C Programming, Part 2: Text Input And Output

Printing to Streams

How do I print strings, ints, chars to the standard output stream?

Use printf. The first parameter is a format string that includes placeholders for the data to be printed. Common format specifiers are %s treat the argument as a c string pointer, keep printing all characters until the NULL-character is reached; %d print the argument as an integer; %p print the argument as a memory address.

A simple example is shown below:

char *name = ... ; int score = ...;
printf("Hello %s, your result is %d\n", name, score);
printf("Debug: The string and int are stored at: %p and %p\n", name, &score );
// name already is a char pointer and points to the start of the array. 
// We need "&" to get the address of the int variable

By default, for performance, printf does not actually write anything out (by calling write) until its buffer is full or a newline is printed.

How else can I print strings and single characters?

Use puts( name ) and putchar( c ) where name is a pointer to a C string and c is just a char

How do I print to other file streams?

Use fprintf( _file_ , "Hello %s, score: %d", name, score); Where _file_ is either predefined 'stdout' 'stderr' or a FILE pointer that was returned by fopen or fdopen

Can I use file descriptors?

Yes! Just use dprintf(int fd, char* format_string, ...); Just remember the stream may be buffered, so you will need to assure that the data is written to the file descriptor.

How do I print data into a C string?

Use sprintf or better snprintf.

char result[200];
int len = snprintf(result, sizeof(result), "%s:%d", name, score);

snprintf returns the number of characters written excluding the terminating byte. In the above example, this would be a maximum of 199. The return value of snprintf is the length that would have been written given enough space, excluding the ending NULL byte.

char x[5];
int size = snprintf(x, 5, "%s%s%s", "12", "34", "56"); // writes "1234" + null
printf("%d\n", size); // output 6

Source: this StackOverflow post and man page.

What if I really really want printf to call write without a newline?

Use fflush( FILE* inp ). The contents of the file will be written. If I wanted to write "Hello World" with no newline, I could write it like this.

int main(){
    fprintf(stdout, "Hello World");
    fflush(stdout);
    return 0;
}

How is perror helpful?

Let's say that you have a function call that just failed (because you checked the man page and it is a failing return code). perror(const char* message) will print the English version of the error to stderr

int main(){
    int ret = open("IDoNotExist.txt", O_RDONLY);
    if(ret < 0){
        perror("Opening IDoNotExist:");
    }
    //...
    return 0;
}

Parsing Input

How do I parse numbers from strings?

Use long int strtol(const char *nptr, char **endptr, int base); or long long int strtoll(const char *nptr, char **endptr, int base);.

What these functions do is take the pointer to your string *nptr and a base (ie binary, octal, decimal, hexadecimal etc) and an optional pointer endptr and returns a parsed value.

int main(){
    const char *nptr = "1A2436";
    char* endptr;
    long int result = strtol(nptr, &endptr, 16);
    return 0;
}

Be careful though! Error handling is tricky because the function won't return an error code. If you give it a string that is not a number it will return 0. This means you cant differentiate between a valid "0" and an invalid string. See the man page for more details on strol behavior with invalid and out of bounds values. A safer alternative is use to sscanf (and check the return value).

int main(){
    const char *input = "0"; // or "!##@" or ""
    char* endptr;
    long int parsed = strtol(input, &endptr, 10);
    if(parsed == 0){
        // Either the input string was not a valid base-10 number or it really was zero!

    }
    return 0;
}

How do I parse input using scanf into parameters?

Use scanf (or fscanf or sscanf) to get input from the default input stream, an arbitrary file stream or a C string respectively. It's a good idea to check the return value to see how many items were parsed. scanf functions require valid pointers. It's a common source of error to pass in an incorrect pointer value. For example,

int *data = (int *) malloc(sizeof(int));
char *line = "v 10";
char type;
// Good practice: Check scanf parsed the line and read two values:
int ok = 2 == sscanf(line, "%c %d", &type, &data); // pointer error

We wanted to write the character value into c and the integer value into the malloc'd memory. However, we passed the address of the data pointer, not what the pointer is pointing to! So sscanf will change the pointer itself. i.e. the pointer will now point to address 10 so this code will later fail e.g. when free(data) is called.

How do I stop scanf from causing a buffer overflow?

The following code assumes the scanf won't read more than 10 characters (including the terminating byte) into the buffer.

char buffer[10];
scanf("%s",buffer);

You can include an optional integer to specify how many characters EXCLUDING the terminating byte:

char buffer[10];
scanf("%9s", buffer); // reads up to 9 charactes from input (leave room for the 10th byte to be the terminating byte)

Why is gets dangerous? What should I use instead?

The following code is vulnerable to buffer overflow. It assumes or trusts that the input line will be no more than 10 characters, including the terminating byte.

char buf[10];
gets(buf); // Remember the array name means the first byte of the array

gets is deprecated in C99 standard and has been removed from the latest C standard (C11). Programs should use fgets or getline instead.

Where each has the following structure respectively:

char *fgets (char *str, int num, FILE *stream); 

ssize_t getline(char **lineptr, size_t *n, FILE *stream);

Here's a simple, safe way to read a single line. Lines longer than 9 characters will be truncated:

char buffer[10];
char *result = fgets(buffer, sizeof(buffer), stdin);

The result is NULL if there was an error or the end of the file is reached. Note, unlike gets, fgets copies the newline into the buffer, which you may want to discard-

if (!result) { return; /* no data - don't read the buffer contents */}

int i = strlen(buffer) - 1;
if (buffer[i] == '\n') 
    buffer[i] = '\0';

How do I use getline?

One of the advantages of getline is that will automatically (re-) allocate a buffer on the heap of sufficient size.

// ssize_t getline(char **lineptr, size_t *n, FILE *stream);

 /* set buffer and size to 0; they will be changed by getline */
char *buffer = NULL;
size_t size = 0;

ssize_t chars = getline(&buffer, &size, stdin);

// Discard newline character if it is present,
if (chars > 0 && buffer[chars-1] == '\n') 
    buffer[chars-1] = '\0';

// Read another line.
// The existing buffer will be re-used, or, if necessary,
// It will be `free`'d and a new larger buffer will `malloc`'d
chars = getline(&buffer, &size, stdin);

// Later... don't forget to free the buffer!
free(buffer);

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