C stdio fscanf() Function
Example
Read a string from a file:
FILE *fptr;
fptr = fopen("filename.txt", "r");
char output[50];
fscanf(fptr, "%49s", output);
printf("%s", output);
fclose(fptr);
Definition and Usage
The fscanf() function reads formatted data from a file and writes it into memory locations specified by the arguments, then moves the position indicator to the file position where it stopped reading.
The fscanf() function is defined in the <stdio.h> header file.
The format parameter is a string that describes the format of the data that is expected from the file. If the content of the file does not match the format then the function stops reading at the point where the first mismatch occurs.
Format specifiers
The format string can contain format specifiers which specify which parts of the data should be written to the arguments. Each format specifier corresponds to one of the additional arguments of the function.
The format specifiers have the form %[*][width][length]specifier. The components in [square brackets] are optional.
An explanation of each of the components:
*- Optional. When present, the format specifier does not correspond to an argument.width- Optional. Specifies the maximum number of characters to read for this specifier.length- Optional. A sequence of characters which changes the data type of the argument. It can be one of the following:hh- Expectchar*type for whole numbers.h- Expectshort*type for whole numbers.l- Expectlong int*type for whole numbers.
Expectwchar_t*type for characters and strings.
Expectdouble*type for floating point numbers.ll- Expectlong long int*type for whole numbers.j- Expectintmax_t*oruintmax_t*type for whole numbers.z- Expectsize_t*type for whole numbers.t- Expectptrdiff_t*type for whole numbers.L- Expectlong double*type for whole numbers.
specifier- Required. A character or sequence which indicates how file data should be interpreted. The list of possible specifiers is shown in the table below.
List of specifiers
| Character | Specifier | Description |
|---|---|---|
i or u |
Integer | Reads a sequence of digits and interprets them as an integer. If the sequence starts with "0x" then it expects hexadecimal digits (0-9 and A-F). If the sequence starts with "0" then it expects octal digits (0-7). The sequence may be preceded by a sign ("+" or "-"). |
d |
Decimal integer | Reads a sequence of digits (0-9) and interprets them as an integer. The sequence may be preceded by a sign ("+" or "-"). |
o |
Octal integer | Reads a sequence of digits (0-7) and interprets them as an octal integer. The sequence may be preceded by a sign ("+" or "-"). |
x |
Hexadecimal integer | Reads a sequence of digits (0-9 and A-F) and interprets them as a hexadecimal integer. It may begin with "0x" The sequence may be preceded by a sign ("+" or "-"). |
f, e, g or a |
Floating point number | Reads a valid sequence of characters and interprets them as a floating point number. A valid sequence has at least one digit, it can be preceded by a sign ("+" or "-") and it can be followed by a decimal point and decimal digits. Scientific notation (a number followed by "e" or "E" and some digits) can also be used. |
c |
Character | Reads a character from the file. If a width is specified then it reads that number of characters. |
s |
String | Reads all of the characters up to the next whitespace (space, tab, line break) in the file. The value written to the argument will have an additional \0 null terminating character appended to it. |
p |
Pointer | Reads a sequence of characters which represent a pointer address. |
n |
No input | Nothing is read from the file. The number of characters that have been read up to this point is written into the argument. The argument must be a pointer to an integer. |
% |
Percent symbol | Reads one character from the file expecting a "%" symbol. This specifier is not associated with an argument. |
[characters] |
Character set | Reads one character which matches one of the characters specified in characters. |
[^characters] |
Excluded character set | Reads one character which is not in the set of characters specified in characters. |
See More Examples below for examples of how to use format specifiers.
Syntax
fscanf(FILE * fptr, const char * format, arg1, arg2...);Parameter Values
| Parameter | Description |
|---|---|
| fptr |
Required. A file pointer, usually created by the fopen() function.
|
| format | Required. A string representing the format of the data expected from the file. |
| arg1, arg2... | Optional. Any number of additional arguments which are pointers to memory where values can be written. |
Technical Details
| Returns: | An int value representing the number of arguments that were written to. It returns the constant EOF if an error occurred. |
|---|
More Examples
Example
Extract numbers from a file containing the sequence "1 + 2 = 3":
FILE *fptr;
fptr = fopen("filename.txt", "r");
int a, b, c;
fscanf(fptr, "%i + %i = %i", &a, &b, &c);
printf("a = %d \n", a);
printf("b = %d \n", b);
printf("c = %d \n", c);
fclose(fptr);
Example
Read a hexadecimal number from a file and output its value in decimal:
FILE *fptr;
fptr = fopen("filename.txt", "r");
int num;
fscanf(fptr, "%x", &num);
printf("%d", num);
fclose(fptr);
Example
Search for either "x", "y" or "z" in the next character of the file:
FILE *fptr;
fptr = fopen("filename.txt", "r");
char c;
int
found = fscanf(fptr, " %[xyz]", &c);
if (found > 0) {
printf("Found %c", c);
} else {
printf("Character not found");
}
fclose(fptr);
