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bool isNumber(char* zahl){
    char flag=0,*p=zahl;
    if(!*p) return false;
    if(*p=='-'||*p=='+')++p;
    for(;*p;++p)
        if(!isdigit(*p))
            if(flag || *p!='.' )return false;
            else ++flag;
    return true;
}

const char * input = … // Eingabe einlesen
char *tailptr = 0;
long int number = strtol(input, &tailptr ,10);
if ((*tailptr) == '\0') {
    // die Eingabe enthielt nur Zahlen
} else {
    // die Eingabe enthielt ungültige Zeichen
}

#include <stdio.h>

/* Workaround to detect non-numerical chars, since the string-cast functions
   don't distinguish between errors and "real" zeros when returning 0 */

int
is_numstring (const char *string)
{
    char *end;

    end = NULL;
    strtol (string, &end, 10);

    return ((*end) == '\0');
}

int
main (int argc, char *argv[])
{
    char *string;

    string = argv[1];
    if (is_numstring (string)) {
        printf ("%i\n", atoi (string));
        return 0;
    } else {
        fprintf (stderr, "%s: found non-numerical chars\n", argv[0]);
        return 1;
    }
}

$ gcc test2.c -o test2
test2.c: In Funktion »string_to_version«:
test2.c:28: Warnung: Initialisierung erzeugt Zeiger von Ganzzahl ohne Typkonvertierung
test2.c:35: Warnung: Initialisierung erzeugt Zeiger von Ganzzahl ohne Typkonvertierung
test2.c:42: Warnung: Initialisierung erzeugt Zeiger von Ganzzahl ohne Typkonvertierung
test2.c:60: Fehler: expected declaration or statement at end of input
test2.c:60: Fehler: expected declaration or statement at end of input
test2.c:60: Fehler: expected declaration or statement at end of input

#include <stdio.h>

/* This holds the version information as strings
   NOTE: minor and micro may be NULL pointers */
typedef struct {
    char *major;
    char *minor;
    char *micro;
} Version;

/* Workaround to detect non-numerical chars, since the string-cast functions
   don't distinguish between errors and "real" zeros when returning 0 */
int
is_numstring (const char *string)
{
    char *end = NULL;
    strtol (string, &end, 10);
    return ((*end) == '\0');
}

/* Parse string like "0.2.3" to Version struct */
Version *
string_to_version (char *version_string)
{
    Version *version;

    /* Major */
    char *major = strtok (version_string, ".");
    if (is_numstring (major)) {
        version->major = major;
    } else {
        version->major = NULL;

    /* Minor */
    char *minor = strtok (NULL, ".");
    if (is_numstring (minor)) {
        version->minor = minor;
    } else {
        version->minor = NULL;

    /* Micro */
    char *micro = strtok (NULL, ".");
    if (is_numstring (micro)) {
        version->micro = micro;
    } else {
        version->micro = NULL;

    return version;
}

int
main (int argc, char *argv[])
{
    char *version_string = argv[1];
    Version *version = string_to_version (version_string);

    printf ("major: %s\nminor: %s\nmicro: %s\n",
            version->major, version->minor, version->micro);
    return 0;
}

#include <stdio.h>
#include <string.h>

/* This holds the version information as strings
   NOTE: minor and micro may be NULL pointers */
typedef struct {
    char *major;
    char *minor;
    char *micro;
} Version;

/* Workaround to detect non-numerical chars, since the string-cast functions
   don't distinguish between errors and "real" zeros when returning 0 */
int
is_numstring (const char *string)
{
    char *end = NULL;
    strtol (string, &end, 10);
    return ((*end) == '\0');
}

/* Parse string like "0.2.3" to Version struct */
Version *
string_to_version (char *version_string)
{
    Version *version;

    /* Major */
    char *major = strtok (version_string, ".");
    if (is_numstring (major)) {
        version->major = major;
    } else {
        return NULL; // return NULL, since there's not even a major number
    }

    /* Minor */
    char *minor = strtok (NULL, ".");
    if (is_numstring (minor)) {
        version->minor = minor;
    } else {
        version->minor = NULL;
    }

    /* Micro */
    char *micro = strtok (NULL, ".");
    if (is_numstring (micro)) {
        version->micro = micro;
    } else {
        version->micro = NULL;
    }

    return version;
}

int
main (int argc, char *argv[])
{
    char *version_string = argv[1];
    Version *version = string_to_version (version_string);

    printf ("major: %s\nminor: %s\nmicro: %s\n",
            version->major, version->minor, version->micro);
    return 0;
}

$ gcc test2.c -o test2 && ./test2 "2.5.4"
Segmentation fault

test.c: In Funktion »is_numstring«:
test.c:18: Warnung: Implizite Deklaration der Funktion »strtol«
test.c: In Funktion »string_to_version«:
test.c:52: Warnung: »version« may be used uninitialized in this function

#include <stdlib.h>

typedef struct {
    int major;
    int minor;
    int micro;
} Version;

Version *string_to_version(const char *str) {
    Version *ret = malloc(sizeof *ret);
    if (!ret) 
        goto fail;
    int len;
    int n = sscanf("%d%n.%d%n.%d%n", &ret->major, &len, &ret->minor, &len, &ret->micro, &len);
    switch (n) {
    case 0:
        goto fail;
    case 2:
        ret->micro = -1;
    case 1:
        ret->minor = -1;
    }
    if (str[len])
        goto fail;

    return ret;

    fail: 
    free(ret);
    return 0; 
}

#include <stdio.h>
#include <stdlib.h>

/* This holds the version information */
typedef struct {
    int major;
    int minor;
    int micro;
} Version;

/* Parse a string like "0.2.3" and return it as a version struct. The fields for
   minor and/or micro are -1, if that information can not be detected. Strings
   like "0.2a" will be interpreted as "0.2". Everything after an non-numeric
   character will be ignored. The function will return at least the major
   version, or NULL if that fails. */ 
Version *
string_to_version (const char *str)
{
    Version *version = malloc (sizeof *version);
    if (!version) {
        goto error;
    }
    int n = sscanf (str, "%d.%d.%d",
                    &version->major, &version->minor, &version->micro);
    switch (n) {
        case 0:
            goto error;
        case 1:
            version->minor = -1;
            version->micro = -1;
        case 2:
            version->micro = -1;
    }
    return version;
    
    error:
        free (version);
        return NULL;
}

int
main (int argc, char *argv[])
{
    char *vstring = argv[1];
    if (!vstring) {
        fprintf (stderr, "An argument is required\n");
        return 1;
    }
    Version *version = string_to_version (vstring);
    if (!version) {
        fprintf (stderr, "Unable to get version information from '%s'\n", vstring);
        return 1;
    }
    printf ("major: %d\nminor: %d\nmicro: %d\n",
            version->major, version->minor, version->micro);
    free (version);

    return 0;
}