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


fgets - get a string from a stream


#include <stdio.h>

char *fgets(char *restrict
s, int n, FILE *restrict stream);


[CX] [Option Start] 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. [Option End]

The fgets() function shall read bytes from stream into the array pointed to by s until n-1 bytes are read, or a <newline> is read and transferred to s, or an end-of-file condition is encountered. A null byte shall be written immediately after the last byte read into the array. If the end-of-file condition is encountered before any bytes are read, the contents of the array pointed to by s shall not be changed.

[CX] [Option Start] The fgets() 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(). [Option End]


Upon successful completion, fgets() shall return s. If the stream is at end-of-file, the end-of-file indicator for the stream shall be set and fgets() shall return a null pointer. If a read error occurs, the error indicator for the stream shall be set, fgets() shall return a null pointer, [CX] [Option Start]  and shall set errno to indicate the error. [Option End]


Refer to fgetc.

The following sections are informative.


Reading Input

The following example uses fgets() to read lines of input. It assumes that the file it is reading is a text file and that lines in this text file are no longer than 16384 (or {LINE_MAX} if it is less than 16384 on the implementation where it is running) bytes long. (Note that the standard utilities have no line length limit if sysconf(_SC_LINE_MAX) returns -1 without setting errno. This example assumes that sysconf(_SC_LINE_MAX) will not fail.)

#include <limits.h>
#include <stdio.h>
#include <unistd.h>
#define MYLIMIT 16384

char *line; int line_max; if (LINE_MAX >= MYLIMIT) { // Use maximum line size of MYLIMIT. If LINE_MAX is // bigger than our limit, sysconf() cannot report a // smaller limit. line_max = MYLIMIT; } else { long limit = sysconf(_SC_LINE_MAX); line_max = (limit < 0 || limit > MYLIMIT) ? MYLIMIT : (int)limit; }
// line_max + 1 leaves room for the null byte added by fgets(). line = malloc(line_max + 1); if (line == NULL) { // out of space ... return error; }
while (fgets(line, line_max + 1, fp) != NULL) { // Verify that a full line has been read ... // If not, report an error or prepare to treat the // next time through the loop as a read of a // continuation of the current line. ... // Process line ... ... } free(line); ...








Standard I/O Streams, fgetc, fopen, fread, fscanf, getc, getchar, getdelim, gets, ungetc

XBD <stdio.h>


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

Issue 6

Extensions beyond the ISO C standard are marked.

The prototype for fgets() is changed for alignment with the ISO/IEC 9899:1999 standard.

Issue 7

Austin Group Interpretation 1003.1-2001 #051 is applied, updating the list of functions that mark the last data access timestamp for update.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0134 [182] and XSH/TC1-2008/0135 [14] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0114 [468] is applied.

End of informative text.


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